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Cambridge IGCSE0620

Formulae, functional groups and terminology

Chemistry 0620 Chapter Notes

What this chapter covers

Formulae, functional groups and terminologyNaming organic compoundsFuelsAlkanesAlkenesAlcoholsCarboxylic acidsPolymers
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Formulae, functional groups and terminology notes

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In this chapter

  1. 1Displayed formulae
  2. 2General formulae
  3. 3Functional groups and homologous series
  4. 4Saturated and unsaturated compounds
  5. 5Structural formulae
  6. 6Structural isomers
  7. 7Characteristics of a homologous series
  8. 8Types of compound from the name
  9. 9Methane, ethane and ethene
  10. 10Ethanol and ethanoic acid
  11. 11Unbranched alkanes and alkenes
  12. 12Unbranched alcohols
  13. 13Unbranched carboxylic acids
  14. 14Esters
  15. 15Fossil fuels and natural gas
  16. 16Hydrocarbons and petroleum
  17. 17Separating petroleum
  18. 18Trends up the fractionating column
  19. 19Uses: refinery gas to kerosene
  20. 20Uses: diesel oil to bitumen
  21. 21Saturated hydrocarbons with single bonds
  22. 22Unreactive, except combustion and substitution
  23. 23Substitution reactions
  24. 24A photochemical reaction
  25. 25Products of monosubstitution
  26. 26Alkenes: unsaturated hydrocarbons
  27. 27Cracking larger alkanes
  28. 28Testing for unsaturation
  29. 29Addition reactions
  30. 30Alkenes and bromine
  31. 31Alkenes with hydrogen and with steam
  32. 32Making ethanol by fermentation
  33. 33Making ethanol from ethene and steam
  34. 34Combustion of ethanol
  35. 35Uses of ethanol
  36. 36Comparing the two methods
  37. 37Ethanoic acid and metals
  38. 38Ethanoic acid and bases
  39. 39Ethanoic acid and carbonates
  40. 40Making ethanoic acid from ethanol
  41. 41Making an ester
  42. 42Naming and writing esters
  43. 43Polymers, monomers and plastics
  44. 44Addition polymerisation of ethene
  45. 45Plastics and the environment
  46. 46Addition polymers: repeat unit and monomer
  47. 47Condensation polymerisation
  48. 48Nylon and PET
  49. 49Proteins

1. Displayed formulae

A displayed formula shows all the atoms and all the bonds in a molecule. Every bond is drawn as a line.

To interpret one, count the atoms of each element. That gives the molecular formula.

Examiner insight

“The structure was well known but many candidates did not show all the bonds, particularly the O–H bond in the functional group.”

Worked example 14 marks

Compound X has two carbon atoms joined by a single bond. One carbon atom is bonded to three hydrogen atoms. The other carbon atom is bonded to two hydrogen atoms and to one -OH group.

(a) Deduce the molecular formula of X.

(b) State the number of bonds in the displayed formula of X.

  1. 1

    (a) carbon = 2, hydrogen = 3 + 2 + 1 = 6, oxygen = 1 [1]

  2. 2

    molecular formula = C₂H₆O [1]

  3. 3

    (b) one C-C, five C-H, one C-O and one O-H [1]

  4. 4

    total = 1 + 5 + 1 + 1 = 8 bonds [1]

2. General formulae

Compounds in the same homologous series have the same general formula. For alkanes, alkenes and alcohols, n is the number of carbon atoms in one molecule.

For carboxylic acids the carbon atom in the -COOH group is extra, so a molecule has n + 1 carbon atoms in total. To use a general formula, put in n and work out each subscript.

alkanes: CnH₂n₊₂

alkenes: CnH₂n

alcohols: CnH₂n₊₁OH

carboxylic acids: CnH₂n₊₁COOH

Examiner insight

“The general formula of alkanes was well known, but candidates needed to be careful in the use of subscript.” “Some candidates did not use correct subscripts and others wrote RCOOH.”

Worked example 14 marks

(a) Hexane is an alkane with six carbon atoms in each molecule. Use the general formula to deduce its molecular formula.

(b) An alcohol has four carbon atoms in each molecule. Deduce its molecular formula.

  1. 1

    (a) alkanes: CnH₂n₊₂ with n = 6, so 2n + 2 = 14 [1]

  2. 2

    molecular formula = C₆H₁₄ [1]

  3. 3

    (b) alcohols: CnH₂n₊₁OH with n = 4 gives C₄H₉OH [1]

  4. 4

    molecular formula = C₄H₁₀O [1]

3. Functional groups and homologous series

A functional group is an atom or group of atoms that determine the chemical properties of a homologous series.

A homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group.

alcohols: -OH

carboxylic acids: -COOH

alkenes: C=C

Examiner insight

“Many candidates did not appear to know the meaning of a functional group because they included large portions of the carbon chain within their ring.” “Some candidates wrote group rather than series or used the term hydrocarbon.”

Worked example 12 marks

A, B, C and D are the first four members of a family of organic compounds with similar chemical properties.

(a) State the term given to a family of organic compounds with similar chemical properties.

(b) Explain why members of a family of organic compounds show similar chemical properties.

  1. 1

    (a) homologous series [1]

  2. 2

    (b) they have the same functional group [1]

4. Saturated and unsaturated compounds

A saturated compound has molecules in which all carbon–carbon bonds are single bonds.

An unsaturated compound has molecules in which one or more carbon–carbon bonds are not single bonds. The usual case is a carbon–carbon double bond, C=C.

Only carbon–carbon bonds count. A C=O bond, as in -COOH, does not make a compound unsaturated.

Examiner insight

“Most candidates chose option B, suggesting they were not clear that an unsaturated compound contains one or more multiple carbon-carbon bonds such as C=C.” “Some candidates stated that unsaturated substances do not contain single bonds.”

Worked example 11 mark

Compound M is unsaturated.

State why compound M is unsaturated.

  1. 1

    contains carbon-carbon double bond(s) / contains C=C bond(s) [1]

  2. 2

    a saturated compound would have carbon-carbon single bonds only

5. Structural formulae

A structural formula is an unambiguous description of the way the atoms in a molecule are arranged. It is not the same as the molecular formula, which only counts atoms.

Examples are CH₂=CH₂, CH₃CH₂OH and CH₃COOCH₃.

Examiner insight

“Some candidates gave an ambiguous structural formula attempt for B, for example CH3C=OOH or CH3CHO2.” “This question proved difficult with few candidates recognising HCOOH as a structural formula.”

Worked example 11 mark

What is the structural formula of propan-1-ol?

A C₃H₇OH · B CH₃CH₂CH₂OH · C C₃H₈O · D CnH₂n₊₁OH

  1. 1

    B: CH₃CH₂CH₂OH shows how the atoms are arranged [1]

  2. 2

    C₃H₇OH is ambiguous, C₃H₈O only counts atoms, and the last option is a general formula

6. Structural isomers

Structural isomers are compounds with the same molecular formula, but different structural formulae.

The two compounds in each pair below have the same molecular formula, but their atoms are joined in a different order.

Writing the same structure from the other end does not give a new isomer: CH₂=CHCH₂CH₃ is the same compound as CH₃CH₂CH=CH₂.

CH₃CH₂CH₂CH₃ and CH₃CH(CH₃)CH₃ (C₄H₁₀)

CH₃CH₂CH=CH₂ and CH₃CH=CHCH₃ (C₄H₈)

Examiner insight

“The word ‘structural’ was often missing from structural isomer. Isotope was seen occasionally as was ‘structural isotope’.”

Worked example 13 marks

Compound X is CH₃CH₂CH₂CH₂CH₃. Compound Y is CH₃CH(CH₃)CH₂CH₃. Show that X and Y are structural isomers.

  1. 1

    both have the molecular formula C₅H₁₂ [1]

  2. 2

    their structural formulae are different: X is a straight chain, Y has a CH₃ branch [1]

  3. 3

    same molecular formula and different structural formulae, so they are structural isomers [1]

7. Characteristics of a homologous series

Members of a homologous series have the same functional group, the same general formula and similar chemical properties. Each member differs from the next by a -CH₂⁻ unit.

They also show a trend in physical properties. In Table 4.1 below, the density of the alkanes increases as the number of carbon atoms increases.

next member = previous member + CH₂

Examiner insight

“Common errors were to mix up chemical and physical properties (giving incorrect answers such as ‘a trend in chemical properties’) and confusion between general and molecular formulae (giving answers such as ‘have the same molecular formula’).” “Common errors included stating ‘Same molecular formula of CnH2n’ where candidates confused molecular and general formula, and ‘differ by a –CH2– molecule’.”

Worked example 12 marks

Table 4.1 shows the properties of some alkanes. A question mark means the value is not given.

(i) Predict the boiling point of propane.

(iii) State the trend in the density of the alkanes as the number of carbon atoms increases.

alkanenumber of carbon atoms in one moleculemelting point in °Cboiling point in °Cdensity of liquid alkane in g / cm³
methane1–182–1620.47
ethane2–183–880.57
propane3–188?0.59
butane4?00.60
pentane5–130360.63
  1. 1

    (i) lies between ethane (–88 °C) and butane (0 °C): any value from –87 °C to –1 °C [1]

  2. 2

    (iii) density increases as the number of carbon atoms increases [1]

Recap

  • A displayed formula shows all the atoms and all the bonds, including O-H.
  • General formulae: alkanes CnH₂n₊₂, alkenes CnH₂n, alcohols CnH₂n₊₁OH, carboxylic acids CnH₂n₊₁COOH.
  • A functional group is an atom or group of atoms that determine the chemical properties of a homologous series.
  • A homologous series is a family of similar compounds with similar chemical properties due to the same functional group.
  • Saturated: all carbon–carbon bonds are single. Unsaturated: one or more are not single.
  • Extended: a structural formula is an unambiguous description of how the atoms are arranged.
  • Extended: structural isomers have the same molecular formula but different structural formulae.
  • Extended: a homologous series has the same functional group and general formula, differs by -CH₂⁻, and shows a trend in physical properties.

8. Types of compound from the name

The ending of a name shows the type of compound. -ane is an alkane, -ene is an alkene, -ol is an alcohol and -oic acid is a carboxylic acid.

The start of the name gives the number of carbon atoms in the chain: meth (1), eth (2), prop (3), but (4).

An alkene has a C=C double bond. An alcohol has an OH group. A carboxylic acid has a COOH group.

ethane C₂H₆ ethene C₂H₄

ethanol C₂H₆O ethanoic acid C₂H₄O₂

Examiner insight

“Option A was the most common incorrect answer where candidates confused ethane with ethene.” “The commonest errors were to suggest ethanol or ethene instead of ethanoic acid.”

Worked example 14 marks

State the type of compound for each of these four names.

(a) propan-2-ol(b) but-1-ene(c) ethanoic acid(d) propane

  1. 1

    propan-2-ol ends in -ol: alcohol [1]

  2. 2

    but-1-ene ends in -ene: alkene [1]

  3. 3

    ethanoic acid ends in -oic acid: carboxylic acid [1]

  4. 4

    propane ends in -ane: alkane [1]

9. Methane, ethane and ethene

A displayed formula shows every atom and every bond. Each carbon atom forms four bonds. Each hydrogen atom forms one.

Methane has one carbon atom bonded to four hydrogen atoms. Ethane has two carbon atoms joined by a single bond, with three hydrogen atoms on each.

Ethene has two carbon atoms joined by a double bond, with two hydrogen atoms on each.

methane CH₄

ethane CH₃CH₃ ethene CH₂=CH₂

Examiner insight

“The most common errors were either leaving off one or more hydrogen atoms or drawing the structure of methane or the addition of a C=C bond.” “Others did not gain credit because they wrote a molecular formula as well as a displayed formula, often with an arrow between the two, so that it was not clear which formula was being selected.”

Worked example 12 marks

Ethane has the molecular formula C₂H₆ and ethene has the molecular formula C₂H₄. State how the displayed formula of ethene differs from the displayed formula of ethane.

  1. 1

    ethene has a C=C double bond between the carbon atoms, ethane has a single bond [1]

  2. 2

    each carbon atom in ethene has two hydrogen atoms, in ethane it has three [1]

10. Ethanol and ethanoic acid

Ethanol is an alcohol. Its structural formula is CH₃CH₂OH, also written C₂H₅OH. Its molecular formula is C₂H₆O: six hydrogen atoms and one oxygen atom.

Ethanoic acid is a carboxylic acid. Its structural formula is CH₃COOH. Its displayed formula has a C=O bond and an O-H bond. Ethanoic acid is the organic product when ethanol is oxidised.

A displayed formula of either compound must show the covalent bond between the oxygen atom and the hydrogen atom.

ethanol CH₃CH₂OH

ethanoic acid CH₃COOH

Examiner insight

“Many errors in the formula of ethanol were seen; many candidates thought that ethanol contained two oxygen atoms or five hydrogen atoms.” “Common errors included incorrect numbers of carbons in the compound, and omitting the O–H bond in the carboxylic group.”

Worked example 12 marks

Ethanol has the structural formula CH₃CH₂OH. State the number of C-H bonds and the number of O-H bonds in the displayed formula of ethanol.

  1. 1

    CH₃ has 3 C-H bonds and CH₂ has 2, so 5 C-H bonds [1]

  2. 2

    the OH group gives 1 O-H bond [1]

11. Unbranched alkanes and alkenes

The unbranched alkanes are methane, ethane, propane and butane. The alkenes are ethene, propene, but-1-ene and but-2-ene.

In an alkene name, the number is the carbon atom where the double bond starts. Number from the end that gives the lower number.

propane CH₃CH₂CH₃ propene CH₂=CHCH₃

but-1-ene CH₂=CHCH₂CH₃ but-2-ene CH₃CH=CHCH₃

Examiner insight

“When naming unbranched alkenes with more than three atoms, it is essential that candidates use the IUPAC convention to identify the position of the double bond, as exemplified in section 11.2.3(a) of the syllabus.”

Worked example 12 marks

Write the structural formula of(a) but-1-ene and(b) but-2-ene.

  1. 1

    (a) CH₂=CHCH₂CH₃ [1]

  2. 2

    (b) CH₃CH=CHCH₃ [1]

12. Unbranched alcohols

Alcohols end in -ol and contain the OH group. The number in the name shows which carbon atom carries the OH group.

In propan-1-ol the OH group is on an end carbon atom. In propan-2-ol it is on the middle one. The same pattern gives butan-1-ol and butan-2-ol.

propan-1-ol CH₃CH₂CH₂OH propan-2-ol CH₃CH(OH)CH₃

butan-1-ol CH₃CH₂CH₂CH₂OH butan-2-ol CH₃CH(OH)CH₂CH₃

Examiner insight

“Quite often, ‘butan-1-ol’ appeared as ‘butanol’ and where numbering was applied, it was often incorrect and names such as ‘but-2-ol’ were frequently seen.” “Most candidates named the alcohol as propanol.”

Worked example 13 marks

Alcohols are a homologous series of organic compounds. Table 6.1 gives some information about alcohols A, B, C and D.

(b)(iii) Name C.

(b)(iv) Name D.

(c) The structural formula of B, CH₃CH₂OH, can also be written as C₂H₅OH. Explain why the structural formula C₃H₇OH does not identify C.

alcoholABCD
number of carbon atoms1233
namemethanolethanol
structural formulaCH₃OHCH₃CH₂OHCH₃CH₂CH₂OHCH₃CH(OH)CH₃
  1. 1

    C is propan-1-ol [1]

  2. 2

    D is propan-2-ol [1]

  3. 3

    C₃H₇– can be CH₃CH₂CH₂– or CH₃(CH₃)CH–, so the formula fits both C and D [1]

13. Unbranched carboxylic acids

Carboxylic acids end in -oic acid. The COOH group is at the end of the chain, and its carbon atom counts in the number of carbon atoms.

So ethanoic acid, with two carbon atoms, is CH₃COOH, and propanoic acid, with three, is CH₃CH₂COOH.

methanoic acid HCOOH ethanoic acid CH₃COOH

propanoic acid CH₃CH₂COOH butanoic acid CH₃CH₂CH₂COOH

Examiner insight

“Some candidates gave a correct displayed formula for propanoic acid instead and others drew alcohols.”

Worked example 11 mark

Which of the named compounds have the correct structural formula?

1 ethanoic acid, CH₃CH₂COOH

2 propan-2-ol, CH₃CH(OH)CH₃

3 but-2-ene, CH₃CH=CHCH₃

A 1 and 2 · B 1 and 3 · C 2 and 3 · D 3 only

  1. 1

    1 is wrong: CH₃CH₂COOH has three carbon atoms, so it is propanoic acid

  2. 2

    2 is right: three carbon atoms with the OH group on the middle one

  3. 3

    3 is right: four carbon atoms with the double bond starting at carbon 2

  4. 4

    C: 2 and 3 only. [1]

14. Esters

An alcohol reacts with a carboxylic acid, in the presence of a catalyst, to make an ester and water.

The name has two words. The first comes from the alcohol: methanol gives methyl, ethanol gives ethyl, propan-1-ol gives propyl, butan-1-ol gives butyl. The second comes from the acid: ethanoic acid gives ethanoate.

In ethyl ethanoate, CH₃COOCH₂CH₃, the CH₃COO part comes from the acid and the CH₂CH₃ part comes from the alcohol. The ester has all the carbon atoms of both.

CH₃CH₂OH + CH₃COOH →(catalyst) CH₃COOCH₂CH₃ + H₂O

ethyl ethanoate butyl ethanoate ethyl butanoate

Examiner insight

“Candidates are required to know the name of esters which can be made from unbranched alcohols and carboxylic acids, each containing up to four carbon atoms so the two expected names were ethyl butanoate and butyl ethanoate.” “Some candidates drew incorrect ester linkages, omitted hydrogens and/or added carbons to the alkyl group.”

Worked example 13 marks

Ethanol reacts with propanoic acid in the presence of a catalyst. An ester and one other product are formed. Name the ester and name the other product.

  1. 1

    first word from the alcohol: ethyl [1]

  2. 2

    second word from the acid: propanoate, so ethyl propanoate [1]

  3. 3

    the other product is water [1]

Recap

  • The ending gives the type: -ane alkane, -ene alkene, -ol alcohol, -oic acid carboxylic acid.
  • Methane CH₄, ethane C₂H₆, ethene C₂H₄. Ethene has a C=C double bond.
  • Ethanol CH₃CH₂OH and ethanoic acid CH₃COOH: draw the O-H bond.
  • Extended: the number in but-1-ene and butan-2-ol is the position of the double bond or OH group.
  • Extended: the COOH carbon counts in the chain, so propanoic acid is CH₃CH₂COOH.
  • Extended: an ester name is alcohol part then acid part, such as ethyl propanoate. The other product is water.

15. Fossil fuels and natural gas

The three fossil fuels are coal, natural gas and petroleum.

Methane is the main constituent of natural gas.

Examiner insight

“Candidates did not know that natural gas is mainly methane. The commonest incorrect answer was to suggest ‘hydrogen’.” “It was anticipated that most candidates would be able to recall the main constituent of natural gas, but the most common answer was carbon dioxide.”

Worked example 11 mark

Four different fuels are listed. 1 ethanol · 2 petroleum · 3 hydrogen · 4 natural gas. Which fuels are fossil fuels?

A 1 and 3 · B 1 and 4 · C 2 and 3 · D 2 and 4

  1. 1

    D: petroleum and natural gas are fossil fuels. [1]

16. Hydrocarbons and petroleum

Hydrocarbons are compounds that contain hydrogen and carbon only.

Petroleum is a mixture of hydrocarbons. Its other name is crude oil.

Examiner insight

“Most omitted the words ‘compound’ or ‘only’ which were needed to gain credit.” “Common incorrect answers were “petrol” (not the same as petroleum) and “oil” (not the same as “crude oil”).”

Worked example 12 marks

State what is meant by the term hydrocarbon.

  1. 1

    a compound [1]

  2. 2

    that contains hydrogen and carbon only [1]

17. Separating petroleum

Petroleum is separated into useful fractions by fractional distillation. The property that allows this is that the fractions have different boiling points.

Petroleum is heated, so the hydrocarbons vaporise. The vapours pass into a fractionating column. The column is hotter at the bottom and cooler at the top.

The vapours condense at different heights in the column and are collected as liquids.

Examiner insight

“Not many candidates knew that for the hydrocarbons to be separated in this way they have to have different boiling points.” “The distinction between fractional distillation and cracking confused many candidates.”

Worked example 14 marks

Outline how petroleum is separated by fractional distillation.

In your answer, include the property of the fractions which allows them to be separated, and a description of how fractional distillation separates the petroleum into fractions.

  1. 1

    property: the fractions have different boiling points [1]

  2. 2

    the petroleum is heated, so it vaporises [1]

  3. 3

    the column is cooler at the top and hotter at the bottom [1]

  4. 4

    the vapours condense and are collected as liquids [1]

19. Uses: refinery gas to kerosene

Refinery gas is the gas used in heating and cooking. Gasoline / petrol is the fuel used in cars.

Naphtha is a chemical feedstock: it is used for making chemicals. Kerosene / paraffin is jet fuel.

Examiner insight

“The commonest error was to suggest that kerosene was used to make chemicals.” “Common errors were that petrol / diesel was used as ‘jet fuel’ and that bitumen was used as ‘ships fuel’.”

Worked example 11 mark

Some of the fractions obtained from the fractional distillation of petroleum are used as fuels for vehicles. Which two fractions are used as fuels for vehicles?

A bitumen fraction and gasoline fraction B bitumen fraction and naphtha fraction C gasoline fraction and kerosene fraction D kerosene fraction and lubricating fraction

  1. 1

    C: gasoline is the fuel used in cars and kerosene is jet fuel. [1]

20. Uses: diesel oil to bitumen

Diesel oil / gas oil is the fuel used in diesel engines. Fuel oil is the fuel used in ships and home heating systems.

The lubricating oil fraction is used for lubricants, waxes and polishes. Bitumen is used for making roads.

Examiner insight

“Few candidates recalled the use of fuel oil as the fuel for ships or home heating systems. The most common answer was fuel for cars.” “The commonest errors were to link naphtha to waxes and polishes or the lubricating fraction to making chemicals.”

Worked example 12 marks

Two of the fractions obtained from petroleum are bitumen and fuel oil. State one use of each of these fractions.

  1. 1

    bitumen: making roads [1]

  2. 2

    fuel oil: fuel for ships, or fuel for home heating [1]

Recap

  • Fossil fuels: coal, natural gas and petroleum.
  • Methane is the main constituent of natural gas.
  • A hydrocarbon is a compound containing hydrogen and carbon only.
  • Petroleum is a mixture of hydrocarbons, separated by fractional distillation because the fractions have different boiling points.
  • Bottom to top of the column: shorter chains, lower boiling points, higher volatility, lower viscosity.
  • Refinery gas: heating and cooking. Gasoline: cars. Naphtha: chemical feedstock. Kerosene: jet fuel.
  • Diesel oil: diesel engines. Fuel oil: ships and home heating. Lubricating oil: lubricants, waxes and polishes. Bitumen: making roads.

21. Saturated hydrocarbons with single bonds

Alkanes are hydrocarbons. A hydrocarbon is a compound containing hydrogen and carbon only.

The bonding in alkanes is single covalent. Alkanes are saturated hydrocarbons: they contain no carbon–carbon double bonds.

A compound that contains carbon–carbon double bond(s) is unsaturated.

C_nH_{2n+2}

Examiner insight

“Most candidates identified ethene as a hydrocarbon but confused the meaning of saturated and unsaturated.”

Worked example 11 mark

Which statement describes ethane?

A It is a saturated covalent compound. · B It is an unsaturated covalent compound. · C It is a saturated ionic compound. · D It is an unsaturated ionic compound.

  1. 1

    A: ethane is a saturated covalent compound. [1]

22. Unreactive, except combustion and substitution

Alkanes are generally unreactive. They react in only two ways: combustion and substitution by chlorine.

In complete combustion an alkane burns in oxygen. The products are carbon dioxide and water.

2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O

Examiner insight

“The syllabus describes alkanes as only reacting in combustion or in substitution reactions with chlorine which gives the answer.”

Worked example 12 marks

Pentane is a member of the alkane homologous series. Pentane reacts in the same way as ethane.

Name the two products of the complete combustion of pentane.

  1. 1

    carbon dioxide [1]

  2. 2

    water [1]

23. Substitution reactions

In a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms.

When an alkane reacts with chlorine, a hydrogen atom of the alkane is replaced by a chlorine atom.

C₃H₈ + Cl₂ → C₃H₇Cl + HCl

Examiner insight

“The frequent transposing of the displacement and substitution reaction equations by candidates demonstrated widespread confusion between the two types of reaction.”

Worked example 11 mark

Pentane is a member of the alkane homologous series. Pentane reacts in the same way as ethane.

Identify the type of reaction that takes place when pentane reacts with chlorine. Choose from: addition · neutralisation · redox · substitution

  1. 1

    substitution [1]

24. A photochemical reaction

The reaction of an alkane with chlorine is photochemical: it needs or uses light.

Ultraviolet light provides the activation energy, E_a, for the reaction. With ultraviolet light the reaction takes place at room temperature.

C₂H₆ + Cl₂ →(ultraviolet light) C₂H₅Cl + HCl

Examiner insight

“In (ii), the idea of ultraviolet light providing the activation energy needed for the reaction was not known.” “However, candidates who performed less well wrote only about the temperature or pressure requirements.”

Worked example 11 mark

The equation for the reaction between butane and chlorine is C₄H₁₀ + Cl₂ → C₄H₉Cl + HCl.

State the condition required for the reaction between butane and chlorine to occur at room temperature.

  1. 1

    ultraviolet light [1]

25. Products of monosubstitution

In monosubstitution only one hydrogen atom is replaced, so each product molecule has one chlorine atom.

The products are a chloroalkane and hydrogen chloride. Propane gives two structural isomers of C₃H₇Cl: 1-chloropropane and 2-chloropropane.

1-chloropropane:CH₃CH₂CH₂Cl

2-chloropropane:CH₃CHClCH₃

Examiner insight

“Many candidates wrote ‘hydrochloric acid’ for their response. This is incorrect. In the absence of water, candidates need to appreciate that the other product in this substitution reaction is gaseous hydrogen chloride.” “Candidates should be taught that in the case of a monosubstitution, a chlorine atom drawn on either end of the molecule is the same isomer.”

Worked example 13 marks

Butane, CH₃CH₂CH₂CH₃, reacts with chlorine in a monosubstitution reaction. Give the structural formula of each of the two organic compounds with the molecular formula C₄H₉Cl and name the other product.

  1. 1

    1-chlorobutane, CH₃CH₂CH₂CH₂Cl [1]

  2. 2

    2-chlorobutane, CH₃CHClCH₂CH₃ [1]

  3. 3

    hydrogen chloride [1]

Recap

  • A hydrocarbon is a compound containing hydrogen and carbon only.
  • Alkanes have single covalent bonds and are saturated hydrocarbons.
  • Alkanes are generally unreactive, except in combustion and in substitution by chlorine.
  • Extended: in a substitution reaction one atom or group of atoms is replaced by another.
  • Extended: alkane and chlorine react in a photochemical reaction; ultraviolet light provides the activation energy, E_a.
  • Extended: monosubstitution makes a chloroalkane and hydrogen chloride, e.g. 1-chloropropane and 2-chloropropane.

26. Alkenes: unsaturated hydrocarbons

A hydrocarbon is a compound containing hydrogen and carbon only.

The bonding in alkenes includes a carbon-carbon double bond, C=C. Alkenes are unsaturated hydrocarbons.

Ethene, CH₂=CH₂, is an alkene.

Examiner insight

“The correct answer was that the ‘compound was unsaturated’ because it had a ‘carbon-carbon double bond’ and not just a ‘double bond’ or ‘carbon double bond’.” “Most candidates identified ethene as a hydrocarbon but confused the meaning of saturated and unsaturated.”

Worked example 11 mark

Which statement about alkenes is correct?

A Alkenes are saturated molecules containing a C=C double bond.

B Alkenes are hydrocarbons with the general formula C_nH_{2n+2}.

C Alkenes react with aqueous bromine to give a colour change from brown to colourless.

D Alkenes are manufactured from larger alkene molecules in a process called combustion.

  1. 1

    C: alkenes react with aqueous bromine and decolourise it. [1]

  2. 2

    This question calls the starting colour brown. In a written answer give orange, the colour the mark scheme credits.

  3. 3

    A is wrong: a molecule with a C=C double bond is unsaturated.

  4. 4

    B is wrong: C_nH_{2n+2} is the general formula of alkanes.

  5. 5

    D is wrong: alkenes are made from larger alkanes, by cracking.

27. Cracking larger alkanes

Cracking breaks larger alkane molecules into smaller molecules. It needs a high temperature and a catalyst.

The products are alkenes and hydrogen, and shorter alkanes.

Cracking is done to make more useful hydrocarbons. These include alkenes, and shorter-chain alkanes used in petrol.

C₁₀H₂₂ →(high temperature, catalyst) C₈H₁₈ + C₂H₄

C₁₀H₂₂ → C₄H₈ + C₆H₁₂ + H₂

Examiner insight

“Most candidates just described what cracking is rather than why it is done.” “The purpose of cracking is to produce a range of more useful hydrocarbons. This can include alkenes and shorter-chain alkanes used in petrol.”

Worked example 13 marks

Decane, C₁₀H₂₂, is cracked. One product is ethene, C₂H₄. Deduce the formula of the other product, write the equation and state the conditions for cracking.

  1. 1

    carbon atoms 10 - 2 = 8 and hydrogen atoms 22 - 4 = 18, so the other product is C₈H₁₈ [1]

  2. 2

    equation: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ [1]

  3. 3

    high temperature and a catalyst [1]

28. Testing for unsaturation

Add aqueous bromine to the hydrocarbon. The test separates saturated from unsaturated hydrocarbons.

An unsaturated hydrocarbon turns aqueous bromine from orange to colourless. A saturated hydrocarbon leaves it orange.

Describe the colour change. Naming the reaction is not enough.

Examiner insight

“Many candidates did not know that the colour of aqueous bromine used in this test should be orange.” “Common errors were: ethene turns colourless; no reaction; stating why the change occurred rather than giving a description of the colour change; or suggesting that the colour change was to “clear” or “to a different colour”.”

Worked example 12 marks

Compound P is unsaturated and colourless. A test-tube contains aqueous bromine. Describe the colour change when excess compound P is added to the test-tube.

  1. 1

    from orange [1]

  2. 2

    to colourless [1]

29. Addition reactions

In an addition reaction only one product is formed.

Two reactants give one product, as in C₄H₈ + H₂ → C₄H₁₀. A reaction that gives two or more products is not an addition reaction.

Alkenes undergo addition reactions with bromine, hydrogen and steam.

Examiner insight

“Candidates were aware of the concept of an addition reaction but did not state the key point that only one product is formed.” “Weaker candidates answered in terms of breaking double bonds, explaining saturation or stating bromine is added.”

Worked example 11 mark

Which equation describes an addition reaction?

A C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

B C₃H₈ + Br₂ → C₃H₇Br + HBr

C C₁₀H₂₂ → C₄H₈ + C₆H₁₂ + H₂

D C₄H₈ + H₂ → C₄H₁₀

  1. 1

    D: two reactants join to give one product. [1]

  2. 2

    A has two products (combustion), B has two products (substitution), and C has three products (cracking).

30. Alkenes and bromine

Alkenes react with bromine or aqueous bromine in an addition reaction. Aqueous bromine turns from orange to colourless.

One bromine atom adds to each of the two carbon atoms of the C=C double bond. The product has no double bond.

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

CH₃CH=CHCH₃ + Br₂ → CH₃CHBrCHBrCH₃

Examiner insight

“Many candidates did not recall that the reaction of an alkene with bromine is an addition reaction.” “A large number of the candidates who gave a dibromo- product attached the bromines each to C1 and C3 whereas others attached both bromines to the same C atom.”

Worked example 11 mark

Which compound reacts rapidly with bromine to produce CH₃CHBrCHBrCH₃?

A CH₃CH₂Br

B CH₂=CHCH₂CH₃

C CH₃CH=CHCH₃

D CH₃CH₂CH₂CH₃

  1. 1

    C: bromine adds across the C=C of CH₃CH=CHCH₃, one Br on each carbon. [1]

  2. 2

    B would give CH₂BrCHBrCH₂CH₃. A and D have no double bond.

31. Alkenes with hydrogen and with steam

Alkenes react with hydrogen in the presence of a nickel catalyst. The product is an alkane.

Alkenes react with steam in the presence of an acid catalyst. The product is an alcohol. For ethene it is ethanol.

Each reaction gives only one product, so both are addition reactions. The reagent is steam, not liquid water.

CH₂=CH₂ + H₂ →(Ni) CH₃CH₃

CH₂=CH₂(g) + H₂O(g) →(acid catalyst) CH₃CH₂OH(g)

Examiner insight

“For the reagent, candidates often incorrectly wrote water rather than steam; although, many rescued this by stating a temperature above 100°C.” “Fermentation and hydrolysis were frequent incorrect names for the conversion of ethene to ethanol.”

Worked example 11 mark

Alkenes can undergo addition reactions with aqueous bromine, hydrogen or steam. Which reactions of ethene need a catalyst?

1 with aqueous bromine

2 with steam

3 with hydrogen

A 1, 2 and 3

B 1 and 2 only

C 1 only

D 2 and 3 only

  1. 1

    D: steam needs an acid catalyst and hydrogen needs nickel. [1]

  2. 2

    Aqueous bromine needs no catalyst.

Recap

  • Alkenes are hydrocarbons with a carbon-carbon double bond. They are unsaturated.
  • Cracking: larger alkanes, a high temperature and a catalyst give alkenes and hydrogen.
  • Cracking is done to make more useful hydrocarbons.
  • Aqueous bromine test: unsaturated turns orange to colourless. Saturated stays orange.
  • Extended: in an addition reaction only one product is formed.
  • Extended: alkene + bromine: one Br adds to each carbon of the double bond.
  • Extended: alkene + hydrogen, nickel catalyst, gives an alkane.
  • Extended: alkene + steam, acid catalyst, gives an alcohol.

32. Making ethanol by fermentation

Ethanol is made by fermentation of aqueous glucose. The conditions are a temperature of 25 – 35 °C, the presence of yeast and the absence of oxygen (anaerobic).

Yeast is the catalyst. Name it: ‘catalyst’ alone is not enough detail.

This is a low-temperature process. At high temperatures the enzymes in the yeast are denatured and fermentation does not occur.

Examiner insight

“Common errors included ‘presence of oxygen’, ‘high temperature’ or ‘high pressure’.” “Candidates must also be careful when quoting a range of temperatures. The highest and lowest values in their range must be included in the correct range.”

Worked example 12 marks

Ethanol is manufactured by the fermentation of aqueous glucose. State two conditions needed for the fermentation of aqueous glucose.

  1. 1

    one condition from: 25 – 35 °C (inclusive of these values), yeast, absence of oxygen / anaerobic [1]

  2. 2

    a second, different condition from the same list [1]

33. Making ethanol from ethene and steam

Ethanol is also made by the catalytic addition of steam to ethene. The conditions are 300 °C, 6000 kPa / 60 atm and an acid catalyst.

The reagent is steam, not liquid water. Ethene and steam join to form ethanol.

C₂H₄(g) + H₂O(g) →(acid catalyst) C₂H₅OH(g)

Examiner insight

“Some gave the correct conditions for fermentation rather than for the process of producing ethanol by hydrating ethene with steam.” “Many candidates thought that the reagent needed to convert ethene to ethanol is ‘water’ rather than ‘steam’.”

Worked example 13 marks

Outline how ethanol is manufactured from ethene and steam. In your answer, include:

• the name of this reaction • two conditions that are needed.

  1. 1

    name of reaction: (catalytic) addition [1]

  2. 2

    one condition from: 300 °C, 6000 kPa / 60 atm, acid (catalyst) [1]

  3. 3

    a second, different condition from the same list [1]

34. Combustion of ethanol

Ethanol burns in oxygen. In complete combustion the only products are carbon dioxide and water.

The reaction is exothermic: it gives out heat. That is why ethanol works as a fuel.

Balance the oxygen last. Remember the oxygen atom already in the ethanol.

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O

Examiner insight

“More practice is needed in remembering the two products formed when ethanol undergoes complete combustion.” “A few candidates confused complete combustion with incomplete combustion and gave incorrect products. For example, carbon monoxide or carbon.”

Worked example 12 marks

Ethanol is a fuel. Write the symbol equation for the complete combustion of ethanol.

  1. 1

    CO₂ and H₂O as the only products [1]

  2. 2

    correct equation: CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O [1]

35. Uses of ethanol

Ethanol is used as a solvent and as a fuel.

As a fuel it burns in the way shown in the last section. Camping stoves burn it for cooking.

Give a specific use. A vague use earns nothing.

Examiner insight

“Other candidates referred vaguely to “used in chemistry labs” or “used for making chemicals”.” “It is a good idea for candidates to know at least two uses of the substances mentioned for uses in the syllabus.”

Worked example 11 mark

State one use of ethanol.

  1. 1

    solvent / fuel [1]

36. Comparing the two methods

Fermentation uses renewable raw materials. That is its advantage. Its disadvantage is that it is slower. The ethanol is left in the reaction mixture, so it is separated by distillation.

The addition of steam to ethene is faster and gives ethanol of greater purity. Its raw materials are not renewable. Do not swap the advantages of the two methods.

Examiner insight

“A minority of candidates were able to recognise that the advantage of fermentation is the use of renewable raw materials.” “Some candidates mixed up the advantages of each method and stated that catalytic hydration used renewable materials.”

Worked example 14 marks

A company can make ethanol by fermentation or by the addition of steam to ethene. Give one advantage and one disadvantage of each method.

  1. 1

    fermentation, advantage: uses renewable raw materials [1]

  2. 2

    fermentation, disadvantage: slower / the ethanol must be separated from the mixture by distillation [1]

  3. 3

    steam, advantage: faster / the ethanol is purer [1]

  4. 4

    steam, disadvantage: the raw materials are not renewable [1]

Recap

  • Fermentation: aqueous glucose, yeast, 25 – 35 °C, absence of oxygen.
  • Steam method: catalytic addition of steam to ethene at 300 °C, 6000 kPa / 60 atm, with an acid catalyst.
  • Do not swap the two sets of conditions.
  • Complete combustion of ethanol gives carbon dioxide and water, and gives out heat.
  • CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O
  • Uses of ethanol: solvent and fuel.
  • Extended: fermentation uses renewable raw materials but is slower. The steam method is faster and gives purer ethanol.

37. Ethanoic acid and metals

Ethanoic acid is a carboxylic acid. It reacts with a metal like other acids do. The products are a salt and hydrogen.

The salt is named from the metal, then ethanoate. Magnesium makes magnesium ethanoate. Sodium makes sodium ethanoate. No water is formed.

Mg(s) + 2CH₃COOH(aq) → Mg(CH₃COO)₂(aq) + H₂(g)

2Na(s) + 2CH₃COOH(aq) → 2CH₃COONa(aq) + H₂(g)

Examiner insight

“Candidates should recall that acids such as ethanoic acid will react to give similar products to those of mineral acids. Its reaction with magnesium will produce hydrogen and not water.” “Many candidates gave near-miss spellings for ethanoate such as ‘ethanate’ or ‘ethanoic’.”

Worked example 12 marks

Ethanol is oxidised to ethanoic acid. Complete the word equation for the reaction of ethanoic acid with magnesium.

ethanoic acid + magnesium → ______ + ______

  1. 1

    metal + acid → salt + hydrogen

  2. 2

    magnesium ethanoate [1]

  3. 3

    hydrogen [1]

38. Ethanoic acid and bases

Ethanoic acid reacts with a base to make a salt and water. Nothing else is formed, so no gas is given off.

With potassium hydroxide the salt is potassium ethanoate. With magnesium oxide it is magnesium ethanoate.

CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)

2CH₃COOH(aq) + MgO(s) → Mg(CH₃COO)₂(aq) + H₂O(l)

Examiner insight

“Candidates should be reminded that ethanoic acid reacts in a similar way to mineral acids when added to alkaline solutions, metals or carbonates.” “Those candidates who used aqueous sodium hydroxide rarely mentioned that heat being produced is the only way to decide that a reaction has occurred. Formation of bubbles was a common incorrect suggestion.”

Worked example 11 mark

Potassium ethanoate is made by reacting ethanoic acid with potassium hydroxide.

What else is formed in the reaction?

hydrogenwatercarbon dioxide
Ayesnoyes
Bnoyesyes
Cyesnono
Dnoyesno
  1. 1

    acid + base → salt + water. No gas forms, so hydrogen and carbon dioxide are both no.

  2. 2

    D [1]

39. Ethanoic acid and carbonates

Ethanoic acid reacts with a carbonate to make a salt, water and carbon dioxide. The carbon dioxide is seen as fizzing.

With sodium carbonate the salt is sodium ethanoate. With calcium carbonate it is calcium ethanoate.

2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)

2CH₃COOH(aq) + CaCO₃(s) → Ca(CH₃COO)₂(aq) + H₂O(l) + CO₂(g)

Examiner insight

“Candidates should be reminded that ethanoic acid will react in a similar way as mineral acids when added to metals, carbonates and metal oxides.”

Worked example 11 mark

Which solids react with dilute ethanoic acid to produce a gas?

1 sodium · 2 sodium hydroxide · 3 sodium oxide · 4 sodium carbonate

A 1 and 2 · B 1 and 4 · C 2 and 3 · D 3 and 4

  1. 1

    Sodium is a metal: it makes hydrogen. Sodium carbonate makes carbon dioxide.

  2. 2

    Sodium hydroxide and sodium oxide are bases: salt and water only.

  3. 3

    B: 1 and 4. [1]

40. Making ethanoic acid from ethanol

Ethanol is oxidised to ethanoic acid. One method heats ethanol with acidified aqueous potassium manganate(VII). Acidified and aqueous are the two conditions.

The other method uses bacteria. Bacterial oxidation of ethanol happens during the production of vinegar.

The name is potassium manganate(VII). The (VII) comes after manganate.

Examiner insight

“Some candidates were able to name potassium manganate(VII) as the oxidising agent but omitted to state that the oxidising agent needed to be acidified.” “Most candidates knew the acidified reagent but often did not give the correct oxidation state or placed it in the wrong position such as potassium(VII) manganate.”

Worked example 13 marks

Ethanol can be oxidised by two methods.

method 1: Heating ethanol with an acidified aqueous oxidising agent.

method 2: The oxidation of ethanol using bacteria.

(i) Name the acidified aqueous oxidising agent used in method 1.

(ii) Give the chemical name of the organic product in method 1.

(iii) Name the food product in method 2.

  1. 1

    (i) potassium manganate(VII) [1]

  2. 2

    (ii) ethanoic acid [1]

  3. 3

    (iii) vinegar [1]

41. Making an ester

A carboxylic acid reacts with an alcohol to make an ester and water. The mixture is heated with an acid catalyst. Concentrated sulfuric acid acts as the catalyst.

The reaction is reversible. Only carboxylic acids do this, not acids in general.

CH₃COOH + CH₃CH₂OH ⇌(acid catalyst) CH₃COOCH₂CH₃ + H₂O

Examiner insight

“The identity of the catalyst needed to convert an alcohol and a carboxylic acid into an ester was rarely known.” “Some candidates omitted water as a product or spread the two names of the ester over both answer lines.”

Worked example 13 marks

D reacts with ethanol to form an ester.

(i) State the type of catalyst needed for this reaction.

(ii) Complete the word equation for this reaction.

butanoic acid + ethanol → ______ + ______

  1. 1

    (i) acid [1]

  2. 2

    (ii) ethyl butanoate [1]

  3. 3

    water [1]

42. Naming and writing esters

An ester is named with the alcohol part first and the acid part second. The acid part ends in -oate. Butanoic acid and ethanol make ethyl butanoate. Ethanoic acid and ethanol make ethyl ethanoate.

Write the acid part, then the alcohol part, in the formula. Ethyl butanoate is C₃H₇COOC₂H₅.

C₃H₇COOH + C₂H₅OH ⇌(acid catalyst) C₃H₇COOC₂H₅ + H₂O

Examiner insight

“The most common error was the misspelling of ‘butanoate’ as ‘butanote’.” “In option A, candidates identified the structural formula of the ester but did not reduce the formula to the simplest whole number ratio to give the empirical formula.”

Worked example 11 mark

Butanoic acid reacts with ethanol to form an ester.

What is the structural formula and the empirical formula of the ester?

structural formulaempirical formula
AC₃H₇COOC₂H₅C₆H₁₂O₂
BC₃H₇COOC₂H₅C₃H₆O
CCH₃COOC₄H₉C₆H₁₂O₂
DCH₃COOC₄H₉C₃H₆O
  1. 1

    The acid part is C₃H₇COO and the alcohol part is C₂H₅. Structural formula: C₃H₇COOC₂H₅.

  2. 2

    Molecular formula C₆H₁₂O₂. Divide by 2: empirical formula C₃H₆O.

  3. 3

    B [1]

Recap

  • Ethanoic acid + metal → salt + hydrogen. Ethanoic acid + base → salt + water. Ethanoic acid + carbonate → salt + water + carbon dioxide.
  • The salts are named ethanoates: magnesium ethanoate, sodium ethanoate, calcium ethanoate.
  • Extended: ethanol is oxidised to ethanoic acid by acidified aqueous potassium manganate(VII).
  • Extended: bacterial oxidation of ethanol makes vinegar.
  • Extended: carboxylic acid + alcohol → ester + water, with an acid catalyst.
  • Extended: name the alcohol part first, then the acid part ending -oate.

43. Polymers, monomers and plastics

A polymer is a large molecule built up from many smaller molecules called monomers.

Monomers join together to form a polymer. Plastics are made from polymers.

Examiner insight

“The best answers usually contained the idea of monomers as the small units which make up polymers and that polymers are long or large molecules.” “Others wrote statements about ‘atoms combining’ or ‘polymers joining’.”

Worked example 11 mark

Which statements are correct?

1 The polymer of ethene is poly(ethane). · 2 Monomers are small molecules. · 3 Monomers join together to form polymers.

A 1 and 3 · B 1 only · C 2 and 3 · D 2 only

  1. 1

    Statement 1 is wrong: the polymer of ethene is poly(ethene).

  2. 2

    C: statements 2 and 3 are correct. [1]

44. Addition polymerisation of ethene

In addition polymerisation, many ethene monomers join to form poly(ethene).

The double bond in each monomer breaks. The monomers join by single bonds and no other product is made.

n CH₂=CH₂ → (-CH₂-CH₂–)_{n}

Examiner insight

“‘Additional’ polymerisation was considered incorrect.” “Most candidates went for the double-bonded structure. These candidates had forgotten that in this type of polymerisation the double bond breaks to give single bonds only.”

Worked example 11 mark

Four statements about poly(ethene) are listed.

1 It is a plastic. · 2 It is formed by addition polymerisation. · 3 It is formed from small molecules called polymers. · 4 It burns to produce carbon dioxide and hydrogen.

Which statements are correct?

A 1 and 2 · B 1 and 3 · C 2 and 3 · D 3 and 4

  1. 1

    Statements 3 and 4 are wrong. Statement 3 swaps the terms: the small molecules are monomers.

  2. 2

    A: statements 1 and 2 are correct. [1]

45. Plastics and the environment

Plastics decompose very slowly in the environment. This is why their disposal is a problem.

Three challenges: most do not decompose in landfill sites. Plastics pollute rivers and the ocean, where they harm wildlife. Burning plastics can produce toxic gases.

Examiner insight

“The best answers referred to animals getting trapped or choking on plastics.” “Some suggested that toxic gases were released but omitted the important words ‘when plastics are burned’.”

Worked example 11 mark

Which statements describe an environmental challenge caused by the disposal of plastics?

1 The burning of the plastics can release toxic gases. · 2 Plastics are harmful to marine life. · 3 Plastics decompose very slowly in the environment.

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

  1. 1

    All three are environmental challenges caused by plastics.

  2. 2

    A: 1, 2 and 3. [1]

46. Addition polymers: repeat unit and monomer

The repeat unit is the part that repeats. Only the two carbon atoms of the double bond are in its chain.

Alkene to polymer: the C=C double bond becomes a single bond and an extension bond is left at each end. Polymer to alkene: put the double bond back and remove the extension bonds.

n CH₃-CH=CH₂ → (-CH(CH₃)-CH₂–)_{n}

Examiner insight

“When addition polymerisation occurs, the only part of the alkene molecule that reacts is the double bond between the carbon atoms. Therefore, a repeat unit can only have two carbon atoms in the chain between the two continuation bonds.” “Many candidates, rather than draw the structure of the monomer, tried to draw the structure of the repeat unit and so drew structures that lacked the essential C to C double bond.”

Worked example 13 marks

Propene, CH₃-CH=CH₂, forms an addition polymer.

(a) Name the polymer.

(b) Write the repeat unit of the polymer, with an extension bond at each end.

  1. 1

    (a) poly(propene) [1]

  2. 2

    (b) the double bond breaks and the two carbon atoms stay in the chain [1]

  3. 3

    repeat unit -CH(CH₃)-CH₂–, with an extension bond at each end [1]

47. Condensation polymerisation

A polyamide is made from a dicarboxylic acid and a diamine. A polyester is made from a dicarboxylic acid and a diol.

In condensation polymerisation, a small molecule, water, is produced as a second product each time two monomers join. The monomers have different functional groups. They are joined by an amide linkage in a polyamide and an ester linkage in a polyester. In addition polymerisation, the monomers have a double bond and no other products are produced.

Polymer to monomers: break every linkage. Each C=O end gains -OH to give -COOH. Each N or O end gains a hydrogen atom to give -NH₂ or -OH. Each of these monomers has the same group at both ends and no extension bonds. Do not copy out a section of the polymer with its linkages.

dicarboxylic acid + diamine longrightarrow polyamide + water

dicarboxylic acid + diol longrightarrow polyester + water

Examiner insight

“Condensation was usually thought to apply in the physical sense of changing a gas to liquid.” “Many candidates named the types of organic compounds as carboxylic acids and amines, or included diols.”

Worked example 11 mark

Polymers are described as addition polymers or condensation polymers.

Which two rows give a correct statement about each type of polymerisation?

A 1 and 3 · B 1 and 4 · C 2 and 3 · D 2 and 4

addition polymerisationcondensation polymerisation
1Monomers have different functional groups.Monomers have a double bond.
2No other products are produced.A small molecule is produced as a second product.
3Monomers have a double bond.Monomers have different functional groups.
4Nylon is an addition polymer.Polyester is a condensation polymer.
  1. 1

    Row 1 is wrong: the two statements are the wrong way round.

  2. 2

    Row 2 is correct: addition makes no other product. Condensation makes a small molecule.

  3. 3

    Row 3 is correct: addition monomers have a double bond. Condensation monomers have different functional groups.

  4. 4

    Row 4 is wrong: nylon is a condensation polymer.

  5. 5

    C: rows 2 and 3. [1]

48. Nylon and PET

Nylon is a polyamide: a dicarboxylic acid and a diamine joined by the amide linkage -CONH–. PET is a polyester: a dicarboxylic acid and a diol joined by the ester linkage -COO–. PET can be converted back into monomers and re-polymerised.

In a block drawing of nylon, one repeat unit is a block with a C=O group at each end, joined to a block with an N-H group at each end. In PET, the second block has an oxygen atom at each end instead.

Examiner insight

“The other common error was to confuse the amide bond in nylon with an ester bond.” “Candidates recalled that PET is a condensation polymer, but candidates who chose option B thought that this polyester was a polyamide or did not recall that PET can be converted back into monomers and re-polymerised.”

Worked example 11 mark

Which statements about the plastic PET are correct?

1 PET is a condensation polymer. · 2 PET is a polyamide. · 3 PET can be converted into monomers and re-polymerised.

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

  1. 1

    Statement 2 is wrong: PET is a polyester.

  2. 2

    C: 1 and 3 only. [1]

49. Proteins

Proteins are natural polyamides: amino acid monomers joined by amide linkages. In condensation polymerisation, each amino acid adds the unit -NH-CH(R)-CO– to the protein chain.

In an amino acid, a central carbon atom is bonded to an -NH₂ group, a -COOH group, a hydrogen atom and a side-chain, R. R represents different types of side-chain.

H₂N-CH(R)-COOH

Examiner insight

“The main confusion appears to be that these candidates thought that proteins are formed by addition rather than condensation polymerisation.”

Worked example 13 marks

In an amino acid monomer, a central carbon atom is bonded to a side-chain, R.

State the three other atoms or groups bonded to the central carbon atom.

  1. 1

    a hydrogen atom, -H [1]

  2. 2

    a carboxylic acid group, -COOH, bonded through its carbon atom [1]

  3. 3

    an -NH₂ group, bonded through its nitrogen atom [1]

Recap

  • A polymer is a large molecule built up from many smaller molecules called monomers. Plastics are made from polymers.
  • Addition polymerisation: ethene monomers join, the double bond breaks and no other product forms.
  • Plastics decompose slowly: problems in landfill, in the oceans and toxic gases when burned.
  • Extended: an addition polymer's repeat unit has two carbon atoms in the chain and no double bond.
  • Extended: condensation polymerisation gives a small molecule, water, as a second product.
  • Extended: polyamide = dicarboxylic acid + diamine. Polyester = dicarboxylic acid + diol.
  • Extended: nylon is a polyamide. PET is a polyester that can be converted back into monomers and re-polymerised.
  • Extended: proteins are natural polyamides made from amino acid monomers.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what is meant by the term functional group.2 marks
  2. Methanol, ethanol and propan-1-ol are alcohols. (a) State the term given to a family of compounds like these. (b) Explain why they have similar chemical properties.2 marks
  3. Molecules of compound R contain one carbon–carbon double bond and no other multiple bonds. Molecules of compound S contain only carbon–carbon single bonds. State which compound is saturated and give a reason. State why the other is unsaturated.3 marks
  4. A student writes CH₃CH₂OH and says it is the displayed formula of ethanol. Explain why it is not, and say what must be added.2 marks
  5. An alkane has eight carbon atoms in each molecule. Deduce its molecular formula.2 marks
  6. A carboxylic acid has a total of five carbon atoms in each molecule. Deduce its molecular formula.3 marks
  7. Propan-1-ol has the molecular formula C₃H₈O. (a) Write its structural formula. (b) Explain why C₃H₈O is not a structural formula.3 marks
  8. Methyl ethanoate has the structural formula CH₃COOCH₃. Propanoic acid has the structural formula CH₃CH₂COOH. (a) Deduce the molecular formula of methyl ethanoate. (b) Show that the two compounds are structural isomers.3 marks
  9. A student is given five structural formulae. J is CH₃CH₂CH=CH₂. K is CH₂=CHCH₂CH₃. L is CH₃CH=CHCH₃. M is CH₃CH₂CH₂COOH. N is CH₃CH(CH₃)CH₃. (a) The student says J, K and L are three different structural isomers. Explain why the student is wrong. (b) The student says M is unsaturated because it contains a double bond. Explain why the student is wrong. (c) State which of L and N is in the same homologous series as J. Give a reason.5 marks
  10. Z is a member of the carboxylic acid homologous series. One molecule of Z contains 12 hydrogen atoms. (a) Use the general formula to deduce the molecular formula of Z. (b) Deduce the molecular formula of the next member of the series after Z. (c) Compound E has the structural formula CH₃CH₂COOCH₂CH₂CH₃. A student says E is in the same homologous series as Z because it has the same molecular formula. Show that the molecular formulae are the same, and explain why the student is wrong.6 marks
  11. State the type of compound whose name ends in -ene.1 mark
  12. State the type of compound for each name. (a) butane (b) butan-1-ol (c) butanoic acid3 marks
  13. A compound has the structural formula CH₃COOH. State its type and its name.2 marks
  14. Ethane has two carbon atoms. State the number of hydrogen atoms in a molecule of ethane and the number of bonds each carbon atom forms.2 marks
  15. Ethanol has the molecular formula C₂H₆O. State how many hydrogen atoms are bonded to carbon atoms and how many to the oxygen atom.2 marks
  16. Name the compound with the structural formula CH₂=CHCH₂CH₃.2 marks
  17. Name the two unbranched alcohols that have four carbon atoms.2 marks
  18. Write the structural formula of propanoic acid.2 marks
  19. An ester in a food flavouring is made from an unbranched alcohol and an unbranched carboxylic acid. One molecule of the ester has six carbon atoms. The acid has twice as many carbon atoms as the alcohol. (a) Deduce the number of carbon atoms in the alcohol and in the acid. (b) Name the acid and write its structural formula. (c) Name the ester. (d) A different ester also has six carbon atoms. It is made from ethanoic acid and an alcohol with the OH group on an end carbon atom. Name this alcohol and this ester.6 marks
  20. Ester X has the structural formula CH₃CH₂COOCH₂CH₂CH₂CH₃. A student writes: ‘X is made from propanol and butanoic acid, so X is propyl butanoate.’ (a) Name the carboxylic acid that X is made from and write its structural formula. (b) Name the alcohol that X is made from and write its structural formula. (c) Name X. (d) Propyl butanoate is CH₃CH₂CH₂COOCH₂CH₂CH₃. Deduce the molecular formula of X and of propyl butanoate, and state whether they are the same.7 marks
  21. Name the three fossil fuels.3 marks
  22. Name the main constituent of natural gas.1 mark
  23. A student writes: ‘A hydrocarbon is a mixture of hydrogen and carbon.’ Correct it.2 marks
  24. Identify the two hydrocarbons: C₂H₆, C₂H₅OH, C₃H₆, CO₂.2 marks
  25. A student says: ‘Petrol is separated into fractions by cracking.’ Correct the two errors.2 marks
  26. Petroleum vapours enter a fractionating column. Explain how the column separates them.4 marks
  27. Fraction P is collected near the top of a fractionating column and fraction Q near the bottom. Compare P with Q in terms of chain length, boiling point, volatility and viscosity.4 marks
  28. Fraction R is more volatile than fraction S, from the same column. State what volatile means, which fraction has the higher boiling points, and which is collected higher in the column.3 marks
  29. A refinery tests four fractions from one fractionating column. The table lists them in no particular order. (a) Name fractions J and L. (b) Write the four letters in order, from the top of the column to the bottom. (c) State which fraction is the most viscous and which is the most volatile. (d) A fifth fraction has 11 to 14 carbon atoms in its chains and is used as jet fuel. Name it, and state the two letters it is collected between. | fraction | carbon atoms in the chains | use | | --- | --- | --- | | J | 15 to 19 | fuel used in diesel engines | | K | 1 to 4 | gas used in heating and cooking | | L | more than 70 | making roads | | M | 5 to 10 | fuel used in cars |7 marks
  30. A technician heats a small sample of petroleum and slowly raises the temperature. She collects the liquid that distils over in each temperature range. (a) State the property of the hydrocarbons that makes this separation possible. (b) State which sample has the longest chains. Give a reason from the table. (c) State which sample is the most viscous. (d) The technician writes: ‘Sample 1 is the most volatile. In a fractionating column it would be collected at the bottom, because the bottom is the coolest part.’ One of her three claims is correct. State which, and correct the other two. | sample | collected between | | --- | --- | | 1 | 30 °C and 100 °C | | 2 | 100 °C and 200 °C | | 3 | 200 °C and 350 °C |7 marks
  31. State what is meant by the term hydrocarbon.1 mark
  32. Ethane has the formula C₂H₆. State the type of bonding in ethane and state why ethane is a saturated hydrocarbon.2 marks
  33. Three compounds are propane, C₃H₈, ethene, C₂H₄, and ethanol, C₂H₅OH. (a) State which one is a saturated hydrocarbon. (b) State which one is not a hydrocarbon and give a reason.3 marks
  34. Hexane is an alkane. State the two types of reaction that hexane undergoes.2 marks
  35. Propane, C₃H₈, burns completely in oxygen. Write the balanced equation for the reaction.2 marks
  36. State what is meant by a substitution reaction.1 mark
  37. A flask of ethane and chlorine is kept in the dark at room temperature and nothing happens. The flask is then placed under an ultraviolet lamp and a reaction begins. Explain the difference.2 marks
  38. Ethane, C₂H₆, reacts with chlorine in ultraviolet light in a monosubstitution reaction. (a) Write the equation for the reaction. (b) Name the two products.3 marks
  39. Pentane, CH₃CH₂CH₂CH₂CH₃, reacts with chlorine in a monosubstitution reaction. A student says that five different compounds with the formula C₅H₁₁Cl form, one for each carbon atom. (a) State the correct number of different compounds. (b) Give the structural formula of each compound. (c) Explain the student's mistake.5 marks
  40. A camping stove burns 2-methylpropane, CH₃CH(CH₃)CH₃, an alkane with the molecular formula C₄H₁₀. A sample of the gas is mixed with chlorine at room temperature under an ultraviolet lamp. A monosubstitution reaction takes place. A student writes: ‘The lamp warms the flask and this heat starts the reaction. Four different compounds C₄H₉Cl form, one for each carbon atom. The other product is hydrochloric acid.’ (a) Identify the three mistakes and correct each one. (b) Give the structural formula of each organic product. (c) Write the equation for the reaction, using molecular formulae.6 marks
  41. Ethene is an unsaturated hydrocarbon. State what unsaturated tells you about its bonding.1 mark
  42. Explain why oil refineries crack large alkane molecules.2 marks
  43. State two conditions needed for cracking.2 marks
  44. Tetradecane, C₁₄H₃₀, is cracked to give propene, C₃H₆, and one other hydrocarbon. Write the balanced equation.2 marks
  45. Ethene gas is bubbled into aqueous bromine. Describe the colour change.2 marks
  46. A student heats a liquid alkane strongly with a catalyst and collects a gas. The gas is shaken with aqueous bromine. (a) Name the process. (b) The aqueous bromine turns colourless. State what this shows about the gas. (c) State the colour of the aqueous bromine at the start.4 marks
  47. State what is meant by an addition reaction, in terms of the products formed.1 mark
  48. Propene, CH₃CH=CH₂, reacts with aqueous bromine. (a) Give the structural formula of the product. (b) Describe the colour change.3 marks
  49. One molecule of dodecane, C₁₂H₂₆, is cracked. The only products are one molecule of octane, C₈H₁₈, and two molecules of an alkene Y. (a) Deduce the molecular formula of Y and write the balanced equation for the cracking. (b) Y reacts with steam. State the type of catalyst and give the structural formula of the product. (c) Write the balanced equation for the reaction in (b). Use it to explain why this is an addition reaction.6 marks
  50. A hydrocarbon Z has the molecular formula C₄H₈. Z reacts rapidly with bromine. The only product is CH₃CHBrCHBrCH₃. (a) Deduce the structural formula of Z. (b) Z reacts with steam. State the type of catalyst and give the structural formula of the product. (c) Z reacts with hydrogen. Name the catalyst and give the structural formula of the product. (d) The product from (c) is shaken with aqueous bromine. State what is seen and explain why.7 marks
  51. Name the process in which yeast changes aqueous glucose into ethanol.1 mark
  52. State two conditions needed to make ethanol by the fermentation of aqueous glucose.2 marks
  53. A factory makes ethanol by passing ethene and steam over an acid catalyst. Name this type of reaction, and state the temperature and the pressure that the factory uses.3 marks
  54. A cleaning spray contains ethanol to dissolve grease. Name the use of ethanol that this shows, and state one other use of ethanol.2 marks
  55. A camping stove burns ethanol for cooking. Name the two products of complete combustion and state what the reaction gives out that makes ethanol useful as a fuel.3 marks
  56. Write the symbol equation for the complete combustion of ethanol, C₂H₅OH.2 marks
  57. A student ferments aqueous glucose with yeast in an open flask in a very hot room. The mixture reaches 50 °C and the fermentation stops. In the write-up the student gives the temperature needed as 20 – 40 °C. (a) Explain why 20 – 40 °C does not earn the mark. (b) Explain why the fermentation stops at 50 °C. (c) State the condition for fermentation that an open flask does not meet.3 marks
  58. Give one advantage and one disadvantage of making ethanol by fermentation instead of by the addition of steam to ethene.2 marks
  59. A company needs a large amount of very pure ethanol quickly, to use as a solvent in perfume. A manager writes: ‘Use the addition of steam to ethene. It is faster than fermentation, the ethanol is purer and the raw materials are renewable.’ (a) One of the manager's three claims is wrong. Identify it and state the method for which it is true. (b) After fermentation, the ethanol is left in the reaction mixture. Name the method used to separate it, and state which of the manager's claims this supports. (c) State the temperature, the pressure and the type of catalyst that the manager's method needs.7 marks
  60. A country grows a great deal of sugar cane, which supplies glucose. It plans to make ethanol and burn it as a fuel in its buses. (a) Name the process it should use on aqueous glucose. Give the temperature range and one other condition. (b) A report says the process was chosen ‘because it is faster’. State whether this is correct, and give the advantage that the report should state. (c) An engineer writes this equation for the complete combustion of ethanol in a bus engine. The equation is balanced. C₂H₅OH + 2O₂ → 2CO + 3H₂O Explain why it is still wrong, and write the correct equation.8 marks
  61. Name the two products formed when ethanoic acid reacts with sodium hydroxide.2 marks
  62. Ethanoic acid reacts with zinc. Name the salt and the gas formed.2 marks
  63. Ethanoic acid is added to solid calcium carbonate. Name the three products.3 marks
  64. Write the balanced symbol equation, with state symbols, for the reaction of aqueous ethanoic acid with magnesium.3 marks
  65. Dilute ethanoic acid is added to separate samples of magnesium, magnesium oxide and magnesium carbonate. State which samples give off a gas and name each gas.3 marks
  66. Ethanol is oxidised to ethanoic acid by heating it with potassium manganate(VII). State the two other conditions needed, and name the type of substance that potassium manganate(VII) is acting as.3 marks
  67. Vinegar is made by changing ethanol into ethanoic acid using bacteria. State the type of chemical change and name the food product.2 marks
  68. Methanol reacts with propanoic acid in the presence of an acid catalyst. Name the two products.2 marks
  69. An ester has the structural formula CH₃CH₂COOCH₃. It was made by heating a carboxylic acid with an alcohol and an acid catalyst. (a) Name the carboxylic acid and the alcohol. (b) Name the ester. (c) Write the symbol equation for the reaction.5 marks
  70. Vinegar is made from wine. A cook uses vinegar to remove limescale from a kettle. Limescale is solid calcium carbonate, CaCO₃. (a) Bacteria change the ethanol in wine into the acid in vinegar. Name the acid and state the type of chemical change. (b) Write the balanced symbol equation, with state symbols, for the reaction of this acid with limescale. (c) A little vinegar is then heated with methanol and concentrated sulfuric acid. Name the two products.7 marks
  71. State what is meant by the term monomer.1 mark
  72. Poly(ethene) is a plastic. (a) Name the monomer from which poly(ethene) is made. (b) Explain why poly(ethene) is called a polymer.2 marks
  73. A factory makes poly(ethene) by addition polymerisation. It uses 56 g of ethene and all of it reacts. Deduce the mass of poly(ethene) formed.2 marks
  74. Waste plastic is put into landfill sites. A different town burns its waste plastic. State one problem caused by each method.2 marks
  75. Plastic bags are found floating far out at sea. Explain why this is an environmental challenge.3 marks
  76. An addition polymer has the repeat unit -CH₂-CH(C₂H₅)–. (a) Write the structural formula of the monomer. (b) Name the monomer.2 marks
  77. A polyester is made by condensation polymerisation. (a) Name the two types of monomer used to make a polyester. (b) Name the second product formed.3 marks
  78. Nylon and PET are both condensation polymers. A drinks bottle is made from PET. (a) Name the linkage in PET. (b) Name the linkage in nylon. (c) State what can be done to the PET from used bottles.3 marks
  79. A fishing line is made from a condensation polymer with the repeat unit -OC⁻(CH₂)₄-CO-NH⁻(CH₂)₆-NH–. (a) Name the linkage in the polymer. (b) Write the structural formula of each of the two monomers. (c) 146 g of one monomer reacts completely with 116 g of the other. 36 g of water is produced. Deduce the mass of polymer formed.5 marks
  80. Glycine, H₂N-CH₂-COOH, and alanine, H₂N-CH(CH₃)-COOH, are amino acids. One short chain molecule forms from exactly five amino acid molecules joined in the order glycine, alanine, glycine, alanine, glycine. (a) Deduce the number of different types of amino acid monomer in the chain. (b) Deduce the number of water molecules produced when the chain forms. (c) Write the structure of the part of the chain made from one glycine unit joined to one alanine unit, with an extension bond at each end. (d) The molecular formula of glycine is C₂H₅NO₂ and of alanine is C₃H₇NO₂. Deduce the molecular formula of the chain molecule.6 marks

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