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Cambridge IGCSE0620

Properties of metals

Chemistry 0620 Chapter Notes

What this chapter covers

Properties of metalsUses of metalsAlloys and their propertiesReactivity seriesCorrosion of metalsExtraction of metals
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In this chapter

  1. 1Thermal and electrical conductivity
  2. 2Malleability and ductility
  3. 3Melting points and boiling points
  4. 4Using data to tell metals from non-metals
  5. 5Reaction with dilute acids
  6. 6Reaction with cold water and steam
  7. 7Reaction with oxygen
  8. 8Aluminium in aircraft: low density
  9. 9Aluminium in overhead cables: two properties
  10. 10Aluminium in food containers: corrosion resistance
  11. 11Copper in electrical wiring
  12. 12Choosing the property that fits the use
  13. 13What an alloy is
  14. 14Brass and stainless steel
  15. 15Why alloys are more useful
  16. 16Stainless steel in cutlery
  17. 17Recognising an alloy from its structure
  18. 18Why alloys are harder and stronger
  19. 19The order of the reactivity series
  20. 20Reactions with water and steam
  21. 21Reactions with dilute hydrochloric acid
  22. 22Deducing an order from results
  23. 23Positive ions and displacement reactions
  24. 24Aluminium and its oxide layer
  25. 25Conditions for rusting
  26. 26Barrier methods
  27. 27Galvanising
  28. 28Sacrificial protection
  29. 29Reactivity and the ease of extraction
  30. 30Iron: heat, carbon monoxide and reduction
  31. 31Limestone and slag
  32. 32Aluminium: bauxite and electrolysis
  33. 33The role of cryolite
  34. 34Reactions at the electrodes
  35. 35Replacing the carbon anodes

1. Thermal and electrical conductivity

Metals are good conductors of heat: this is thermal conductivity. They are good conductors of electricity: this is electrical conductivity. All metals conduct electricity when solid.

Non-metals are generally poor conductors of both. Always say which kind of conductivity you mean.

Examiner insight

“The correct statement that all metals conduct electricity when solid was mentioned rarely.” “Others gave electrical and heat conductivity or high melting point and high boiling point. These two pairs were only given one mark per pair.”

Worked example 11 mark

Copper is sometimes used to make cooking utensils. Three properties of copper are listed.

1 corrosion resistant

2 good conductor of electricity

3 good conductor of heat

Which properties make copper a suitable metal for making cooking utensils?

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

  1. 1

    A utensil must carry heat from the cooker to the food: property 3.

  2. 2

    It must not corrode when used with food: property 1.

  3. 3

    A utensil does not need to carry electricity: property 2 is not needed.

  4. 4

    The answer is A. [1]

2. Malleability and ductility

Metals are malleable: they can be hammered or bent into shape. Metals are ductile: they can be drawn out into wires.

These are physical properties. They are shared by most metals, though some are less malleable or less ductile than others.

Examiner insight

“Some candidates chose malleability or ductility but did not gain credit because they suggested that transition elements were not malleable or ductile rather than less malleable or less ductile.” “The correct answer of ductility was the most often seen response, although ‘malleability’ was also seen. It was not uncommon to see both responses with one being crossed through, showing uncertainty amongst many candidates.”

Worked example 11 mark

Which property of a metal is necessary for the named use?

A car bodies: ductile

B cutlery: conducts heat

C food containers: resists corrosion

D overhead electrical cables: high density

Choose the correct pairing.

  1. 1

    A car body is pressed into shape from sheet metal. That needs a malleable metal, not a ductile one, so A is wrong.

  2. 2

    Food containers must not corrode, so a metal that resists corrosion is necessary.

  3. 3

    The answer is C. [1]

3. Melting points and boiling points

Most metals have high melting points and high boiling points. Most non-metal elements have low ones.

Some elements break the pattern. Group I metals have relatively low melting points. Graphite and diamond are non-metals with high melting points, and graphite conducts electricity when solid.

Examiner insight

“Candidates should realise that Group I metals have relatively low density and low melting point and are relatively soft.” “Common errors included reference to reactivity, brittleness and/or low melting point.”

Worked example 11 mark

Which substances have a high melting point and conduct electricity when solid?

1 zinc

2 diamond

3 sulfur

4 graphite

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

  1. 1

    Zinc is a metal: high melting point, conducts when solid. Include 1.

  2. 2

    Diamond has a high melting point but does not conduct. Leave out 2.

  3. 3

    Sulfur has a low melting point and does not conduct. Leave out 3.

  4. 4

    Graphite has a high melting point and conducts. Include 4. The answer is B. [1]

4. Using data to tell metals from non-metals

Test the solid first. Metals are good conductors when solid and when melted. Most non-metal elements are poor conductors both when solid and when melted. Graphite is the exception.

Do not decide from melting point alone. Use conductivity and melting point together.

Worked example 11 mark

The table shows some physical properties of four different substances. Which row describes the properties of a non-metallic element?

melting point / °Cconductivity when solidconductivity when melted
A63goodgood
B119poorpoor
C659goodgood
D808poorgood
  1. 1

    A and C conduct when solid and when melted: metals.

  2. 2

    D conducts only when melted. A metal conducts when solid too, and a non-metallic element is poor in both states, so D is neither.

  3. 3

    B is a poor conductor solid and melted, with a low melting point: a non-metal. The answer is B. [1]

5. Reaction with dilute acids

Metals react with dilute acids to give a salt and hydrogen. Hydrogen is the gas formed.

Not every metal reacts. Copper does not react with dilute acids.

metal + acid → salt + hydrogen

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Examiner insight

“Candidates did not realise that copper metal does not react with dilute sulfuric acid.” “Most candidates recognised that sodium reacts with cold water but thought that copper reacted with dilute hydrochloric acid.”

Worked example 11 mark

Which row identifies properties of zinc?

reacts with dilute acidthermal conductivity
Anopoor
Bnogood
Cyespoor
Dyesgood
  1. 1

    Zinc is a metal, so its thermal conductivity is good. Rules out A and C.

  2. 2

    Zinc reacts with dilute acid. Rules out B.

  3. 3

    The answer is D. [1]

6. Reaction with cold water and steam

Reactive metals such as calcium react with cold water to give a metal hydroxide and hydrogen. Less reactive metals such as magnesium react with steam to give a metal oxide and hydrogen.

Metals react more vigorously with steam or with a dilute acid than with cold water.

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Examiner insight

“Candidates should recall that metals will react more vigorously with steam or with a dilute acid than with cold water.” “Almost one in four of the candidates did not recall the reaction of calcium with water and/or the reaction of magnesium with steam.”

Worked example 13 marks

Magnesium is heated in steam. Write a balanced equation for the reaction, with state symbols.

  1. 1

    Reactants and products: Mg + H₂O → MgO + H₂. [1]

  2. 2

    Balanced: one Mg, two H and one O on each side. [1]

  3. 3

    State symbols: Mg(s) + H₂O(g) → MgO(s) + H₂(g). [1]

7. Reaction with oxygen

Metals react with oxygen to form a metal oxide.

When a question asks for a word equation, give a word equation. Examiners report a greater chance of error when candidates write a symbol equation instead.

metal + oxygen → metal oxide

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Examiner insight

“Where word equations were given, they were often incorrect. The commonest correct word equations referred to the reaction of metals with either oxygen or acids.” “Some candidates attempted to write symbol equations instead of word equations. This should be discouraged since there is a greater chance of error.”

Worked example 13 marks

Aluminium burns in oxygen.(a) Write the word equation.(b) Write the balanced symbol equation, with state symbols.

  1. 1

    (a) aluminium + oxygen → aluminium oxide [1]

  2. 2

    (b) Formulas: Al, O₂ and Al₂O₃. [1]

  3. 3

    Balanced with states: 4Al(s) + 3O₂(g) → 2Al₂O₃(s). [1]

Recap

  • Metals are good conductors of heat and of electricity. They conduct electricity when solid.
  • Metals are malleable (hammered into shape) and ductile (drawn into wires).
  • Most metals have high melting and boiling points. Group I metals are the low exception.
  • Metal and dilute acid give a salt and hydrogen. Copper does not react.
  • Calcium reacts with cold water to give its hydroxide and hydrogen. Magnesium reacts with steam to give its oxide and hydrogen.
  • Metal and oxygen give a metal oxide. Give a word equation when one is asked for.

8. Aluminium in aircraft: low density

Aluminium is used in the manufacture of aircraft because of its low density.

The property to name is density. Do not write “light”, and do not compare it with air.

Examiner insight

“A few candidates disadvantaged themselves by suggesting that aluminium is less dense than air.”

Worked example 11 mark

Which metal is used in aircraft manufacture because it has a low density?

A aluminium · B copper · C iron · D potassium

  1. 1

    A: aluminium. It is the metal used in aircraft because of its low density. [1]

9. Aluminium in overhead cables: two properties

Aluminium is used in overhead electrical cables because of its low density and its good electrical conductivity. Both properties are needed, and each earns its own mark.

Examiner insight

“A few candidates confused the properties and answered with ‘high density’ and ‘does not conduct electricity’.”

Worked example 12 marks

Give two properties of aluminium that make it suitable for the manufacture of overhead electrical cables.

  1. 1

    low density [1]

  2. 2

    good electrical conductivity / conducts electricity [1]

10. Aluminium in food containers: corrosion resistance

Aluminium is used in food containers because of its resistance to corrosion, not its low density.

Worked example 11 mark

Which property of a metal is necessary for the named use?

use of the metalproperty of the metal
Acar bodiesductile
Bcutleryconducts heat
Cfood containersresists corrosion
Doverhead electrical cableshigh density
  1. 1

    C: a food container needs a metal that resists corrosion. [1]

11. Copper in electrical wiring

Copper is used in electrical wiring because of its good electrical conductivity and its ductility.

The term is ductility, not malleability. Name the use as “electrical wiring”. “Wires” alone is too vague.

Examiner insight

“The correct answer of ductility was the most often seen response, although ‘malleability’ was also seen.” “The best answers referred to electrical wiring. Many candidates simply suggested ‘wires’ which was considered too vague.”

Worked example 12 marks

Copper is used in electrical wiring. Give two properties of copper that make it suitable for this use.

  1. 1

    good electrical conductivity [1]

  2. 2

    ductility [1]

12. Choosing the property that fits the use

Every correct property must fit the use in the question. A true fact about the metal can still earn no mark.

Copper is a good conductor of electricity, but that has no bearing on a cooking utensil. There the useful properties are corrosion resistance and good conduction of heat.

Examiner insight

“Copper is a good conductor of electricity but this has no bearing on its use in cooking utensils.”

Worked example 11 mark

Copper is sometimes used to make cooking utensils. Three properties of copper are listed.

1 corrosion resistant · 2 good conductor of electricity · 3 good conductor of heat

Which properties make copper a suitable metal for making cooking utensils?

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

  1. 1

    A: 1 and 3. Corrosion resistance and good conduction of heat suit a utensil. [1]

Recap

  • Aluminium in aircraft: low density.
  • Aluminium in overhead electrical cables: low density and good electrical conductivity.
  • Aluminium in food containers: resistance to corrosion.
  • Copper in electrical wiring: good electrical conductivity and ductility.
  • Name the property that fits the use. Say “low density”, not “light”.

13. What an alloy is

An alloy is a mixture of a metal with other elements. The other elements can be metals or non-metals.

There must be a metal. It is a mixture, not a compound: the elements are not chemically combined.

Examiner insight

“The best answers mentioned a mixture of a metal with another element.” “Most candidates recalled that a metal was required, although many did not use the word ‘mixture’ or thought that an alloy was a metal compound.”

Worked example 13 marks

Stainless steel is an alloy that contains nickel.

(i) State the meaning of the term alloy.

(ii) Name two elements in stainless steel, other than nickel.

  1. 1

    (i) a mixture of a metal with another element [1]

  2. 2

    (ii) any two of: chromium, iron, carbon [2]

14. Brass and stainless steel

Brass is a mixture of copper (Cu) and zinc (Zn).

Stainless steel is a mixture of iron and other elements such as chromium, nickel and carbon.

Both are alloys, so both are mixtures.

Examiner insight

“Most candidates knew that copper was the other substance in brass. Common incorrect answers were tin and iron.” “A number of candidates wrongly selected B, thinking that brass (an alloy) is a compound.”

Worked example 12 marks

Brass is an alloy.

Name the two metals in brass.

  1. 1

    copper [1]

  2. 2

    zinc [1]

15. Why alloys are more useful

Alloys can be harder and stronger than the pure metals. This makes them more useful. Always write a comparison with the pure metal.

An alloy can also be more resistant to corrosion. Write ‘corrosion’ for alloys in general, because rusting only relates to iron.

Examiner insight

“Many candidates gave statements which did not refer to all alloys, e.g. ‘they do not rust’ or ‘they are more reactive’.” “A comparative statement such as stronger or harder was required.”

Worked example 11 mark

Stainless steel is an alloy of iron.

Give one reason why alloys are more useful than pure metals.

  1. 1

    any one of: stronger, harder, (more) resistant to corrosion [1]

16. Stainless steel in cutlery

The use of an alloy follows from its physical properties.

Stainless steel is used to make cutlery because it is hard and resistant to rusting.

Name a real use such as cutlery. ‘Utensils’ is too vague to be credited.

Examiner insight

“Many others gave answers which were too vague such as ‘for utensils’ or ‘for machines’.” “A small number suggested properties of metals but not one which would be useful in the manufacture of cutlery for example, some candidates suggested ‘ductile’ or ‘electrical/thermal conduction’.”

Worked example 11 mark

Which statements explain why stainless steel is used to make cutlery?

1 It is a good conductor of thermal energy.

2 It is ductile.

3 It is corrosion resistant.

4 It is hard.

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

  1. 1

    D: statements 3 and 4. Stainless steel is used for cutlery because it is corrosion resistant and hard. [1]

17. Recognising an alloy from its structure

In a pure metal the atoms are all the same size and sit in regular layers.

In an alloy, atoms of a second element are mixed in among them. The structure has different sized atoms and the second atoms are spread at random.

A structure in which the second atoms sit in a completely regular pattern is not the alloy.

Examiner insight

“Candidates did not know that an alloy shows a random distribution of the atoms of the second metal.” “The commonest incorrect answer was to choose B in which there is complete regularity in the layers of iron, chromium and nickel, which would be impossible in a bulk manufactured alloy.”

Worked example 12 marks

Metal X has atoms of one size only, in regular layers. Metal Y has the same atoms with some larger atoms of another element mixed in at random.

State which of X and Y is an alloy and give a reason.

  1. 1

    Y is the alloy [1]

  2. 2

    it has atoms of different sizes, with the second atoms spread at random [1]

18. Why alloys are harder and stronger

In a pure metal the atoms are all the same size. The layers of atoms can slide over each other.

In an alloy the different sized atoms mean the layers can no longer slide over each other. This is how an alloy can be harder and stronger than the pure metal.

For the same reason an alloy is less malleable than the pure metal.

Examiner insight

“When describing the difference between alloys and pure metals, the size and movement of particles is essential information.”

Worked example 11 mark

Which row describes the difference in malleability between alloys and pure metals and explains the difference?

malleability of an alloy compared with pure metalexplanation
Aless malleabledifferent-sized atoms slide more easily over each other
Bless malleabledifferent-sized atoms slide less easily over each other
Cmore malleabledifferent-sized atoms slide more easily over each other
Dmore malleabledifferent-sized atoms slide less easily over each other
  1. 1

    B: the alloy is less malleable because the different-sized atoms slide less easily over each other. [1]

Recap

  • An alloy is a mixture of a metal with other elements.
  • Brass is copper and zinc. Stainless steel is iron with other elements such as chromium, nickel and carbon.
  • Alloys can be harder and stronger than the pure metals, so they are more useful.
  • Stainless steel is used in cutlery because it is hard and resists rusting.
  • The structure of an alloy has different sized atoms, with the second atoms spread at random.
  • Extended: different sized atoms mean the layers can no longer slide over each other.

19. The order of the reactivity series

The reactivity series lists metals from most reactive to least reactive. Learn it in this order: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.

A metal higher in the series is more reactive than any metal below it. The series also includes carbon and hydrogen, which are not metals.

In an exam, read whether the question wants most reactive or least reactive at the top before you write the list.

Examiner insight

“The commonest errors were to reverse the order of copper and zinc and/or to reverse the order of magnesium and potassium.” “The most common error was to completely reverse the order.”

Worked example 12 marks

(b) Five metals: copper, gold, iron, magnesium, zinc.

Put these metals in order of their reactivity. Put the most reactive metal at the top.

  1. 1

    most reactive: magnesium, zinc, iron, copper, gold: least reactive [2]

  2. 2

    One pair reversed, or the whole order reversed, earns 1 mark only.

20. Reactions with water and steam

Potassium, sodium and calcium react with cold water. The higher the metal in the series, the more readily it reacts.

Magnesium reacts with steam and forms magnesium oxide and hydrogen. Metals react more vigorously with steam or with a dilute acid than with cold water.

Calcium and cold water form calcium hydroxide and hydrogen.

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Examiner insight

“Candidates should recall that metals will react more vigorously with steam or with a dilute acid than with cold water.” “Almost one in four of the candidates did not recall the reaction of calcium with water and/or the reaction of magnesium with steam.”

Worked example 11 mark

Which word equation is correct?

A calcium + water → calcium hydroxide + hydrogen

B zinc + hydrochloric acid → zinc chloride + water

C magnesium + oxygen → magnesium oxide + water

D aluminium + sulfuric acid → aluminium nitrate + hydrogen

  1. 1

    A: calcium reacts with cold water and forms calcium hydroxide and hydrogen. [1]

  2. 2

    B and C are wrong because no water is formed. D is wrong because sulfuric acid cannot form a nitrate.

21. Reactions with dilute hydrochloric acid

Magnesium, zinc and iron react with dilute hydrochloric acid. Hydrogen is formed.

Copper, silver and gold do not react with it. They are below hydrogen in the series.

A metal above hydrogen in the reactivity series reacts with acids. A metal below hydrogen does not.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Examiner insight

“Some candidates assumed that silver would react with a dilute acid and chose option C.” “Most candidates recalled that graphite does not react with dilute acids but did not recall the reactivity of iron and copper.”

Worked example 11 mark

Cobalt is a metal. It is more reactive than hydrogen, but less reactive than iron.

Which statements are correct?

1 Cobalt reacts with dilute hydrochloric acid.

2 Cobalt reacts rapidly with cold water to form hydrogen gas.

3 Cobalt is more reactive than zinc.

4 Cobalt is more reactive than copper.

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

  1. 1

    Cobalt is above hydrogen, so statement 1 is correct. It is below iron, so statements 2 and 3 are wrong. Hydrogen is above copper, so statement 4 is correct.

  2. 2

    B: 1 and 4. [1]

22. Deducing an order from results

To rank metals, compare how readily each one reacts. A metal that reacts with cold water is more reactive than one that needs steam or acid. A metal that reacts with a dilute acid is more reactive than one that does not.

Metals which are easy to extract from their oxide are less reactive metals.

Use the results you are given, not the order you remember. Check which end of your list is the most reactive.

Examiner insight

“Candidates confused the ease of extraction of a metal from its oxide and the reactivity of the metal.” “Some candidates gave the correct order of reactivity but many reversed the order completely. Others seemed to rely on their existing knowledge of the reactivity series rather than using the information in the table.”

Worked example 11 mark

Nickel reacts with dilute hydrochloric acid. The reaction is slower than the reaction of iron with dilute hydrochloric acid.

Strontium reacts with cold water more slowly than sodium reacts with cold water, but faster than iron reacts with cold water.

Which statement correctly describes the order of reactivity?

A Nickel is the most reactive and strontium is the least reactive.

B Strontium is the most reactive and iron is the least reactive.

C Iron is the most reactive and strontium is the least reactive.

D Sodium is the most reactive and nickel is the least reactive.

  1. 1

    Cold water: sodium is more reactive than strontium, and strontium is more reactive than iron.

  2. 2

    Dilute acid: iron is more reactive than nickel.

  3. 3

    Order: sodium, strontium, iron, nickel. D: sodium is the most reactive and nickel is the least reactive. [1]

23. Positive ions and displacement reactions

A metal atom reacts by releasing electrons to become a positive ion. A metal higher in the series has a greater tendency to form positive ions.

In a displacement reaction a more reactive metal displaces a less reactive metal from a solution of its ions. The ion of the less reactive metal is reduced.

A less reactive metal cannot displace a more reactive one, so there is no reaction.

Mg(s) + CuSO₄(aq) → Cu(s) + MgSO₄(aq)

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

Examiner insight

“Candidates should be reminded that metal atoms react by releasing electrons to become cations. In displacement reactions, the ion of the least reactive metal is reduced.” “Better-performing candidates discussed the displacement of copper by iron, which was the more reactive metal.”

Worked example 11 mark

The equation for the reaction between magnesium and aqueous copper(II) sulfate is Mg + CuSO₄ → Cu + MgSO₄.

Which statement is correct?

A Copper ions are oxidised to copper metal during the reaction.

B Copper is displaced because it is more reactive than magnesium.

C The oxidation number of magnesium decreases during the reaction.

D The reaction happens because magnesium has a greater tendency to form positive ions than copper.

  1. 1

    Magnesium is more reactive than copper, so it displaces copper from its solution.

  2. 2

    Copper ions gain electrons and are reduced, so A is wrong. Copper is less reactive, so B is wrong.

  3. 3

    Magnesium atoms release electrons, so the oxidation number of magnesium increases and C is wrong.

  4. 4

    D: magnesium has a greater tendency to form positive ions than copper. [1]

24. Aluminium and its oxide layer

Aluminium is above zinc in the reactivity series, but it appears unreactive. The reason is not its position.

Aluminium is coated with a layer of its oxide, Al₂O₃. This inert coating covers the metal.

So never write that aluminium is unreactive. Write that its oxide layer makes it appear so.

Examiner insight

“The existence of an inert coating of aluminium oxide on the surface of aluminium was not well known. A common error was for candidates to claim that aluminium was unreactive.”

Worked example 11 mark

Aluminium does not react with dilute hydrochloric acid.

Zinc reacts with dilute hydrochloric acid to produce hydrogen gas.

Which statement explains these observations?

A Aluminium has a greater tendency than zinc to form positive ions.

B Only aluminium is coated with a layer of its oxide.

C Aluminium is above zinc in the reactivity series.

D Zinc has a greater tendency than aluminium to form positive ions.

  1. 1

    Aluminium is above zinc, so A and C describe true facts but do not explain the observation; D is false.

  2. 2

    B: only aluminium is coated with a layer of its oxide. [1]

Recap

  • Order: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
  • Potassium, sodium and calcium react with cold water. Magnesium reacts with steam.
  • Metals above hydrogen react with dilute hydrochloric acid and form hydrogen. Copper, silver and gold do not.
  • To order metals, use the results given. Reacting with cold water ranks a metal above one that needs acid.
  • Extended: a more reactive metal has a greater tendency to form positive ions and displaces a less reactive metal from its solution.
  • Extended: aluminium appears unreactive because it is coated with a layer of its oxide.

25. Conditions for rusting

Iron and steel rust when they are in contact with oxygen and water. Both are needed. Take either one away and the iron does not rust.

The rust that forms is hydrated iron(III) oxide. Only iron and steel rust. Other metals corrode, but they do not rust.

Examiner insight

“Oxygen or air was more commonly given than water. A significant number of candidates suggested that heat was necessary.” “The commonest error was to state hydrogen and oxygen.”

Worked example 11 mark

Which substances in the air are needed for iron to rust?

A oxygen and water · B oxygen only · C water and carbon dioxide · D water only

  1. 1

    Iron needs both oxygen and water to rust, so options B and D are incomplete.

  2. 2

    A: oxygen and water. [1]

26. Barrier methods

A barrier method covers the iron with a layer that keeps oxygen and water away from it. Common barrier methods are painting, greasing and coating with plastic.

The layer works by excluding oxygen or water from the surface of the iron. With one of them missing, the iron cannot rust.

Examiner insight

“Many candidates wrote vague statements about coating with an unreactive substance without mentioning the type of substance involved.” “Most candidates could name one barrier method to prevent iron from rusting. The most common incorrect answer was ‘sacrificial protection’.”

Worked example 16 marks

Iron nails rust slowly in the presence of water and air. When an iron nail rusts, insoluble solid hydrated iron(III) oxide forms. The solid hydrated iron(III) oxide falls off the iron nail. The rusting of iron nails is slowed down by coating the nails with oil or paint. Plan an investigation to show which of the two coatings, oil or paint, slows down rusting the most. Your plan must include how the results will show which coating slows down rusting the most. You are provided with iron nails, oil, paint, water and common laboratory apparatus.

  1. 1

    Use the same mass of nails for each coating. [1]

  2. 2

    Place the oiled nails and the painted nails in water, in a suitable container. [1]

  3. 3

    Leave them for a stated suitable length of time. [1]

  4. 4

    Filter out the rust, the hydrated iron(III) oxide. [1]

  5. 5

    Dry the rust. [1]

  6. 6

    Measure the mass of rust. [1]

  7. 7

    The least mass of rust shows the coating that slows down rusting the most. (The mark scheme lists seven points; any six score.)

27. Galvanising

Galvanising is coating iron with zinc. It is an example of a barrier method and of sacrificial protection.

As a barrier, a complete zinc layer keeps oxygen and water off the iron. If the layer is scratched, the zinc still protects the iron as sacrificial protection.

Examiner insight

“Zinc is used as both a barrier protector of iron and a sacrificial protector of iron. Provided the zinc is in contact with at least part of the iron, the iron will be protected from corrosion.” “Most candidates talked about sacrificial methods of rust protection rather than the obvious barrier to water / oxygen that a complete layer on the surface of the iron would afford.”

Worked example 11 mark

Steel is an alloy of iron. Zinc is used to protect a steel car body from rusting. The steel body is completely coated in zinc. Which statements about this use of zinc are correct?

1 The layer of zinc prevents oxygen reaching the steel to cause rust. 2 Zinc is used because it is a more reactive metal than iron. 3 If the zinc layer is scratched, the steel will rust.

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

  1. 1

    1 is correct: the zinc is a barrier. 2 is correct: zinc is more reactive than iron, so it is a sacrificial protector.

  2. 2

    3 is wrong: scratched zinc still protects the steel. Answer B: 1 and 2 only. [1]

28. Sacrificial protection

In sacrificial protection a metal that is more reactive than iron is kept in contact with the iron. The more reactive metal is higher in the reactivity series, so it corrodes in preference to iron. Copper is less reactive than iron, so copper cannot protect iron this way.

In terms of electrons, the more reactive metal loses electrons more readily than iron does. For zinc:

Zn → Zn²⁺ + 2e⁻

Examiner insight

“At this level it was sufficient to state that zinc is more reactive than iron and therefore will corrode in preference to iron.” “Further explanation often included the phrase ‘sacrificial protection’ without explaining its meaning.”

Worked example 13 marks

Steel is an alloy of iron. Blocks of magnesium are bolted to the steel hull of a ship. Explain, in terms of the reactivity series and electron loss, why this protects the hull from rusting.

  1. 1

    Magnesium is more reactive than iron: it is higher in the reactivity series. [1]

  2. 2

    So magnesium loses electrons more readily than iron does. [1]

  3. 3

    The magnesium corrodes in preference to the iron, so the hull does not rust. [1]

Recap

  • Iron and steel rust in contact with oxygen and water. The rust is hydrated iron(III) oxide.
  • Only iron and steel rust. Other metals corrode.
  • Barrier methods: painting, greasing, coating with plastic.
  • A barrier works by excluding oxygen or water.
  • Extended: galvanising is coating iron with zinc. Zinc is a barrier and a sacrificial protector.
  • Extended: a more reactive metal loses electrons more readily, so it corrodes in preference to iron.

29. Reactivity and the ease of extraction

The more reactive a metal is, the more difficult it is to extract from its ore. The position of the metal in the reactivity series decides the method.

A metal less reactive than carbon can be obtained by reduction of its oxide with carbon or carbon monoxide. A metal more reactive than carbon needs electrolysis.

Whichever route is used, the lump of ore is first crushed to a powder.

Examiner insight

“The correct answer should be determined by their knowledge of the reactivity series and that more reactive metals are more difficult to extract from their ore.” “However, all three routes required the ore to be broken down into small pieces (preferably a powder) at the start – this step was missing from many answers.”

Worked example 16 marks

Tin is a metal that is between copper and iron in the reactivity series. The ore cassiterite contains the compound tin(IV) oxide, SnO₂.

Tin(IV) oxide is insoluble in water and reacts with dilute acids to form an aqueous solution of a salt. The ore cassiterite contains no other compounds that are insoluble in water and no other compounds that react with acids to form aqueous solutions.

Describe how you could obtain a sample of tin metal starting with a large lump of the ore cassiterite.

You are provided with common laboratory apparatus and chemicals.

  1. 1

    Crush the lump of ore into a powder. [1]

  2. 2

    Use a pestle and mortar. [1]

  3. 3

    Mix the powder with a reducing agent. [1]

  4. 4

    Use carbon. Carbon monoxide, hydrogen or a metal more reactive than tin, such as zinc, also earns the mark. [1]

  5. 5

    Heat the mixture. [1]

  6. 6

    Tin(IV) oxide is reduced to tin. [1]

30. Iron: heat, carbon monoxide and reduction

Iron is extracted from its ore, hematite, in the blast furnace. Hematite is mainly iron(III) oxide.

Coke is carbon. It burns in air to provide heat and to produce carbon dioxide. The carbon dioxide is reduced by more carbon to carbon monoxide.

Carbon monoxide reduces iron(III) oxide to iron.

C + O₂ → CO₂

C + CO₂ → 2CO

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Examiner insight

“The commonest error was to suggest that carbon monoxide is formed directly by incomplete combustion. Few candidates mentioned that the oxygen reacts with the coke to form carbon dioxide as a first step.” “Many candidates incorrectly referred to the iron and not the iron(III) oxide.”

Worked example 13 marks

Iron is extracted from its ore in a blast furnace.

(i) Name the ore used in the blast furnace to extract iron.

(ii) State one reason why coke is added to the blast furnace during the extraction process.

(iii) Name the compound used to reduce iron(III) oxide in the blast furnace.

  1. 1

    (i) hematite [1]

  2. 2

    (ii) to provide heat, or to produce carbon dioxide, or to produce carbon monoxide [1]

  3. 3

    (iii) carbon monoxide [1]

31. Limestone and slag

Limestone is calcium carbonate. In the blast furnace it is heated and undergoes thermal decomposition to calcium oxide and carbon dioxide.

Calcium oxide is a base. It reacts with silicon dioxide, an impurity, to form slag, which is calcium silicate.

Slag is less dense than iron. It floats on the iron and is removed from above it.

CaCO₃ →(heat) CaO + CO₂

CaO + SiO₂ → CaSiO₃

Examiner insight

“Few candidates realised that calcium carbonate decomposes to calcium oxide before reacting with silicon dioxide. Many candidates suggested, incorrectly, that calcium carbonate ‘is a catalyst’.” “Common errors included references to calcium oxide reacting with the iron or calcium oxide reacting with carbon monoxide to produce carbon dioxide.”

Worked example 11 mark

Equations 1 and 2 represent two reactions that occur inside a blast furnace.

1 Fe₂O₃ + 3CO → 2Fe + 3CO₂

2 CaCO₃ → CaO + CO₂

Which type of reactions are 1 and 2?

12
Aredoxredox
Bredoxthermal decomposition
Cthermal decompositionredox
Dthermal decompositionthermal decomposition
  1. 1

    Reaction 1: iron(III) oxide loses oxygen and carbon monoxide gains it. This is redox.

  2. 2

    Reaction 2: one compound breaks down on heating into two products. This is thermal decomposition.

  3. 3

    B: redox, thermal decomposition. [1]

32. Aluminium: bauxite and electrolysis

The main ore of aluminium is bauxite. Aluminium is extracted by electrolysis.

Bauxite is purified first. The purified ore is mainly aluminium oxide, Al₂O₃, which is a compound, not an ore.

Aluminium is more reactive than carbon, so it is not extracted by heating with carbon.

Examiner insight

“The commonest incorrect answers were hematite, aluminium oxide (which is not an ore) or aluminium ore.”

Worked example 12 marks

Aluminium is extracted from its purified main ore. The purified ore is mainly aluminium oxide. During this extraction process aluminium oxide is broken down into its elements.

(i) Name the main ore of aluminium.

(ii) Name the process in which aluminium oxide is broken down into its elements.

  1. 1

    (i) bauxite [1]

  2. 2

    (ii) electrolysis [1]

33. The role of cryolite

For the extraction, purified aluminium oxide is dissolved in molten cryolite.

Cryolite is a solvent for aluminium oxide. It lowers the operating temperature and increases the conductivity of the electrolyte. It is not a catalyst, and it does not change the melting point of aluminium oxide.

Examiner insight

“Cryolite as a catalyst was commonly seen.” “There were many references to cryolite as an electrical conductor itself rather than stating that cryolite increased the conductivity of the electrolyte.”

Worked example 12 marks

Aluminium is extracted by electrolysis using cryolite.

State two reasons why cryolite is used in this electrolysis.

  1. 1

    It is a solvent for aluminium oxide. [1]

  2. 2

    It lowers the operating temperature, or it increases conductivity. [1]

34. Reactions at the electrodes

The electrodes are carbon. Aluminium ions, Al³⁺, are attracted to the negative cathode. They gain electrons to form aluminium.

Oxide ions, O²⁻, are attracted to the positive anode. They lose electrons to form oxygen.

The electrons gained and lost must balance the charges in each half-equation.

cathode: Al³⁺ + 3e⁻ → Al

anode: 2O²⁻ → O₂ + 4e⁻

Examiner insight

“Candidates found the half-equations difficult and many attempts were incorrect due to an imbalance of charge. This often resulted from using an incorrect number of electrons.” “Some candidates gave the two correct products at the wrong electrodes.”

Worked example 11 mark

Aluminium is manufactured from aluminium oxide by electrolysis.

Which row shows the ionic half-equations at each electrode and describes the role of cryolite in the process?

reaction at anodereaction at cathoderole of cryolite
A2O²⁻ → O₂ + 4e⁻Al³⁺ + 3e⁻ → 3Alcatalyst
BAl³⁺ + 3e⁻ → Al2O²⁻ → O₂ + 4e⁻solvent for aluminium oxide
C2O²⁻ → O₂ + 4e⁻Al³⁺ + 3e⁻ → Alsolvent for aluminium oxide
DAl³⁺ + 3e⁻ → 3Al2O²⁻ → O₂ + 4e⁻catalyst
  1. 1

    Oxide ions go to the anode, so the anode reaction is 2O²⁻ → O₂ + 4e⁻. This rules out B and D.

  2. 2

    Aluminium forms at the cathode as Al³⁺ + 3e⁻ → Al. A has 3Al, which is not balanced.

  3. 3

    Cryolite is a solvent, not a catalyst. The answer is C. [1]

35. Replacing the carbon anodes

The anodes are made of carbon. The oxygen formed at the anode reacts with the carbon of the anode.

This forms carbon dioxide, so the anodes burn away. They have to be replaced regularly.

C + O₂ → CO₂

Examiner insight

“Many answers stated that the anode corroded, without further qualification. Those who knew that the carbon anode reacted with oxygen did not always give the product.” “Common weaker answers referred to the carbon coming from the electrolyte or the oxygen from the air.”

Worked example 12 marks

Aluminium is extracted when electricity is used to decompose an electrolyte containing a purified ore of aluminium. The electrodes are made of carbon.

Explain why the carbon anodes have to be replaced.

  1. 1

    The carbon anodes react with the oxygen made at the anode. [1]

  2. 2

    They form carbon dioxide: C + O₂ → CO₂. [1]

Recap

  • The more reactive the metal, the harder it is to extract from its ore.
  • Iron: hematite is reduced by carbon monoxide in the blast furnace.
  • Coke burns to give heat and carbon dioxide. Carbon dioxide and more coke make carbon monoxide.
  • Limestone decomposes to calcium oxide, which reacts with silicon dioxide to form slag.
  • Aluminium: the ore is bauxite and the method is electrolysis.
  • Extended: Fe₂O₃ + 3CO → 2Fe + 3CO₂, CaCO₃ → CaO + CO₂, CaO + SiO₂ → CaSiO₃.
  • Extended: cryolite is a solvent. Cathode Al³⁺ + 3e⁻ → Al, anode 2O²⁻ → O₂ + 4e⁻. Oxygen reacts with the carbon anodes.

End-of-chapter exercise

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

  1. State what is meant by a metal being ductile.1 mark
  2. A metal spoon left in hot tea gets a warm handle. Name the property of metals this shows.1 mark
  3. Name the property that lets copper be (a) drawn into wires and (b) hammered into sheets.2 marks
  4. The table shows three elements. State which are metals and give a reason. | element | melting point / °C | conductivity when solid | | --- | --- | --- | | P | 1535 | good | | Q | 44 | poor | | R | 98 | good |2 marks
  5. Magnesium is added to dilute hydrochloric acid. State one observation and name the gas formed.2 marks
  6. Zinc and copper are each added to dilute sulfuric acid. Describe what happens to each metal.2 marks
  7. Zinc reacts with dilute hydrochloric acid. (a) Write the word equation. (b) Write the balanced symbol equation with state symbols.3 marks
  8. Calcium reacts with cold water. Name the two products and write the balanced symbol equation.3 marks
  9. A student tests four solid elements. The results are in the table. (a) State which two elements are metals. (b) Explain why M is not classed as a metal, although it conducts electricity. (c) Another student says L cannot be a metal because it does not react with the acid. Explain why this is wrong. (d) The chloride of J is JCl₂. Write the balanced symbol equation, with state symbols, for J and the acid. | element | conducts when solid | when hammered | in dilute hydrochloric acid | | --- | --- | --- | --- | | J | good | flattens | bubbles of gas | | K | poor | shatters | no change | | L | good | flattens | no change | | M | good | shatters | no change |5 marks
  10. Strontium reacts with cold water in the same way as calcium. Zinc reacts with steam in the same way as magnesium. The metals form the ions Sr²⁺ and Zn²⁺. (a) Write the balanced symbol equation, with state symbols, for strontium and cold water. (b) Write the balanced symbol equation, with state symbols, for zinc and steam. (c) A student predicts that copper reacts with oxygen to form CuO and with dilute sulfuric acid to form hydrogen. State whether each prediction is correct, and write a balanced symbol equation for any reaction that happens.7 marks
  11. Name the metal used to make aircraft bodies because of its low density.1 mark
  12. State the property of aluminium that makes it suitable for food containers.1 mark
  13. Give two properties of copper that make it suitable for electrical wiring.2 marks
  14. Give two properties of aluminium that make it suitable for overhead electrical cables.2 marks
  15. Which row correctly pairs a metal with a use and a property that suits it? A aluminium · aircraft bodies · high density B aluminium · food containers · resistant to corrosion C copper · electrical wiring · low density D copper · cooking utensils · good conductor of electricity1 mark
  16. A student writes: “Aluminium is used in aircraft because it is light and is less dense than air.” Identify the two faults in this answer and correct them.2 marks
  17. Copper is used to make cooking utensils. Which two of these properties explain this? 1 corrosion resistant · 2 good conductor of electricity · 3 good conductor of heat2 marks
  18. Metal P is a good electrical conductor with a low density, but it is not ductile. Metal Q is a ductile, good electrical conductor with a high density. A company needs one of them for the electrical wiring in a house. Choose the metal and give two reasons.3 marks
  19. A power company tests four metals for a new overhead electrical cable. A density below 5.0 g/cm³ counts as low. A relative electrical conductivity above 50 counts as good. (a) Choose the most suitable metal and give the two properties that make it suitable. [3] (b) A student chooses W because it is the best conductor. Use the table to explain why W is less suitable than your choice. [1] | metal | density in g/cm³ | relative electrical conductivity | resistance to corrosion | | --- | --- | --- | --- | | W | 8.9 | 100 | good | | X | 2.7 | 63 | good | | Y | 1.7 | 12 | poor | | Z | 7.9 | 17 | poor |4 marks
  20. A solar-powered research aircraft has a metal body, electrical wiring joining its solar panels to its motors, and metal food containers for the crew. A student writes three sentences about it. 1 The body is aluminium because it is strong. 2 The wiring is copper because it is malleable and corrodes easily. 3 The food containers are aluminium because it is a good conductor of electricity. (a) Rewrite each sentence so that it gives the correct property or properties for that use. [4] (b) Name the use of aluminium for which good electrical conductivity does earn a mark, and state the other property needed for that use. [2]6 marks
  21. State the meaning of the term alloy.1 mark
  22. Name the two metals in brass.2 marks
  23. Stainless steel is a mixture of iron and other elements. Name two other metals that it can contain, and the non-metal that it can contain.3 marks
  24. A student says: ‘Brass is made when copper reacts with zinc to form a new compound.’ Explain what is wrong with this statement.2 marks
  25. A student is asked why alloys are more useful than pure metals. The student writes: ‘Alloys are strong and they do not rust.’ Explain why neither part of this answer earns a mark.2 marks
  26. Stainless steel is used to make cutlery. Give two properties of stainless steel that make it suitable for this use.2 marks
  27. The structures of three solids are described. P has atoms of one size, in regular layers. Q has atoms of two sizes. The second atoms sit in a completely regular repeating pattern. R has atoms of two sizes. The second atoms are spread at random among the others. State which structure best represents an alloy and give a reason. State why P does not represent an alloy.3 marks
  28. Explain, in terms of structure, why an alloy can be harder than the pure metal.2 marks
  29. Pure gold bends too easily to be used for rings. A jeweller mixes a small amount of copper into melted gold and lets it cool. Copper atoms are smaller than gold atoms. A student writes: ‘The gold and copper form a compound. The bonds are stronger, so the ring is hard.’ Write a correct explanation of why the ring is harder than pure gold. Include the type of substance the ring is made of.4 marks
  30. A company tests four rods of the same size. One rod is pure iron. The others are iron mixed with chromium. The same weight is dropped on each rod and the depth of the dent is measured. Each rod is then left in damp air for one week. The table gives the results. (a) State which rod is not an alloy. Give a reason. (b) State which rod is the most suitable for making cutlery. Give two reasons, using the results. (c) Explain, in terms of structure, why the rod chosen in (b) is harder than the pure iron rod. | percentage of chromium | depth of dent / mm | rust seen after one week | | --- | --- | --- | | 0 | 2.0 | yes | | 6 | 1.6 | yes | | 12 | 1.1 | no | | 18 | 0.8 | no |6 marks
  31. Put in order of reactivity, most reactive first: potassium, zinc, iron, magnesium, copper.2 marks
  32. Which of these metals is the least reactive: aluminium, calcium, iron, silver?1 mark
  33. Write a balanced equation, with state symbols, for the reaction of magnesium with steam.2 marks
  34. Zinc, copper and magnesium are each added to dilute hydrochloric acid. State which metals react, name the gas formed, and explain why copper behaves differently.3 marks
  35. The table gives results for three metals. Put X, Y and Z in order, most reactive first. | metal | reacts with cold water | reacts with dilute hydrochloric acid | | --- | --- | --- | | X | yes | yes | | Y | no | no | | Z | no | yes |2 marks
  36. Metal P reacts with cold water less vigorously than sodium but more vigorously than calcium. Metal Q reacts with dilute hydrochloric acid but not with cold water. Metal R does not react with dilute hydrochloric acid. Put sodium, calcium, P, Q and R in order, most reactive first.3 marks
  37. An iron nail is placed in aqueous copper(II) sulfate. Describe what happens and explain it in terms of positive ions.3 marks
  38. A piece of copper is placed in aqueous magnesium sulfate. Nothing happens. Explain why.2 marks
  39. A student adds aluminium foil and zinc foil to separate samples of dilute hydrochloric acid. Only the zinc gives bubbles. The student concludes that zinc is more reactive than aluminium. A new piece of aluminium foil is rubbed with sandpaper and added to the acid at once. It gives bubbles faster than the zinc did. (a) Explain why the first piece of aluminium gave no bubbles. (b) State which metal is more reactive. Explain why the student's conclusion was wrong. (c) Sandpapered aluminium is added to aqueous zinc sulfate. A grey solid forms on it. Explain this in terms of positive ions and write the ionic equation.6 marks
  40. Strips of three unknown metals J, L and M are added to aqueous solutions of five metal ions. The table gives the results: ‘yes’ means a reaction takes place. (a) Put magnesium, zinc, iron, copper, silver, J, L and M in order, most reactive first. (b) Predict whether J and L react with dilute hydrochloric acid. Explain each prediction. (c) J reacts with aqueous copper(II) ions. Name the particle that is reduced, and explain the reaction in terms of the tendency to form positive ions. | metal added | Mg²⁺(aq) | Zn²⁺(aq) | Fe²⁺(aq) | Cu²⁺(aq) | Ag⁺(aq) | | --- | --- | --- | --- | --- | --- | | J | no | no | yes | yes | yes | | L | no | no | no | no | yes | | M | no | yes | yes | yes | yes |7 marks
  41. State the two substances needed for iron to rust.2 marks
  42. Name the compound that forms when iron rusts.1 mark
  43. State three barrier methods used to prevent iron from rusting.3 marks
  44. Three iron nails are left for 14 days. Nail P is in dry air. Nail Q is in water with a layer of oil on top, so that no air reaches the nail. Nail R is in damp air. State which nail rusts and explain your answer.3 marks
  45. Explain how a layer of grease stops an iron axle from rusting.2 marks
  46. A steel bridge is painted. The paint is scratched down to the steel. Explain why the steel rusts at the scratch.2 marks
  47. State what is meant by galvanising and give the two ways that galvanising protects iron.3 marks
  48. A galvanised steel fence is scratched so that some steel is showing. Explain why the steel does not rust.3 marks
  49. A steel water tank is buried in damp soil. A rod of magnesium is bolted to the outside of the tank. Magnesium is above iron in the reactivity series and forms Mg²⁺ ions. Write the ionic half-equation for the change to the magnesium. Then explain, in terms of the reactivity series and electron loss, why the tank does not rust. A student writes: “the magnesium rusts instead of the iron”. State what is wrong with this sentence.5 marks
  50. The order of reactivity of four metals, most reactive first, is magnesium, zinc, iron, copper. Three steel plates are left outdoors in damp air for one month. Plate X is completely coated in zinc. Plate Y is completely coated in copper. Plate Z has no coating, but a small block of magnesium is bolted to it. None of the plates rusts. A deep scratch is then made in the coating of X and of Y, so that steel shows, and all three plates are left for another month. Explain why none of the plates rusts in the first month. Then state which plate rusts in the second month, and explain why it rusts and why plate X does not.6 marks
  51. Name the ore from which iron is extracted in the blast furnace.1 mark
  52. Iron can be extracted by heating its oxide with carbon. Aluminium cannot. Use the reactivity series to explain why.2 marks
  53. Magnesium, copper and iron are obtained from their ores. Put the metals in order, starting with the one that is easiest to extract.2 marks
  54. Describe how carbon monoxide is made in the blast furnace, starting from coke.3 marks
  55. Iron(III) oxide reacts with carbon monoxide in the blast furnace to form iron and carbon dioxide. Explain why this is reduction of iron(III) oxide.2 marks
  56. Calcium carbonate is added to the blast furnace. Describe what happens to it and name the substance made with silicon dioxide.3 marks
  57. A student writes: ‘Aluminium is made by heating its ore, aluminium oxide, with carbon.’ Give two corrections.2 marks
  58. A plant makes aluminium by electrolysis. Purified aluminium oxide is dissolved in molten cryolite in a cell with carbon electrodes. The positive electrodes shrink and have to be replaced. The negative electrodes do not shrink. (a) Name the ore from which the aluminium oxide is purified. (b) State two reasons why cryolite is used. (c) Explain why the positive electrodes shrink. Include a symbol equation.6 marks
  59. In an aluminium cell the electrolyte contains Al³⁺ ions and O²⁻ ions. The electrodes are carbon. In a short time, 12 electrons leave the anode and travel through the wires to the cathode. (a) Write the ionic half-equation for the reaction at the cathode. (b) Write the ionic half-equation for the reaction at the anode. (c) Deduce the number of aluminium atoms and the number of oxygen molecules formed by the 12 electrons. (d) All of this oxygen reacts with the anode. Write the symbol equation for this reaction and deduce the number of carbon atoms the anode loses.8 marks
  60. Tin is between copper and iron in the reactivity series. An ore of tin contains tin(IV) oxide, SnO₂, with silicon dioxide as an impurity. A furnace is loaded with the crushed ore, coke and limestone, and hot air is blown in. The reactions in this furnace are of the same types as those in the blast furnace for iron. (a) Write symbol equations for the two reactions that turn coke into the reducing agent. (b) Write a balanced symbol equation for the reduction of tin(IV) oxide by this reducing agent. (c) A student writes: ‘The limestone is a catalyst.’ Describe what really happens to the limestone and to the impurity. Include two symbol equations. (d) Explain why a furnace like this cannot extract aluminium from aluminium oxide.8 marks

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