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Chemistry: Changes to materials

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Chemistry: Changes to materials
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In this chapter

  1. 1Exothermic Reactions: Releasing Energy
  2. 2Endothermic Reactions: Absorbing Energy
  3. 3Reactions of Metals with Oxygen
  4. 4Reactions of Metals with Water
  5. 5Reactions of Metals with Dilute Acids

1. Exothermic Reactions: Releasing Energy

An exothermic reaction is a chemical reaction that releases energy into its surroundings. This energy is usually transferred as heat, causing the temperature of the surroundings (like the test tube or beaker the reaction is in) to increase. In terms of chemical bonds, more energy is released when new bonds are formed in the products than is absorbed to break the bonds in the reactants. Common examples of exothermic reactions include all combustion (burning) reactions, neutralisation reactions between acids and alkalis, and respiration in living cells.

Reactants → Products + Energy

Acid + Alkali → Salt + Water

Key term

Exothermic reaction: A chemical reaction that releases energy to the surroundings, usually in the form of heat, causing the temperature of the surroundings to rise.

Examiner insight

Examiners reward clear descriptions that link the release of energy by the reaction to the observed temperature increase in the surroundings.

Common pitfall

Stating that 'heat is created'. Energy is conserved, not created; in an exothermic reaction, chemical potential energy is converted into thermal energy.

Fun fact

Self-heating coffee cans use an exothermic reaction, often between calcium oxide and water, in a separate compartment to heat the drink without mixing with it.

Worked example 12 marks

A student adds 5g of anhydrous copper(II) sulfate powder to 25 cm³ of water in a polystyrene cup and stirs. They record the temperature before and after. The initial temperature was 20°C and the final temperature was 35°C. Is the process exothermic or endothermic? Explain your reasoning.

  1. 1

    Step 1: Identify the temperature change. The temperature increased from 20°C to 35°C.

  2. 2

    Step 2: Relate the temperature change to energy transfer. An increase in temperature means that heat energy has been released into the surroundings (the water).

  3. 3

    Step 3: Conclude the type of reaction. Since energy has been released, the reaction is exothermic.

Recap

  • Exothermic reactions release energy into the surroundings.
  • The temperature of the surroundings increases during an exothermic reaction.
  • Combustion and neutralisation are common examples of exothermic reactions.
  • In an exothermic reaction, the energy released from forming new bonds is greater than the energy absorbed to break old bonds.
  • Energy is a product in an exothermic reaction.

Quick check

  1. If you mix two chemicals and the test tube feels warm, what type of reaction has occurred?1 mark

2. Endothermic Reactions: Absorbing Energy

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings. This energy is usually taken in as heat, causing the temperature of the surroundings to decrease. The reaction will feel cold. For an endothermic reaction to occur, more energy must be absorbed to break the bonds in the reactants than is released when new bonds are formed in the products. Key examples include the thermal decomposition of calcium carbonate, photosynthesis, and dissolving certain salts like ammonium nitrate in water.

Reactants + Energy → Products

CaCO₃(s) → CaO(s) + CO₂(g)

Key term

Endothermic reaction: A chemical reaction that absorbs energy from the surroundings, usually in the form of heat, causing the temperature of the surroundings to fall.

Examiner insight

Students who can explain *why* the temperature drops (because the reaction is taking in energy from the solution/surroundings for bond breaking) gain higher marks.

Common pitfall

Assuming that all dissolving is endothermic. Dissolving substances like sodium hydroxide or concentrated sulfuric acid in water are strongly exothermic.

Fun fact

Instant cold packs, used for sports injuries, contain a pouch of water and ammonium nitrate crystals. When the pack is squeezed, the pouch breaks, the salt dissolves, and the endothermic process rapidly drops the pack's temperature to near 0°C.

Worked example 12 marks

When sherbet is placed on the tongue, it feels cold. Sherbet contains citric acid and sodium bicarbonate. Explain why it feels cold, using ideas about energy.

  1. 1

    Step 1: Identify the observation. The sherbet feels cold, which means the temperature on the tongue has decreased.

  2. 2

    Step 2: Relate temperature change to energy transfer. A decrease in temperature means that heat energy has been absorbed from the surroundings (the tongue).

  3. 3

    Step 3: Conclude the type of reaction. The reaction between citric acid and sodium bicarbonate must be endothermic because it is absorbing heat energy from the tongue to proceed.

Recap

  • Endothermic reactions absorb energy from the surroundings.
  • The temperature of the surroundings decreases during an endothermic reaction.
  • Thermal decomposition and photosynthesis are key examples of endothermic processes.
  • In an endothermic reaction, the energy absorbed to break bonds is greater than the energy released from forming bonds.
  • Energy is a reactant in an endothermic reaction.

Quick check

  1. What is the general term for a reaction that breaks down a compound using heat?1 mark

3. Reactions of Metals with Oxygen

Most metals react with oxygen from the air to form metal oxides. This reaction is a form of oxidation. The speed and vigour of the reaction depend on the metal's reactivity. Very reactive metals like potassium and sodium react rapidly, while less reactive metals like iron react slowly (rusting). Unreactive metals like gold do not react at all. When metals burn in oxygen, it is a combustion reaction which is always exothermic, releasing light and heat. For example, magnesium burns with a brilliant white flame to form a white powder, magnesium oxide.

Metal + Oxygen → Metal Oxide

2Mg(s) + O₂(g) → 2MgO(s)

4Fe(s) + 3O₂(g) + H₂O(l) → 2Fe₂O₃·xH₂O(s) (Rusting)

Key term

Oxidation: A chemical process involving the gain of oxygen by a substance or the loss of electrons.

Examiner insight

Marks are often awarded for writing correctly balanced symbol equations for these reactions, including state symbols. Practice these regularly.

Common pitfall

Forgetting that water is essential for iron to rust. Simply stating 'iron and oxygen' is an incomplete answer and will not get full marks.

Worked example 13 marks

Write the balanced chemical equation, including state symbols, for the reaction when calcium metal is burned in air.

  1. 1

    Step 1: Identify the reactants. The reactants are calcium (Ca) and oxygen (O₂).

  2. 2

    Step 2: Determine the product. A metal reacting with oxygen forms a metal oxide. Calcium is in Group 2, so it forms a Ca²⁺ ion. Oxygen forms an O²⁻ ion. The formula for calcium oxide is CaO.

  3. 3

    Step 3: Write the unbalanced equation: Ca + O₂ → CaO.

  4. 4

    Step 4: Balance the equation. There are 2 oxygen atoms on the left and 1 on the right. Place a 2 in front of CaO: Ca + O₂ → 2CaO. Now there are 2 calcium atoms on the right and 1 on the left. Place a 2 in front of Ca: 2Ca + O₂ → 2CaO.

  5. 5

    Step 5: Add state symbols. Calcium is a solid (s), oxygen is a gas (g), and calcium oxide is a solid (s). Final answer: 2Ca(s) + O₂(g) → 2CaO(s).

Recap

  • Metals react with oxygen to form metal oxides.
  • This reaction is called oxidation.
  • The more reactive the metal, the faster it reacts with oxygen.
  • The rusting of iron requires both oxygen and water.
  • Burning a metal in oxygen is a highly exothermic combustion reaction.

Quick check

  1. What is the chemical name for rust?1 mark
  2. What is the general name for the compound formed when a metal reacts with oxygen?1 mark

4. Reactions of Metals with Water

The reaction of a metal with water is a key indicator of its reactivity. A reactivity series can be established by observing these reactions.

  • Very reactive metals (e.g., potassium, sodium, lithium) react vigorously with cold water. They produce a metal hydroxide and hydrogen gas.
  • Less reactive metals (e.g., magnesium, zinc, iron) do not react with cold water but will react with steam at high temperatures. They produce a metal oxide and hydrogen gas.
  • Unreactive metals (e.g., copper, silver, gold) do not react with water or steam at all. The production of hydrogen gas can be confirmed by the 'squeaky pop' test.

Metal + Cold Water → Metal Hydroxide + Hydrogen

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Metal + Steam → Metal Oxide + Hydrogen

Zn(s) + H₂O(g) → ZnO(s) + H₂(g)

Key term

Reactivity Series: A list of metals ranked in order of their reactivity, determined by how readily they take part in chemical reactions.

Examiner insight

Examiners look for precise observations. For example, describing sodium's reaction with water should include 'fizzes', 'melts into a sphere', and 'moves around on the surface'.

Common pitfall

Confusing the products from a metal's reaction with cold water (metal hydroxide) versus its reaction with steam (metal oxide).

Worked example 13 marks

A small piece of calcium is dropped into a beaker of water. Describe two observations you would expect to see and write a word equation for the reaction.

  1. 1

    Step 1: Recall the position of calcium in the reactivity series. Calcium is a reactive metal (Group 2).

  2. 2

    Step 2: Predict the reaction with cold water. It will react to form calcium hydroxide and hydrogen.

  3. 3

    Step 3: Describe the observations. The production of hydrogen gas will cause fizzing/bubbles/effervescence. The calcium will react and disappear. The solution may turn cloudy as calcium hydroxide is only sparingly soluble.

  4. 4

    Step 4: Write the word equation. Metal + Water → Metal Hydroxide + Hydrogen. So, Calcium + Water → Calcium hydroxide + Hydrogen.

Recap

  • A metal's reaction with water indicates its reactivity.
  • Reactive metals react with cold water to form a metal hydroxide and hydrogen.
  • Less reactive metals react with steam to form a metal oxide and hydrogen.
  • Unreactive metals like copper and gold do not react with water or steam.
  • Hydrogen gas is always a product and can be tested for with a lit splint.

Quick check

  1. Which gas is produced when a reactive metal reacts with water?1 mark
  2. What is the difference in the products when magnesium reacts with cold water versus steam?2 marks

5. Reactions of Metals with Dilute Acids

The reaction between a metal and a dilute acid also provides evidence for the reactivity of the metal. Metals that are above hydrogen in the reactivity series will react with dilute acids like hydrochloric acid (HCl) and sulfuric acid (H₂SO₄). Metals below hydrogen (like copper, silver, gold) will not react. The general products of the reaction are a salt and hydrogen gas. The speed of the reaction, observed by the rate of fizzing (effervescence), shows the metal's reactivity. For example, magnesium reacts very rapidly with dilute acid, zinc reacts steadily, and lead reacts very slowly.

Metal + Acid → Salt + Hydrogen

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Key term

Salt: An ionic compound formed when the hydrogen ion in an acid is replaced by a metal ion or an ammonium ion.

Examiner insight

Being able to correctly name the salt produced is a key skill. Remember: hydrochloric acid → chloride salt; sulfuric acid → sulfate salt; nitric acid → nitrate salt.

Common pitfall

Writing the formula for hydrogen gas as 'H'. Hydrogen is diatomic, so its formula is always H₂ when it is an element.

Worked example 13 marks

A student adds a piece of zinc metal to a test tube containing dilute sulfuric acid.(a) Name the two products formed.(b) Write a balanced chemical equation for the reaction.

  1. 1

    Step 1 (a): Apply the general rule: Metal + Acid → Salt + Hydrogen. The metal is zinc and the acid is sulfuric acid. The gas produced is hydrogen.

  2. 2

    Step 2 (a): Name the salt. The first name comes from the metal (zinc). The second name comes from the acid (sulfuric acid becomes sulfate). So the salt is zinc sulfate.

  3. 3

    Step 3 (b): Write the chemical formulas. Zinc is Zn. Sulfuric acid is H₂SO₄. Zinc sulfate is ZnSO₄ (Zn²⁺ and SO₄²⁻ ions). Hydrogen gas is H₂.

  4. 4

    Step 4 (b): Write and balance the equation: Zn + H₂SO₄ → ZnSO₄ + H₂. The equation is already balanced. So the final answer is Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).

Recap

  • Metals above hydrogen in the reactivity series react with dilute acids.
  • The products of the reaction are a salt and hydrogen gas.
  • The rate of bubbling indicates the metal's reactivity.
  • The name of the salt depends on the metal and the acid used.
  • Metals below hydrogen, like copper, do not react with dilute acids.

Quick check

  1. What salt is produced when magnesium reacts with hydrochloric acid?1 mark
  2. Why doesn't copper react with dilute sulfuric acid?1 mark

End-of-chapter exercise

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

  1. Define an endothermic reaction and give one example of a process that is endothermic.2 marks
  2. Write the word equation for the reaction of zinc with sulfuric acid.1 mark
  3. A student observes that calcium reacts more vigorously with water than magnesium does, and that copper does not react at all. Place calcium, magnesium, and copper in order of reactivity, from most reactive to least reactive. Explain your reasoning.3 marks
  4. Write a balanced chemical equation, including state symbols, for the reaction of magnesium burning in oxygen.3 marks
  5. Describe the chemical test to confirm the presence of hydrogen gas.2 marks
  6. When ammonium nitrate dissolves in water, the temperature of the solution drops significantly. Explain this observation in terms of the energy changes that occur when chemical bonds are broken and made.3 marks
  7. A student adds 1g of zinc powder to 50 cm³ of copper(II) sulfate solution in a polystyrene cup. The temperature increases from 21°C to 45°C. (a) Is this reaction exothermic or endothermic? Explain your answer. (b) Write a word equation for the reaction.3 marks
  8. Iron rusts in the presence of oxygen and water. (a) What is the chemical name for rust? (b) Galvanising is a process where iron is coated with zinc to prevent rusting. Explain, with reference to the reactivity series, how this method provides protection even if the zinc coating is scratched.4 marks
  9. Design an experiment to compare the reactivity of magnesium, iron, and copper using dilute hydrochloric acid. Your plan should state the key variables you would control to make it a fair test, and the measurement you would take to compare the rates of reaction.5 marks
  10. When a small piece of sodium metal reacts with water, sodium hydroxide and hydrogen gas are formed. (a) Write a balanced chemical equation for this reaction. (b) Describe two observations you would make during this reaction.4 marks

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