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Chemistry · Practice book · Senior 2 · Unit 3 of 9

Categories of chemical reactions

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 3 Categories of chemical reactions

Key unit competence: Classify different types of chemical reactions based on reactant behaviour and energy changes, and relate chemical reactions to real-life applications.

Key ideas

  • Combination (synthesis): A + B → AB, e.g. 2Mg + O2 → 2MgO. Decomposition: AB → A + B, often by heat (thermal), electricity or light, e.g. CaCO3 → CaO + CO2.
  • Single displacement: a more reactive element displaces a less reactive one from its compound, e.g. Zn + CuSO4 → ZnSO4 + Cu. Double displacement: ions swap partners, often forming a precipitate, e.g. AgNO3 + NaCl → AgCl + NaNO3.
  • Neutralisation: acid + base → salt + water. Combustion: burning in oxygen releasing heat and light. Redox: one substance is oxidised (gains oxygen/loses electrons) and another reduced.
  • Exothermic reactions give out heat (temperature rises): combustion, neutralisation, respiration. Endothermic reactions take in heat (temperature falls): photosynthesis, thermal decomposition, dissolving ammonium nitrate.
  • An ionic equation shows only the ions that react; spectator ions are left out. Example: Ag+(aq) + Cl−(aq) → AgCl(s); neutralisation: H+(aq) + OH−(aq) → H2O(l).
  • Applications: combustion of fuels for energy, decomposition of limestone to make lime, displacement in extraction of metals, precipitation in water treatment, neutralisation in medicine and farming.

Common misconceptions

  • "Every reaction that needs heating is endothermic." No: Many exothermic reactions need heat to start (e.g. burning) but then release more heat.
  • "Spectator ions take part in the reaction." No: Spectator ions remain unchanged in solution and are left out of the ionic equation.
  • "Copper can displace zinc from zinc sulfate." No: Only a more reactive metal displaces a less reactive one; copper is less reactive than zinc.

Worked examples

Example 1. Classify each reaction: (a) 2KClO3 → 2KCl + 3O2, (b) Fe + CuSO4 → FeSO4 + Cu, (c) HNO3 + KOH → KNO3 + H2O.

Solution

(a) Thermal decomposition: one compound breaks into two substances.

(b) Single displacement: iron displaces copper.

(c) Neutralisation: acid + base → salt + water.

Example 2. Write the ionic equation and name the spectator ions for: CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq).

Solution

Full ionic: Cu2+ + SO42− + 2Na+ + 2OH− → Cu(OH)2 + 2Na+ + SO42−.

Cancel the ions that appear on both sides.

Ionic equation: Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s).

Spectator ions: Na+ and SO42−.

Practice questions

  1. Define a decomposition reaction.

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    A reaction in which one compound breaks down into two or more simpler substances.

    Common mistake: Saying 'two substances join'.

  2. Give one example each of an exothermic and an endothermic process.

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    Exothermic: combustion/neutralisation/respiration. Endothermic: photosynthesis/thermal decomposition/dissolving ammonium nitrate.

    Common mistake: Calling melting ice a reaction.

  3. Complete: Mg + ZnSO4 →

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    MgSO4 + Zn.

    Common mistake: Writing no reaction.

  4. What is a precipitate?

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    An insoluble solid formed when two solutions react.

    Common mistake: Saying it is a gas.

  5. Write the ionic equation for neutralisation.

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    H+(aq) + OH−(aq) → H2O(l).

    Common mistake: Including Na+ and Cl−.

  6. Explain why copper does not react with zinc sulfate solution.

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    Copper is less reactive than zinc, so it cannot displace it.

    Common mistake: Saying the solution is too dilute.

  7. Classify: N2 + 3H2 → 2NH3.

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    Combination (synthesis).

    Common mistake: Calling it decomposition.

  8. State what happens to the temperature in an endothermic reaction and why.

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    It falls, because heat is taken from the surroundings.

    Common mistake: Saying it rises.

  9. Write the ionic equation for the reaction of BaCl2 with H2SO4.

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    Ba2+(aq) + SO42−(aq) → BaSO4(s).

    Common mistake: Leaving the spectator ions in.

  10. Explain why burning charcoal needs a match to start even though it is exothermic.

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    Energy (activation energy) is needed to start the reaction; then the heat released keeps it going.

    Common mistake: Saying burning is endothermic.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Mixing silver nitrate and sodium chloride solutions gives a white solid. The reaction is:a. an exothermic combination reaction
    2. Which reaction is exothermic?b. Burning charcoal
    3. Dissolving ammonium nitrate in water makes the water cold. This process is:c. double displacement (precipitation)
    4. Making slaked lime: CaO + H2O → Ca(OH)2 gives out much heat. It is:d. endothermic
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    1 → c (double displacement (precipitation)) · 2 → b (Burning charcoal) · 3 → d (endothermic) · 4 → a (an exothermic combination reaction)

    • Ions swap partners and insoluble AgCl precipitates.
    • Burning releases heat.
    • It absorbs heat from the water.
    • Two substances combine and heat is released.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    releases heat  ·  falls  ·  absorbs light energy  ·  thermal decomposition
    1. In an endothermic reaction, the temperature of the surroundings .
    2. Photosynthesis is endothermic because it .
    3. Heating calcium carbonate to form calcium oxide and carbon dioxide is .
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    1: falls · 2: absorbs light energy · 3: thermal decomposition

    • Heat is taken in from the surroundings.
    • Plants absorb energy from sunlight.
    • One compound breaks into simpler substances by heat.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which metal can displace copper from copper(II) sulfate solution?a. photodecomposition
    2. The ionic equation for the precipitation of silver chloride is:b. SO42−
    3. Silver chloride darkens in light: 2AgCl → 2Ag + Cl2. This is:c. Ag+(aq) + Cl−(aq) → AgCl(s)
    4. The spectator ions in Zn(s) + Cu2+(aq) + SO42−(aq) → Zn2+(aq) + SO42−(aq) + Cu(s) are:d. Iron
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    1 → d (Iron) · 2 → c (Ag+(aq) + Cl−(aq) → AgCl(s)) · 3 → a (photodecomposition) · 4 → b (SO42−)

    • Iron is more reactive than copper.
    • Na+ and NO3− are spectator ions.
    • Light causes the decomposition.
    • Sulfate appears unchanged on both sides.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    form the precipitate  ·  do not change during the reaction  ·  single displacement reaction  ·  exothermic
    1. Zn + CuSO4 → ZnSO4 + Cu is a .
    2. Respiration in cells is .
    3. Spectator ions are ions that .
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    1: single displacement reaction · 2: exothermic · 3: do not change during the reaction

    • Zinc displaces copper from its salt.
    • It releases energy from glucose.
    • They stay in solution unchanged.

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