11.2 Heat of Reaction

Short Answer Questions

1. What is the heat of reaction?
   Answer
   The heat of reaction refers to the heat energy that is either absorbed or released during a chemical reaction.
2. What is another name for the heat of reaction?
   Answer
   The heat of reaction is also known as the enthalpy change (ΔH).
3. What is the unit for measuring enthalpy change (ΔH)?
   Answer
   Kilojoules per mole (kJ mol⁻¹).
4. What does a negative enthalpy change (ΔH) indicate?
   Answer
   A negative ΔH indicates an exothermic reaction, meaning heat is released to the surroundings.
5. What does a positive enthalpy change (ΔH) indicate?
   Answer
   A positive ΔH indicates an endothermic reaction, meaning heat is absorbed from the surroundings.
6. What is the formula for enthalpy change (ΔH)?
   Answer
   ΔH = H_products – H_reactants
7. What is the heat of combustion?
   Answer
   The heat of combustion is the heat released when one mole of a substance is completely burned in excess oxygen.
8. Why is the heat of combustion always negative?
   Answer
   Combustion is an exothermic process, meaning it always releases heat.
9. Give an example of a combustion reaction with its enthalpy change.
   Answer
   C(p) + O₂(g) → CO₂(g) ΔH = -393.5 kJ mol⁻¹
10. What is the heat of neutralisation?
    Answer
    The heat of neutralisation is the heat released when one mole of water is formed from the reaction between an acid and a base.
11. Why is the heat of neutralisation for strong acids and bases always around -57 kJ mol⁻¹?
    Answer
    The reaction between strong acids and bases always involves the formation of water from H⁺ and OH⁻ ions, which releases a constant amount of heat.
12. What is the heat of precipitation?
    Answer
    The heat of precipitation is the heat released when one mole of a precipitate is formed from a reaction in an aqueous solution.
13. Give an example of a precipitation reaction with its enthalpy change.
    Answer
    Ag⁺(aq) + Cl⁻(aq) → AgCl(s) ΔH = -65.5 kJ mol⁻¹
14. What is the heat of displacement?
    Answer
    The heat of displacement is the heat released when one mole of a metal is displaced from its solution by a more reactive metal.
15. Give an example of a displacement reaction with its enthalpy change.
    Answer
    Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) ΔH = -210 kJ mol⁻¹
16. What is a thermochemical equation?
    Answer
    A thermochemical equation is a chemical equation that includes the enthalpy change (ΔH) for the reaction.
17. How can you tell if a thermochemical equation represents an exothermic reaction?
    Answer
    An exothermic reaction will have a negative ΔH value.
18. How can you tell if a thermochemical equation represents an endothermic reaction?
    Answer
    An endothermic reaction will have a positive ΔH value.
19. Why does the decomposition of calcium carbonate have a positive ΔH value?
    Answer
    The decomposition of calcium carbonate (CaCO₃ → CaO + CO₂) is an endothermic reaction that absorbs heat.
20. What is the enthalpy change for the decomposition of calcium carbonate?
    Answer
    ΔH = +178.3 kJ mol⁻¹

Explanation Questions

1. Why does the heat of reaction depend on bond breaking and bond formation?
   Answer
   Bond breaking requires energy (endothermic), while bond formation releases energy (exothermic). The heat of reaction depends on whether more energy is released or absorbed.
2. Why do stronger bonds release more energy when they form?
   Answer
   Stronger bonds require more energy to break, so when they form, they release a larger amount of energy, making reactions more exothermic.
3. Why is the heat of combustion higher for larger hydrocarbons?
   Answer
   Larger hydrocarbons have more carbon and hydrogen atoms, meaning more bonds can be broken and formed, releasing more heat.
4. Why do weak acids have a lower heat of neutralisation than strong acids?
   Answer
   Weak acids do not fully ionise in solution, so some energy is used to ionise them before neutralisation, reducing the overall heat released.
5. Why is displacement an exothermic process?
   Answer
   A more reactive metal displaces a less reactive metal from its solution, releasing energy as new bonds form in the solid metal.
6. Why do thermochemical equations include physical states of reactants and products?
   Answer
   The physical states affect enthalpy change, as different states have different amounts of stored energy.
7. Why is the heat of neutralisation between a strong acid and a strong base always approximately -57 kJ mol⁻¹?
   Answer
   The neutralisation of a strong acid and a strong base always results in the formation of water (H₂O) from H⁺ and OH⁻ ions. Since this reaction is always the same, the energy released is constant at about -57 kJ mol⁻¹.
8. Why does the heat of displacement vary for different metals?
   Answer
   The heat of displacement depends on the reactivity difference between the metals involved. More reactive metals release more energy when displacing a less reactive metal from its solution.
9. Why do thermochemical equations show enthalpy change (ΔH) values?
   Answer
   Thermochemical equations include ΔH values to indicate whether a reaction is exothermic or endothermic and to quantify the amount of heat absorbed or released during the reaction.
10. Why is the enthalpy change of combustion always negative?
    Answer
    Combustion is an exothermic process where chemical energy stored in the fuel is converted into heat energy, releasing it to the surroundings. Therefore, the enthalpy change (ΔH) is always negative.