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Exothermic and Endothermic Reactions: Question 8

Syllabus 5.1

Structured Extended 6 marks

Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas:

H2(g)+Cl2(g)→2HCl(g)\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)

The bond energies are:

  • H–H bond: 436 kJ/mol436\,\text{kJ/mol}
  • Cl–Cl bond: 243 kJ/mol243\,\text{kJ/mol}
  • H–Cl bond: 431 kJ/mol431\,\text{kJ/mol}

(a) Calculate the total energy absorbed in breaking all the bonds in the reactants. [2]

(b) Calculate the total energy released in forming all the bonds in the products. [2]

(c) Calculate the overall energy change, ΔH\Delta H, for this reaction, and state whether the reaction is exothermic or endothermic. [2]

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Worked solution

Part (a): Energy absorbed breaking bonds in the reactants

One mole of H2\text{H}_2 contains one H–H bond, and one mole of Cl2\text{Cl}_2 contains one Cl–Cl bond. Breaking bonds always absorbs energy:

1×(H–H)+1×(Cl–Cl)=436+243=679 kJ/mol1 \times (\text{H–H}) + 1 \times (\text{Cl–Cl}) = 436 + 243 = 679\,\text{kJ/mol}

So 679 kJ/mol679\,\text{kJ/mol} of energy is absorbed breaking the bonds in the reactants.

Part (b): Energy released forming bonds in the products

Two moles of HCl\text{HCl} are formed, and each HCl\text{HCl} molecule contains one H–Cl bond, so two H–Cl bonds are formed in total. Forming bonds always releases energy:

2×(H–Cl)=2×431=862 kJ/mol2 \times (\text{H–Cl}) = 2 \times 431 = 862\,\text{kJ/mol}

So 862 kJ/mol862\,\text{kJ/mol} of energy is released forming the bonds in the products.

Part (c): Overall energy change and classification

ΔH=(energy absorbed breaking bonds)−(energy released forming bonds)\Delta H = (\text{energy absorbed breaking bonds}) - (\text{energy released forming bonds})

ΔH=679−862=−183 kJ/mol\Delta H = 679 - 862 = -183\,\text{kJ/mol}

Since ΔH\Delta H is negative, more energy is released forming the new bonds than is absorbed breaking the old ones, so the reaction is exothermic.

Final answers

  • (a) Energy absorbed breaking bonds == 679 kJ/mol679\,\text{kJ/mol}
  • (b) Energy released forming bonds == 862 kJ/mol862\,\text{kJ/mol}
  • (c) ΔH=−183 kJ/mol\Delta H = \boxed{-183}\,\text{kJ/mol}; the reaction is exothermic.