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Exothermic and Endothermic Reactions: Question 10

Syllabus 5.1

Multiple choice Extended 1 mark

In the Haber process, nitrogen and hydrogen react to form ammonia:

N2(g)+3H2(g)→2NH3(g)\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)

The bond energies are:

  • N≡N bond: 944 kJ/mol944\,\text{kJ/mol}
  • H–H bond: 436 kJ/mol436\,\text{kJ/mol}
  • N–H bond: 391 kJ/mol391\,\text{kJ/mol}

Using these bond energies, which of the following gives the correct value of ΔH\Delta H for this reaction and correctly classifies it?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Energy absorbed breaking bonds in the reactants

One mole of N2\text{N}_2 contains one N≡N bond, and three moles of H2\text{H}_2 contain three H–H bonds:

1×(N≡N)+3×(H–H)=944+(3×436)=944+1308=2252 kJ/mol1 \times (\text{N}{\equiv}\text{N}) + 3 \times (\text{H–H}) = 944 + (3 \times 436) = 944 + 1308 = 2252\,\text{kJ/mol}

Step 2: Energy released forming bonds in the products

Two moles of NH3\text{NH}_3 are formed, and each NH3\text{NH}_3 molecule contains three N–H bonds, giving six N–H bonds in total:

6×(N–H)=6×391=2346 kJ/mol6 \times (\text{N–H}) = 6 \times 391 = 2346\,\text{kJ/mol}

Step 3: Calculate ΔH\Delta H

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

ΔH=2252−2346=−94 kJ/mol\Delta H = 2252 - 2346 = -94\,\text{kJ/mol}

Since ΔH\Delta H is negative, the reaction is exothermic.

Why the other options are wrong

  • Option B has the correct size (94 kJ/mol94\,\text{kJ/mol}) but the wrong sign, which would come from subtracting in the reverse order; a negative ΔH\Delta H means exothermic, not endothermic.
  • Option C uses only the total energy released forming bonds (2346 kJ/mol2346\,\text{kJ/mol}, rounded in the option text) without subtracting the energy absorbed breaking bonds, so it is not a valid enthalpy change.
  • Option D uses only the total energy absorbed breaking bonds (2252 kJ/mol2252\,\text{kJ/mol}) without subtracting the energy released forming bonds, and also wrongly labels this endothermic.

Final answers

  • ΔH=−94 kJ/mol\Delta H = \boxed{-94}\,\text{kJ/mol}; the reaction is exothermic, option A\boxed{\text{option A}}.