Bond energy calculation

Energy changes

GCSE Chemistry (8462) · 4.5.1.3 · Higher

ORIGINAL

Energy in to break every reactant bond, energy out making every product bond; overall change = broken − made.

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Question

Methane burns in oxygen:

\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}

. Use the bond energies to calculate the overall energy change for the reaction, and state whether it is exothermic or endothermic. Bond energies in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464.

Every step worked, with the reasoning.

1
Bonds broken: four C–H and two O=O
Count every bond in the reactants: CH₄ has four C–H bonds, and each of the two O₂ molecules has one O=O bond.
2
$\text{energy taken in} = 4(413) + 2(498) = 1652 + 996 = 2648\ \text{kJ/mol}$
Breaking bonds takes in energy.
3
Bonds made: two C=O and four O–H, so $\text{energy released} = 2(805) + 4(464) = 1610 + 1856 = 3466\ \text{kJ/mol}$
CO₂ has two C=O bonds, and 2H₂O has 2 × 2 = 4 O–H bonds. Common error: forgetting the balancing number and counting only two O–H bonds.
4
$\text{overall energy change} = 2648 - 3466 = -818\ \text{kJ/mol}$
Energy change = sum of bonds broken − sum of bonds made.
5
Negative, so the reaction is exothermic
More energy is released making the new bonds than is taken in breaking the old ones.

Answer: $-818\ \text{kJ/mol}$ ; exothermic

Question

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