Calculation of enthalpy changes of reaction using bond energies
3 marks · EasyThe table below gives bond energies (kJ/mol):
| Bond | Bond energy / kJ mol^{-1} |
|---|---|
| C–C | 151 |
| C–H | 413 |
| H–H | 436 |
Use these values to answer the following about the reaction:
List all the bonds broken and all formed in this reaction.
Calculate the total energy required to break bonds in the reactant.
Calculate the enthalpy change (ΔH) for the reaction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Bonds broken: 1 C–C bond and 2 C–H bonds; Bonds formed: 1 C=C bond (not given, so omitted, but required energy ignored for this specific question), 1 H–H bond.
(b) 977 kJ
(c) 541 kJ/mol
Common mistakes: Confusing bonds broken with bonds formed
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly lists all relevant bonds broken and formed.
M1 (b) · 1 mark · Calculates total energy required to break bonds (151 + 2×413).
A1 (c) · 1 mark · Calculates ΔH (977 – 436 = 541 kJ/mol), with sign.
Worked solution
a) Bonds broken: 1 C–C bond, 2 C–H bonds. Bonds formed: 1 H–H bond. (If C=C energy given, include C=C bond formation.)