Why do reactions either give out or absorb heat?
4 marks · NormalThe table below gives average bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| C–H | 413 |
| O=O | 498 |
| C=O | 799 |
List the bonds broken and bonds formed when methane reacts with oxygen to form carbon dioxide and water.
Suggest, using the bond energies, why this reaction is highly exothermic.
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: 4 C–H in CH4, 2 O=O in O2. Bonds formed: 2 C=O in CO2, 4 O–H in 2H2O.
(b) Much more energy is released making multiple C=O and O–H bonds than is needed to break C–H and O=O bonds, so overall energy is given out.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies all correct bonds broken (C–H, O=O).
B2 (a) · 1 mark · Identifies bonds formed (C=O, O–H).
B3 (b) · 1 mark · Recognises more energy is released in bonds made than used breaking bonds.
B4 (b) · 1 mark · Links this to exothermic reaction (much energy given out).
Worked solution
a) Bonds broken: 4 C–H (CH4) and 2 O=O (O2). Bonds formed: 2 C=O (CO2) and 4 O–H (H2O).
b) The total energy released forming C=O and O–H bonds is much greater than the energy needed to break C–H and O=O bonds, making the reaction highly exothermic.