Why do reactions either give out or absorb heat?
3 marks · EasyThe table shows average bond energies for some bonds:
| Bond | Bond energy (kJ/mol) |
|---|---|
| C–C | 348 |
| C–O | 358 |
| O=O | 498 |
Ethanol (C2H5OH) can be combusted in oxygen to form carbon dioxide and water. Use the data to answer the following:
State which bond in the table requires the most energy to break.
Complete the sentence: The combustion of ethanol is exothermic because the total energy released from making bonds is ____ than the energy needed to break bonds.
Explain what the term 'average bond energy' means.
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(a) O=O
(b) greater
(c) It is the energy needed to break one mole of a given type of bond averaged over many compounds.
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Marking points
B1 (a) · 1 mark · States O=O
B2 (b) · 1 mark · Correctly completes with 'greater' (accept bigger/larger/more).
B3 (c) · 1 mark · Gives correct definition of average bond energy.
Worked solution
a) O=O (498 kJ/mol, the highest energy in the table). b) greater c) Average bond energy is the energy required to break 1 mol of that bond across different molecules.