Why do reactions either give out or absorb heat?
2 marks · HardA reaction is found to be exothermic overall. Explain, in terms of bond energies, why this is the case.
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.
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Also accept: The energy released when new bonds are made is greater than the energy absorbed when bonds are broken
Common mistakes: Reverse explanation (confuse breaking with making); Only mention energy without connecting to bond processes
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Marking points
M1 · 1 mark · Explains that breaking bonds requires energy (endothermic) and making bonds releases energy (exothermic)
A1 · 1 mark · States that total energy released when making bonds in products is greater than the energy needed to break bonds in reactants
Worked solution
Breaking bonds in the reactants needs energy (endothermic), while forming new bonds in the products releases energy (exothermic).
If the total energy released making new bonds is greater than the total energy absorbed breaking the old bonds, the reaction is exothermic.