Why do reactions either give out or absorb heat?
4 marks · NormalRead the following reaction:
You are given these average bond energies (kJ/mol): S=O = 523, O=O = 498
a) Write down all bonds that are broken and formed during the reaction.
b) Explain, using these bond energies, what determines whether the reaction is exothermic or endothermic.
List all bonds broken and formed.
Explain, using bond energies, what determines if the reaction is exothermic or endothermic.
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: S=O in SO2, O=O in O2. Bonds formed: S=O in SO3 (but more per molecule).
(b) The reaction is exothermic if the total energy released by forming S=O bonds in SO3 is greater than the energy used to break S=O and O=O bonds in the reactants. If less, the reaction is endothermic.
Common mistakes: Reversing reference to bond breaking/making.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Lists correct bonds broken.
B2 (a) · 1 mark · Lists correct bonds formed.
B3 (b) · 1 mark · Links exothermic energy to bonds formed releasing more energy than breaking bonds uses.
B4 (b) · 1 mark · Links endothermic outcome to bonds broken requiring more energy than is released making bonds.
Worked solution
a) Bonds broken are S=O in SO2 and O=O in O2, bonds formed are S=O in SO3.
b) If more energy is released making S=O bonds in SO3 than absorbed breaking the S=O and O=O bonds in the reactants, the reaction is exothermic; otherwise, it is endothermic.