AQA GCSE Chemistry (8462), Higher tier · Energy changes › Exothermic and endothermic reactions
Practise Energy change of reactions (bond energies). 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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What does the bond energy of a covalent bond measure?
The energy needed to break one mole of that bond.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Energy changes in chemical reactions can be explained using ideas about chemical bonds.
(a) Which statement about bond breaking and bond making is correct? Tick (✓) one box.[1]
Breaking bonds releases energy and making bonds needs energy.
Breaking bonds needs energy and making bonds releases energy.
Breaking bonds and making bonds both need energy.
Breaking bonds and making bonds both release energy.
(b) For one reaction:
energy needed to break the bonds in the reactants = 2650 kJ/mol
energy released when the bonds in the products form = 3120 kJ/mol.
Calculate the overall energy change for the reaction.[2]
(c) Explain, in terms of bonds, why this reaction is exothermic.[1]
Show the answer and mark scheme
(a)Answer: Breaking bonds needs energy and making bonds releases energy.
(b)Answer: −470 kJ/mol
2650 − 3120
−470 (kJ/mol)
(c)Answer: More energy is released when the new bonds form than is needed to break the old bonds.
more energy is released when (new) bonds form than is needed to break the (old) bonds
Question 2Medium2 marks
(a) Which equation is used to calculate the overall energy change of a reaction from bond energies? Tick (✓) one box.[1]
overall energy change = energy needed to break bonds in the reactants − energy released making bonds in the products
overall energy change = energy released making bonds in the products − energy needed to break bonds in the reactants
overall energy change = energy needed to break bonds in the reactants + energy released making bonds in the products
overall energy change = energy released making bonds in the reactants − energy needed to break bonds in the products
(b) What does the bond energy of a covalent bond measure?[1]
Show the answer and mark scheme
(a)Answer: overall energy change = energy needed to break bonds in the reactants − energy released making bonds in the products
(b)Answer: The energy needed to break one mole of that bond.
the energy needed to break (one mole of) the bond
Question 3Hard6 marks
Hydrogen reacts with iodine vapour in a reversible reaction. H–H + I–I ⇌ 2 H–I The overall energy change for the forward reaction is −9 kJ/mol. Bond energy of H–H = 436 kJ/mol Bond energy of I–I = 151 kJ/mol
(a) Calculate the bond energy of the H–I bond.[4]
(b) The bond energy of H–Cl is 432 kJ/mol. Use your answer to suggest what this shows about the strength of the H–I bond compared with the H–Cl bond. Suggest a reason for this, based on the position of iodine and chlorine in the periodic table.[2]
Show the answer and mark scheme
(a)Answer: 298 kJ/mol
bonds broken: 436 + 151 = 587 (kJ/mol)
let the H–I bond energy be X: 587 − 2X = −9
2X = 587 + 9 = 596
X = 298 (kJ/mol)
(b)Answer: The H–I bond (298 kJ/mol) is weaker than the H–Cl bond (432 kJ/mol); iodine atoms are much bigger than chlorine atoms, so the bonding pair of electrons is further from the iodine nucleus and less strongly attracted.
the H–I bond is weaker than the H–Cl bond
iodine is below chlorine in Group 7, so iodine atoms are larger, meaning the shared (bonding) electrons are further from the nucleus and less strongly attracted