Bond breaking is always endothermic (energy required).
Bond forming is always exothermic (energy released).
Bond energy calculations
Energy required to break bonds in reactants → endothermic.
Energy released when forming bonds in products → exothermic.
Overall ΔH = energy in (breaking) – energy out (forming).
If ΔH is negative → exothermic overall.
If ΔH is positive → endothermic overall.
Bond energies are given in kJ/mol for each type of bond (C–H, O=O, C=O, H–O, etc.).
Reaction profiles (energy diagrams)
Show energy of reactants and products on a graph.
Activation energy — energy needed to start the reaction (height of the hump from reactants).
Exothermic profile: products lower than reactants. Endothermic profile: products higher.
Catalyst lowers activation energy → provides alternative route → reaction faster.
Catalyst does NOT change ΔH (energy difference between reactants and products stays the same).
Exam questions — 4 questions · 12 marks
⚡ This module includes extended questions. Read the hint. Write in full sentences. The AI marks against the real AQA mark scheme.
1 markBond energy calculationAQA 8462 P1 style
In the reaction H₂ + Cl₂ → 2HCl, bond energies are: H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 432 kJ/mol. What is the overall energy change?
A +185 kJ/mol (endothermic)
B –185 kJ/mol (exothermic)
C +679 kJ/mol (endothermic)
D –864 kJ/mol (exothermic)
1 markActivation energyAQA 8462 P1 style
What effect does a catalyst have on a reaction?
A It increases the activation energy making the reaction faster
B It provides an alternative reaction pathway with lower activation energy, increasing the rate without being used up
C It changes the energy difference between reactants and products
D It increases the temperature of the reaction
4 marksBond energy calculation extendedAQA 8462 P1 — real exam style
Methane burns in oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O
Bond energies (kJ/mol): C–H = 413, O=O = 498, C=O = 805, O–H = 464
Calculate the overall energy change and state whether the reaction is exothermic or endothermic. Show all working. (4 marks)
Hint: Step 1: count and break all bonds in reactants. Step 2: count and form all bonds in products. Step 3: ΔH = energy in – energy out. Negative = exothermic.
Sketch and describe the reaction profile for an exothermic reaction. Label: reactants, products, activation energy, overall energy change (ΔH). (3 marks)
Hint: In an exothermic profile: reactants are at a higher energy level than products. The hump peak represents the transition state. Activation energy = height from reactants to peak.