Energy Changes
What's covered
Key facts
Activation energy is shown as a hump on a reaction profile.
Overall energy change = energy to break bonds − energy released forming bonds.
Breaking chemical bonds requires energy (endothermic step); forming bonds releases energy (exothermic step).
A fuel cell generates electricity directly from a chemical reaction between a fuel and oxygen (rather than via combustion + generator).
Activation energy is the minimum energy needed for a reaction to occur.
Bond energies are measured in kilojoules per mole (kJ/mol).
Cold packs use endothermic reactions to absorb heat from the surroundings.
A fuel cell generates electricity from H₂ + O₂. The reverse process (using electricity to split water) is electrolysis. A fuel cell is not an electrolytic cell.
Catalysts affect activation energy by providing an alternative reaction path with a lower activation energy.
Breaking bonds is endothermic; making bonds is exothermic.
Sample questions
A taste of the 42 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
What is activation energy?
- •Energy lost as heat from products
- •Energy released by a reaction
- •Energy stored in chemical bonds
- ✓Minimum energy needed for a reaction
Bonds broken: 600 kJ; bonds formed: 800 kJ. What is ΔH and the reaction type?
- •+1400 kJ; exothermic (add bond energies together)
- •+200 kJ; endothermic (more energy absorbed than released)
- •−200 kJ; endothermic (breaking bonds releases more energy)
- ✓−200 kJ; exothermic (more energy released than absorbed)
What is the difference between an exothermic and an endothermic reaction?
- •Exothermic absorbs energy (temp falls); endothermic releases energy (temp rises)
- •Exothermic finishes in seconds; endothermic always takes hours to complete
- •Exothermic needs a catalyst to begin; endothermic starts spontaneously
- ✓Exothermic releases energy (temp rises); endothermic absorbs energy (temp falls)
What is the overall reaction in a hydrogen fuel cell?
- ✓2H₂ + O₂ → 2H₂O; only water, no CO₂, no moving parts
- •C + O₂ → CO₂; produces more energy per gram than petrol
- •CH₄ + O₂ → CO₂ + H₂O; works at lower temperatures than combustion
- •H₂ + Cl₂ → 2HCl; makes electricity directly from salt water
How do catalysts affect activation energy?
- •Increase activation energy required for reaction
- •Make activation energy fall to exactly zero
- ✓Provide alternative path with lower activation
- •Replace activation energy entirely with heat
What is ΔH for H₂ + Cl₂ → 2HCl? [H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol]
- •ΔH = +185 kJ/mol; endothermic
- •ΔH = +247 kJ/mol; endothermic
- ✓ΔH = −185 kJ/mol; exothermic
- •ΔH = −193 kJ/mol; exothermic
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