Energy Changes
What's covered
Key facts
Activation Energy
Activation energy is shown as a hump on a reaction profile.
Bond Energy Calculations
Bond energies are measured in kilojoules per mole (kJ/mol).
Exothermic and Endothermic Reactions
Cold packs use endothermic reactions to absorb heat from the surroundings.
Fuel Cells
A fuel cell generates electricity directly from a chemical reaction between a fuel and oxygen (rather than via combustion + generator).
Activation Energy
On an energy profile diagram, a catalyst lowers the activation energy peak by providing an alternative reaction pathway.
Bond Energy Calculations
Breaking bonds is endothermic; making bonds is exothermic.
Exothermic and Endothermic Reactions
On a reaction profile, an endothermic reaction is shown with products higher in energy than reactants.
Fuel Cells
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.
Activation Energy
A catalyst lowers activation energy by providing an alternative reaction pathway.
Bond Energy Calculations
Breaking chemical bonds requires (absorbs) energy; forming bonds releases energy.
Practice questions with answers
A taste of the 36 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is activation energy?
- •The difference in energy between reactants and final products
- •The energy stored in the chemical bonds of the reactants
- ✓The minimum energy colliding particles must have to react
- •The total energy released as products form from reactants
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
On an energy profile diagram for an exothermic reaction, what does the activation energy represent?
- •Difference between products and reactants (ΔH), shown as the gap between levels
- ✓Energy from reactants to the peak: the minimum to react
- •Overall energy released, shown as the drop from reactants to products
- •Total energy stored in the products' bonds, shown as their line height
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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