Some cars are powered by hydrogen fuel cells - AQA - GCSE Chemistry - Question 6 - 2014 - Paper 3

1. Using hydrogen saves petrol and diesel, which are non-renewable resources, thereby promoting sustainability.

2. Hydrogen does not cause pollution when used in fuel cells, which supports environmental protection.

To find the energy change, calculate the total bond energies of the reactants and subtract the total bond energies of the products:

• Energy required to break bonds in reactants:

  • 2 H-H bonds: $2 \times 436 = 872$ kJ
  • 1 O=O bond: $498$ kJ
  • Total: $872 + 498 = 1370$ kJ

• Energy released when forming bonds in products:

  • 4 H-O bonds: $4 \times 464 = 1856$ kJ

Energy change = Bond energy of reactants - Bond energy of products: $1370 - 1856 = -486 \, \text{kJ}$

In the energy level diagram, label the activation energy above the reactants. The products should be lower than the reactants, indicating that energy is released in the process.