6 (a) What type of energy is released by hydrogen fuel cells? 6 (b) Owners of cars powered by fuel cells buy hydrogen from hydrogen filling stations - AQA - GCSE Chemistry - Question 6 - 2014 - Paper 3

1. Using hydrogen saves petrol and diesel, which are non-renewable resources.
2. Hydrogen does not cause pollution when used in fuel cells, contributing to cleaner air.

To calculate the energy change, we first identify the bonds broken and formed:

• Bonds broken: 2 H–H bonds and 1 O–O bond:

  Energy required = 2 × 436 kJ + 498 kJ = 1370 kJ

• Bonds formed: 4 O–H bonds:

  Energy released = 4 × 464 kJ = 1856 kJ

The overall energy change is:

$ext{Energy change} = ext{Energy used} - ext{Energy released} = 1370 ext{ kJ} - 1856 ext{ kJ} = -486 ext{ kJ}$

In the energy level diagram, label the activation energy, indicating that it represents the energy barrier that must be overcome for the reaction to occur. The diagram should show that the energy of the products (water) is lower than the energy of the reactants (hydrogen and oxygen).