Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Function
A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product. Unlike a battery with a fixed reactant supply, it can continue operating while fuel and oxygen are supplied.
Energy conversion
Chemical energy is converted into electrical energy through separated redox processes. Electrons travel through an external circuit and can power a motor or other device.
Overall equation
The overall chemical equation is 2H2 + O2 -> 2H2O. The exact electrode half-equations depend on the cell design and electrolyte, so the syllabus focuses on the overall product and evaluation.
Water as vehicle product
At the vehicle, water is the only chemical product. This can reduce local carbon monoxide, hydrocarbon, particulate and carbon dioxide emissions compared with direct petrol combustion.
Worked example
One mole of O2 reacts with two moles of H2 to form two moles of water. The mole ratio follows directly from the balanced equation.
Practical or data skill
Label a fuel-cell diagram with reactant inlets, water outlet, external circuit and direction of electron flow when supplied.
Examination tip
Do not call hydrogen the energy source without considering how the hydrogen was produced.
Review questions and suggested answers
Question 1
What is the only chemical product?
Suggested answer
Water.
Question 2
Write the overall equation.
Suggested answer
2H2 + O2 -> 2H2O.
Question 3
How can a fuel cell operate continuously?
Suggested answer
Reactants are continuously supplied.
