In a hydrogen fuel cell, hydrogen gas and oxygen gas are combined to form water. Write the balanced chemical equation describing this reaction (using lowest whole-number coefficients).
Identify the oxidizing agent and reducing agent.
Determine the number of electrons transferred in the balanced chemical equation.
2 H2 O2 → 2 H2O
Hydrogen is the reducing agent. Oxygen is the oxidizing agent (which ought not be a surprise).
Four electrons are transferred – one for each hydrogen atom.