A voltaic cell utilizes the following reaction: 4Fe2 + (aq) + O2 (g) + 4H + (aq) →4Fe3 + (aq) + 2H2O (l). What is the emf of this cell under standard conditions?

0.46 V

Explanation:

The emf for the cell is given by:

Eº cell = Eº oxidation + Eº reduction

From the given balanced chemical equation, we can deduce that Fe²⁺ has been oxidized to Fe³⁺, and O reduced from 0 to negative 2, according to the half cell reactions:

4Fe²⁺ ⇒ Fe³⁺ + 4e⁻ oxidation

O₂ + 4H⁺ + 4 e⁻ ⇒ 2 H₂O reduction

From reference tables for the standard reduction potential, we get

Eº red Fe³⁺ / Fe²⁺ Eºred = 0.77 V

Eº red O₂ / H₂O Eºred = 1.23 V

Now all we need to do is change the sign of Eº reduction for the species being oxidized (Fe²⁺) and add it to Eº reduction O₂:

Eº cell = Eº oxidation + Eº reduction = - (0.77 V) + 1.23 V = 0.46 V