Question

Calculate E°_cell for the following reaction:
2Fe²⁺(aq) + Cd²⁺(aq) ? 2Fe³⁺(aq) + Cd(s)

A. -0.37 V
B. 0.37 V
C. -1.17 V
D. 1.17 V
E. none of these

Answer 1

Concepts and reason

Two electrodes that are connected by a metallic conducting wire and dipped in electrolytes form an electrochemical cell. Reduction occurs at one electrode and oxidation occurs at the other.

The electric potential applied onto an electrode determines whether it hosts a reduction or oxidation reaction. As a reference, specific potentials have been deduced for many metals by connecting an electrode in a cell where the other electrode is a Standard Hydrogen Electrode (SHE). The metal electrode is dipped in its own salt solution, which is of 1M concentration. A Pt electrode is continuously flushed with hydrogen gas at 1 atm pressure. This setup is dipped in a 1M HCl solution. This forms the SHE. At a temperature of 298 K, the potential acting on the metal electrode in such a cell becomes the standard electrode potential of the electrode .

In essence, the reduction or oxidation potential of an electrode determines the tendency of the electrode to undergo oxidation or reduction. Conventionally, the standard electrode potentials are written as the reduction potentials.

Fundamentals

A metal M dipped in its electrolyte MX and coupled against a SHE is given by the following equilibrium:

At the SHE:

Say, M oxidizes and is reduced in this cell.

The reaction can be written as follows

The cell can be described using a ‘cell notation’ as follows:

The direction of spontaneity is from the left to right across the cell notation. Here, M oxidizes to spontaneously, while is reduced to hydrogen gas.

The symbol ‘||’ divides the cell into two half cells. On the left of the symbol ‘||’ is the oxidation half-cell; on the right of the symbol is the reduction half-cell.

The oxidation potential of the oxidation half-cell is written as.

The reduction potential of the reduction half-cell is written as.

The cell potential is calculated as .

Hence, Cd oxidizes to Cd(II), while Fe(III) reduces to Fe(II).

The cell notation could be written as follows:

= - 0.77 - 0.40 = - 1.17 V

Ans:

The standard potential of the given cell is - 1.17 V.