Chemistry · Ch 2 — Electrochemistry

Galvanic Cells

2.2

Galvanic Cells

The basic idea

A galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction directly into electrical energy. The Gibbs energy released by the spontaneous reaction is converted into electrical work, which can drive a motor, a fan, a heater, or any similar device.

The Daniell cell is the standard example. Its overall spontaneous reaction is

\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \longrightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}

This single reaction is really the sum of two separate half reactions:

\text{Cu}^{2+} + 2e^{-} \longrightarrow \text{Cu(s)} \qquad \text{(reduction half reaction)}

\text{Zn(s)} \longrightarrow \text{Zn}^{2+} + 2e^{-} \qquad \text{(oxidation half reaction)}

Half-cells and electrodes

The two half reactions occur in two physically separate parts of the cell:

the reduction half reaction occurs on the copper electrode,
the oxidation half reaction occurs on the zinc electrode.

Each of these portions is called a half-cell (or redox couple). The copper electrode is the reduction half-cell; the zinc electrode is the oxidation half-cell.

Countless galvanic cells can be built on this same pattern by pairing different half-cells. In general:

each half-cell is a metallic electrode dipped into an electrolyte,
the two half-cells are joined externally by a metallic wire (through a voltmeter and a switch),
the two electrolytes are joined internally by a salt bridge.

When both electrodes happen to dip into the same electrolyte solution, no salt bridge is needed.

In a galvanic cell, the half-cell where oxidation occurs is called the anode, and it carries a negative potential with respect to the solution. The half-cell where reduction occurs is called the cathode, and it carries a positive potential with respect to the solution. Once the external circuit is closed, electrons flow from the negative electrode to the positive electrode; conventional current flows in the opposite direction.

Why an electrode develops a potential

At every electrode–electrolyte interface, two opposing tendencies compete:

metal ions from the solution tend to deposit on the electrode, making it more positive;
metal atoms of the electrode tend to leave as ions into the solution, leaving electrons behind and making the electrode more negative.

At equilibrium a fixed separation of charge is established, and depending on which tendency dominates, the electrode becomes positively or negatively charged relative to the solution. This potential difference between the electrode and the electrolyte is the electrode potential.

Note

When the concentrations of every species involved in a half-cell are unity, the electrode potential is called the standard electrode potential. By IUPAC convention, standard reduction potentials are now simply called standard electrode potentials.

Cell potential (EMF)

The potential difference between the two electrodes of a galvanic cell is the cell potential, measured in volts. It equals the difference between the electrode (reduction) potentials of the cathode and the anode. When no current is drawn from the cell, this is called the cell electromotive force (emf).

By convention, the anode is written on the left and the cathode on the right. In this representation:

E_{\text{cell}} = E_{\text{right}} - E_{\text{left}}

Written this way, the emf of the cell comes out positive.

Cell representation notation

A galvanic cell is written using a compact line notation:

a single vertical line | separates a metal from its electrolyte solution (a phase boundary),
a double vertical line || separates the two electrolytes, standing for the salt bridge connecting them.

For the reaction

\text{Cu(s)} + 2\text{Ag}^{+}\text{(aq)} \longrightarrow \text{Cu}^{2+}\text{(aq)} + 2\text{Ag(s)}

the half-cell reactions are …

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