Upon completion of this topic, learners will be able to:

• write the cell reaction from the cell notation.
• define cell potential.
• calculate the quantity of work from a given amount of cell reactant.
• define standard cell potential and standard electrode potential.
• interpret the table of standard reduction potentials.
• determine the relative strengths of oxidizing and reducing agents.
• determine the direction of spontaneity from electrode potentials.
• calculate cell potential from standard potentials.

Cell notation

How do you represent galvanic cells without drawing a diagram?

A cell notation allows us to describe a Galvanic cell without drawing a diagram. It is a shorthand representation of a Galvanic cell. For example, a Daniell cell in which the electrolyte concentrations are each 1 molar is represented as

Zn(s) | Zn²⁺(1M) || Cu²⁺(1M) | Cu(s)

The anode is written on the left. The cathode on the right and concentrations and other data are given in parentheses. The vertical line (|) indicates phase boundaries, and the double vertical line (||) indicates a salt bridge or a porous partition.

Standard reduction potentials

What is the difference between standard reduction potential and cell potential?

The cell potential is the difference between two electrode potentials, one associated with the cathode and the other associated with the anode.

By convention, the potential associated with each electrode is chosen to be the potential for reduction to occur at the electrode. Thus, standard electrode potentials (electrode potentials at a concentration of 1 M) are tabulated for reduction reactions and are denoted as E^o_red. The cell potential (E^o_cell) isE^o_red (cathode) minus the standard reduction potential of the anode reaction, Eored (anode):

E^o_cell = E^o_red (cathode) – E^o_red (anode)

Because every Galvanic cell involves two half-cells, it is not possible to directly measure the standard reduction potential of a half-reaction.

Electromotive force (emf)

When electrons move through a wire, they encounter resistance from localised atoms in their paths. The driving force that allows the electrons to overcome this resistance and move around the circuit is called electromotive force (emf). The electromotive force in a Galvanic cell comes from the redox reaction that pushes electrons from the anode to the cathode through the external circuit.

The energy of the moving electrons is used to overcome resistance in the external circuit, and it can be used to do various forms of electrical work. Energy that overcomes resistance appears as heat or as light emitted by a glowing filament. Electrical work is used for starting cars, running watches, radios, and computers, etc.