Electrode potentials and electrochemical cells (A-level only) revision notes

Electrode Potentials and Electrochemical Cells

Standard Hydrogen Electrode (SHE)

The standard hydrogen electrode is the reference electrode against which all other electrode potentials are measured. It consists of a platinum electrode in contact with 1 M H⁺ ions and hydrogen gas at 1 atm pressure. The standard electrode potential of the SHE is defined as 0 V.

Standard Electrode Potential

Standard electrode potential (E°) is the voltage measured under standard conditions (1 M concentration, 1 atm pressure, and 25°C) when a half-cell is connected to the SHE. It indicates the tendency of a species to gain electrons (be reduced).

Calculating Cell Potentials

The cell potential (E°cell) for a galvanic cell can be calculated using the formula:

E°cell = E°(cathode) - E°(anode)

Where the cathode is the electrode where reduction occurs and the anode is where oxidation occurs.

Electrochemical Cell Diagrams

Electrochemical cell diagrams represent the components of the cell. The anode is written on the left and the cathode on the right, separated by a double line indicating the salt bridge. For example:

Zn | Zn²⁺ || Cu²⁺ | Cu

Measuring EMF

The electromotive force (EMF) of an electrochemical cell can be measured using a voltmeter. This practical involves setting up the cell and recording the voltage.

Predicting Redox Reactions

The feasibility of redox reactions can be predicted using standard electrode potentials. A positive E°cell indicates a spontaneous reaction, while a negative value suggests non-spontaneity.

Limitations of Predictions

Predictions based on standard conditions may not hold true in real-world scenarios due to factors like concentration changes, temperature variations, and the presence of other species.

Fuel Cells vs. Rechargeable and Non-Rechargeable Cells

Fuel cells convert chemical energy directly into electrical energy and can operate continuously as long as fuel is supplied. In contrast, rechargeable cells can be restored by applying an external current, while non-rechargeable cells cannot be recharged once depleted.

Effect of Concentration and Conditions

The cell potential can be affected by the concentration of reactants and products, temperature, and pressure. According to the Nernst equation, changes in concentration can shift the equilibrium position, thus altering the cell potential.