Standard Electrode Potentials and the Hydrogen Half-Cell HL Only

Full Notes

This page builds on the ideas outlined in R3.2.5, see here.

Recap – a half-cell is part of an electrochemical cell where either oxidation or reduction happens.

• A metal dipped into a solution of its own ions
• A solution containing ions of the same element in different oxidation states (with a platinum electrode)

Two half-cells are connected together to form a full electrochemical cell, allowing electrons to flow from one half-cell (oxidation) to the other (reduction), generating electricity.

Different half-cells are more or less likely to undergo oxidation or reduction, and this can be measured by their electrode potentials.

Oxidation and Reduction in a Half-Cell

Each half-cell contains two forms of a species — one in a higher oxidation state and one in a lower oxidation state.

• Reduction happens when the oxidised form gains electrons.
• Oxidation happens when the reduced form loses electrons.

Example: Copper Half-Cell

Cu²⁺(aq) + 2e⁻ ⇌ Cu(s)

• Cu²⁺ can gain electrons to form Cu (reduction).
• Cu can lose electrons to form Cu²⁺ (oxidation).

Measuring Electrode Potentials

The electrical potentials of each electrode in the half-cells cannot be measured directly. However, we can compare electrode potentials for different half-cells by connecting them to a reference half-cell and measuring the potential difference.

This then gives us a way of comparing how easily oxidation or reduction occurs in different half-cells.

The reference used is the Standard Hydrogen Electrode (SHE).

The Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode (SHE) is used as the universal reference point and consists of:

• Hydrogen gas at 100 kPa
• Bubbled over a platinum electrode
• In 1.00 mol dm⁻³ H⁺ solution
• At 298 K (25 °C)

All standard electrode potentials (E°) are measured under these conditions and describe the potential of a half-cell compared to the Standard Hydrogen Electrode.

The Standard Hydrogen Electrode is assigned a potential of 0.00 V. All this means is that when two standard hydrogen electrodes are connected together, the potential difference is 0.00V.

If the right-hand half-cell is replaced with another, a potential difference (voltage) is measured.

This measured value is called the standard electrode potential (E° value) of the right hand half-cell.

The temperature, concentration and pressure (for gases) must be the same as the standard hydrogen electrode (1 mol dm^-3, 298K and 100kPa of pressure), otherwise positions of equilibrium in each half cell will be affected and comparisons between measured potentials won’t be representative.

Standard electrode potentials are often put into a table called the electrochemical series (see diagram below).

Electrochemical Series

The more positive the E°, the more likely a species is to be reduced.

The more negative the E°, the more likely a species is to be oxidised.

Definition of Standard Electrode Potential (E°)

The standard electrode potential, E°, measures the tendency of a species to gain electrons under standard conditions. It is always measured relative to the SHE, which is assigned a potential of 0.00 V.

Standard conditions:

• Temperature = 298 K
• Pressure = 100 kPa (for gases)
• Ion concentration = 1.00 mol dm⁻³

A more positive E° indicates a greater tendency to be reduced (gain electrons).

Types of Electrodes

Metal or Non-Metal Half-Cells

Example A zinc rod in Zn²⁺ solution

Different Oxidation States of the Same Element

Example Fe³⁺/Fe²⁺ half-cell with platinum electrode

Summary

• Standard electrode potentials measure a species’ tendency to be reduced.
• The hydrogen half-cell is the standard reference with E° = 0.00 V.
• More positive E° = more likely to be reduced; more negative E° = more likely to be oxidised.
• Values are measured under standard conditions: 298 K, 100 kPa, 1 mol dm⁻³.