24.2 Standard Electrode Potentials E⦵, Standard Cell Potentials E⦵cell and the Nernst Equation

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Define standard electrode (reduction) potential and standard cell potential
  2. Describe the standard hydrogen electrode
  3. Describe how to measure the standard electrode potential of (a) metals or non-metals with their ions, and (b) ions of the same element in different oxidation states
  4. Calculate a standard cell potential by combining two standard electrode potentials
  5. Use standard cell potentials to (a) deduce electrode polarity and direction of electron flow, and (b) predict whether a reaction is feasible
  6. Deduce the relative reactivity of species as oxidising or reducing agents from E⦵ values
  7. Construct redox equations using half-equations
  8. Predict qualitatively how electrode potential E varies with ion concentration
  9. Use the Nernst equation to calculate E under non-standard conditions
  10. Use the equation ΔG⦵ = −nE⦵cellF

1. Electrode Potential and Standard Electrode Potential

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What is electrode potential?

When a metal is placed in a solution of its own ions, something interesting happens. Metal atoms can lose electrons and enter the solution as ions, while at the same time, metal ions in the solution can gain electrons and deposit back as solid metal atoms. This sets up a redox equilibrium — a balance between oxidation and reduction happening at the same time.

For example, for zinc:

Zn²⁺(aq) + 2e⁻ ⇌ Zn(s)

This equilibrium creates a tiny electrical potential (voltage) between the metal and the solution. This voltage is called the electrode potential (E) — it tells you how easily a species is reduced.

Half-equations for electrode potentials are always written with electrons on the left-hand side, showing reduction:

Oxidised species + ne⁻ ⇌ Reduced species

  • The more positive the electrode potential, the more easily the species on the left is reduced (the equilibrium lies to the right).
  • The more negative the electrode potential, the less easily the species is reduced — it is more easily oxidised (the equilibrium lies to the left).

Why do we need standard conditions?

The electrode potential depends on temperature, pressure, and the concentration of ions in the solution. To compare different electrode potentials fairly, all measurements must be made under the same conditions. These are called standard conditions:

  • Ion concentration = 1.00 mol dm⁻³
  • Temperature = 298 K (25 °C)
  • Pressure = 1 atm

Under standard conditions, the electrode potential is given the symbol E⦵ (read as "E standard").

Definition: Standard electrode (reduction) potential (E⦵)

The standard electrode (reduction) potential (E⦵) is the voltage measured when a standard half-cell is connected to a standard hydrogen electrode under standard conditions. It shows how easily a species is reduced compared to hydrogen.

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