Standard Electrode Potential

Learning Objectives (OCR Spec 5.2.3)

By the end of this lesson you should be able to:

• Describe the standard hydrogen electrode (SHE) and explain its role as the reference
• Define standard electrode potential, E°
• Describe how E° is measured using a SHE and a test half-cell
• Use cell notation (conventional IUPAC representation) to describe a cell
• Interpret values in the electrochemical series

Why Do We Need Electrode Potentials?

A redox reaction involves transfer of electrons from a reducing agent to an oxidising agent. In principle, this electron transfer can be harnessed to do work if the two half-reactions are physically separated and electrons are forced to flow through an external wire. The driving force for this electron flow is measured by the electrode potential.

Every redox system has a potential associated with it, but potentials can only be measured relative to a reference. By international agreement, that reference is the standard hydrogen electrode (SHE), assigned a potential of exactly 0.00 V.

The Standard Hydrogen Electrode

The SHE consists of:

• A platinum electrode (inert, conducts electrons, catalyses the reaction)
• Coated with platinum black (a finely divided form, providing a large surface area)
• Immersed in a solution containing H+(aq) at 1 mol dm^-3 (activity strictly 1)
• With H2 gas at 100 kPa (1 bar) bubbled over the electrode
• At 298 K (25 °C)

The half-equation is:

2H+(aq) + 2e- <=> H2(g) E° = 0.00 V (by definition)

The platinum does not take part in the reaction; it is the place where electrons enter or leave and where H2/H+ exchange electrons.

graph TD
  A[H2 gas inlet<br/>100 kPa] --> B[Glass tube]
  B --> C[Platinum electrode<br/>coated in Pt black]
  C --> D[H+ solution<br/>1 mol dm-3]
  D --> E[Salt bridge to other half-cell]

Definition of Standard Electrode Potential

The standard electrode potential E° of a half-cell is the voltage (emf) measured between that half-cell and the standard hydrogen electrode under standard conditions (298 K, 100 kPa, all solutions 1 mol dm^-3), with the SHE on the left-hand side of the cell.

Standard conditions:

• Temperature 298 K
• Pressure 100 kPa (1 bar) for any gases
• All solution concentrations 1 mol dm^-3
• Pure solids and liquids in their standard states

Measuring E°: The Zinc Half-Cell

To measure E°(Zn^2+/Zn):

1. Set up a zinc half-cell: a strip of pure Zn metal dipping into 1 mol dm^-3 Zn^2+(aq) (e.g. ZnSO4).
2. Set up a SHE alongside.
3. Connect the two half-cells with a salt bridge (e.g. saturated KNO3 in an agar gel or a strip of filter paper soaked in KNO3). The salt bridge allows ions to flow between the half-cells to balance charge, without the two solutions mixing directly.
4. Connect the two electrodes through a high-resistance voltmeter (negligible current, so the cell is at equilibrium).
5. Read the voltage and note the polarity.

The reading for Zn gives E°(Zn^2+/Zn) = -0.76 V. The negative sign means zinc is a STRONGER reducing agent than hydrogen: at equilibrium, zinc tends to lose electrons and hydrogen ions tend to gain them.

Measuring E°: The Copper Half-Cell

For Cu^2+/Cu the setup is analogous: a copper strip in 1 mol dm^-3 Cu^2+(aq). The voltage is:

E°(Cu^2+/Cu) = +0.34 V

The positive sign means copper is a WEAKER reducing agent than hydrogen: at equilibrium, H2 can reduce Cu^2+ to Cu (in principle) and the Cu electrode is at positive potential relative to the SHE.

Half-Cells Without a Metal

Some redox systems involve only ions in solution (e.g. Fe^3+/Fe^2+, MnO4-/Mn^2+). These cannot use a metal electrode that participates in the reaction. Instead, an inert platinum electrode is used to transfer electrons between the solution species.

Example: Fe^3+/Fe^2+ half-cell

• Solution: 1 mol dm^-3 Fe^3+(aq) AND 1 mol dm^-3 Fe^2+(aq)
• Electrode: platinum wire dipped into the solution
• Half-equation: Fe^3+(aq) + e- <=> Fe^2+(aq), E° = +0.77 V

Example: MnO4-/Mn^2+ half-cell

• Solution: 1 mol dm^-3 MnO4-, 1 mol dm^-3 Mn^2+, 1 mol dm^-3 H+
• Electrode: platinum wire
• Half-equation: MnO4-(aq) + 8H+(aq) + 5e- <=> Mn^2+(aq) + 4H2O(l), E° = +1.51 V

Gas Half-Cells

For gas-related redox systems (other than H2/H+) a platinum electrode is used with gas bubbled over it, exactly as in the SHE. Example:

Cl2/Cl-: Cl2(g) + 2e- <=> 2Cl-(aq), E° = +1.36 V

Cell Notation (Cell Diagrams)

The IUPAC convention for writing a cell is a compact notation that captures all relevant information:

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