Mini Chemistry
Part of Mini EducationSwitch subject

Standard Electrode Potentials and the SHE

Define standard electrode potential and measure a half-cell against the standard hydrogen electrode.

  • GCE A-Level H2 Chemistry 9476-2027

Reading settings

On this page

This lesson makes the data booklet table usable: what E⦵ measures (reduction tendency), what “standard conditions” actually are, and why the SHE is the reference for all values.

Before the harder applications, review Writing Redox Equations from Half-Equations and keep the Electrochemistry hub as your route map.

Definitions (Must Know)

A. Standard electrode potential, E⦵

The standard electrode potential, E⦵, measures the tendency of a half-cell to be reduced compared to the standard hydrogen electrode (SHE), under standard conditions.

B. Standard conditions (electrochemistry)

Unless stated otherwise:

  • temperature: 298 K
  • solutions: 1.0 mol dm⁻³
  • gases: 1 bar (or 100 kPa) pressure

C. Standard hydrogen electrode (SHE)

The SHE is defined as having E⦵ = 0.00 V and is used as the reference half-cell.

SHE setup (key features to remember):

  • platinum electrode (inert)
  • H₂ gas bubbled over Pt
  • acidic solution containing H⁺ (1.0 mol dm⁻³)

Half-equation (written as a reduction): 2H⁺ + 2e⁻ ⇌ H₂

D. Inert electrode

An inert electrode (Pt/graphite) is a conductor that does not react. It is used when both redox species are aqueous (e.g. Fe³⁺/Fe²⁺), so there is no solid metal electrode.

For a definition question, state the measurement explicitly: the standard electrode potential of a half-cell is the electromotive force of a cell formed by coupling that half-cell to the standard hydrogen electrode under standard conditions.

Key Ideas (What Earns Marks)

  • E⦵ values in data booklets are almost always reduction potentials.
  • A more positive E⦵ means a greater tendency to be reduced (gain electrons).
  • If you reverse a half-equation, you reverse the sign of E⦵ for that half-equation’s direction (because the table is for reductions).
  • An inert electrode (Pt / graphite) is used when no solid conductor is present in a half-cell (e.g. Fe³⁺/Fe²⁺).
Quick Recall (Standard Conditions)
  • 298 K - 1.0 mol dm⁻³ (aqueous) - 1 bar (gases)
Selected Standard Reduction Potentials (E°)Example data-booklet values: more positive E° means a greater tendency to be reduced (stronger oxidising agent).Selected Standard Reduction Potentials (E°)Half-cell (reduction)E° (V)
Example data-booklet values: more positive E° means a greater tendency to be reduced (stronger oxidising agent).
Data table
Half-cell (reduction)E°
Zn2+/Zn-0.76
H+/H2 (SHE)0
Cu2+/Cu0.34
I2/I-0.54
Fe3+/Fe2+0.77
Ag+/Ag0.8
Cl2/Cl-1.36

Detailed Explanations

A. What a more positive E⦵ means (reduction table logic)

Because the data booklet lists reduction half-equations, therefore a more positive E⦵ means the species has a stronger tendency to gain electrons (be reduced) under standard conditions.

B. Workflow: reading a half-cell and choosing the electrode

  1. If the half-cell includes a metal (e.g. Zn²⁺/Zn), the metal itself is the electrode.
  2. If both redox species are aqueous (e.g. Fe³⁺/Fe²⁺), use an inert electrode (Pt/graphite).

Mini example:

  • Fe³⁺(aq)/Fe²⁺(aq) needs Pt because there is no solid conductor in the redox pair.

C. How E⦵ is measured (the idea)

You connect an unknown half-cell to the SHE:

  • use a salt bridge to complete the circuit (ion flow)
  • connect electrodes with a wire + voltmeter (electron flow)

The voltmeter reading (with sign) gives the potential difference between the two half-cells under standard conditions, letting you assign the unknown E⦵ value.

D. What a salt bridge does

A salt bridge:

  • allows ions to move to maintain charge balance
  • completes the circuit without mixing solutions directly

E. Measuring the three assessed half-cell types

Unknown half-cellElectrode usedStandard setup
metal/metal ion, e.g. Cu²⁺/Cuthe metal itselfmetal in 1.0 mol dm⁻³ ion solution
non-metal/ion, e.g. Cl₂/Cl⁻inert Ptgas at 1 bar contacting its 1.0 mol dm⁻³ ion solution
two aqueous oxidation states, e.g. Fe³⁺/Fe²⁺inert Ptboth ions at 1.0 mol dm⁻³

In each case, connect the unknown half-cell to the SHE through a salt bridge and high-resistance voltmeter at 298 K. The salt bridge maintains electrical neutrality; it does not carry electrons. The terminal polarity gives the reaction direction, so the voltage is assigned the correct sign relative to E⦵_SHE = 0.00 V.

Worked Examples

Modelled example 1

State the SHE Reference

Core

Problem

State the half-equation for the standard hydrogen electrode and its standard electrode potential.
Study the worked solution

  1. Write the reduction convention

    Method

    Place hydrogen ions and electrons on the left.

    Reason

    Standard electrode potentials are tabulated as reduction half-equations.

    Working

    2H + (aq) + 2e⁻ ⇌ H₂(g)

  2. State the defined reference

    Method

    Give E⦵ = 0.00 V.

    Reason

    The SHE is assigned zero potential so other half-cells can be measured relative to it.

    Working

    E⦵_SHE = 0.00 V

Common misconception 2

Interpret a Negative Electrode Potential

Find and correct the mistake

Learner claim

For Zn²⁺ + 2e⁻ ⇌ Zn, E⦵ = -0.76 V. A learner says the negative sign means zinc ions cannot be reduced. Correct the interpretation.

Try this before viewing the solution

Relative reduction tendency

View solution step by step

  1. Use the reference comparison

    Method

    Compare the zinc couple with the SHE at 0.00 V.

    Reason

    Electrode potentials express relative reduction tendency under standard conditions.

    Working

    -0.76 V < 0.00 V

  2. Correct the direction claim

    Method

    State that Zn²⁺ has a lower tendency to be reduced, while zinc tends to oxidise when paired with the SHE.

    Reason

    The sign predicts the favoured paired direction; it does not say reduction is impossible in every cell.

    Working

    Zn → Zn²⁺ + 2e⁻ is favoured against the SHE under standard conditions.

Common Mistakes

  • Treating E⦵ as “oxidation potential” (tables are reduction potentials).
  • Forgetting standard conditions (especially concentration = 1.0 mol dm⁻³).
  • Missing the need for an inert electrode when both redox species are in solution.

Use the Electrochemistry topic check to practise and check your understanding.

Exam Tips

  • If the half-cell is Xⁿ⁺(aq)/X(s), you have a metal electrode.
  • If the half-cell is A(aq)/B(aq), you need Pt/graphite.
  • When asked “what does E⦵ tell you?”, say “tendency to be reduced compared to SHE”.

Mind Stretchers

Mind stretcher 1Extension

Why is platinum used in the SHE even though platinum does not appear in the half-equation?

Show Hint

Separate chemical participation from electrical function: ask what surface allows the aqueous/gaseous redox pair to exchange electrons.

Show Answer

Mark scheme:

  • Platinum is an inert conductor that provides a surface for electron transfer.
  • It does not react (not consumed) but allows the H + /H₂ redox couple to exchange electrons with the external circuit.

Mind stretcher 2: Designing an electrode-potential measurementExtension

Question. A half-cell contains only Fe³⁺(aq) and Fe²⁺(aq). Describe how its standard electrode potential is measured, including every component needed and how the sign of the reading is established.

Show Hint

Build the comparison from two complete half-cells, a salt bridge and a signed voltmeter reading under standard conditions.

Show Answer

Use both ions at 1.0 mol dm⁻³ and 298 K with an inert platinum electrode. Connect this half-cell by a salt bridge to a standard hydrogen electrode containing H + (aq) at 1.0 mol dm⁻³, H₂(g) at 1 bar and platinum. Connect the electrodes through a high-resistance voltmeter. The polarity of the terminals identifies which half-cell is reduced, and the signed potential difference relative to the SHE value of 0.00 V gives the standard electrode potential.

Syllabus and review details

Last reviewed: