Standard Electrode Potentials and the SHE
Learn and apply Standard Electrode Potentials and the SHE in the published Chemistry course sequence.
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The core idea
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Standard Electrode Potentials and SHE: Orientation
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.
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
- If the half-cell includes a metal (e.g. Zn²⁺/Zn), the metal itself is the electrode.
- 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-cell | Electrode used | Standard setup |
|---|---|---|
| metal/metal ion, e.g. Cu²⁺/Cu | the metal itself | metal in 1.0 mol dm⁻³ ion solution |
| non-metal/ion, e.g. Cl₂/Cl⁻ | inert Pt | gas at 1 bar contacting its 1.0 mol dm⁻³ ion solution |
| two aqueous oxidation states, e.g. Fe³⁺/Fe²⁺ | inert Pt | both 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
Problem
Study the worked solution
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)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
Learner claim
Try this before viewing the solution
View solution step by step
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 VCorrect 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.
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.