A-Level Chemistry: Electrode Potentials – Making Sense of Redox

If there’s one topic in A-Level Chemistry that feels abstract at first glance, it’s electrode potentials. Lots of half-equations. Lots of numbers. A mysterious hydrogen electrode at 0.00 V.

But once students see what’s really going on, it becomes beautifully logical.

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🔋 What Is an Electrode Potential?

An electrode potential (E°) measures the tendency of a species to gain electrons (be reduced).

All values are measured relative to the:

Standard Hydrogen Electrode (SHE)

• Defined as 0.00 V

• 1 mol dm⁻³ H⁺

• 100 kPa H₂ gas

• 298 K (25°C)

• Platinum electrode

Every other half-cell is compared to this.

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📈 What Do the Values Mean?

• More positive E° → greater tendency to be reduced.

• More negative E° → greater tendency to lose electrons (be oxidised).

For example:

• Cu²⁺ + 2e⁻ → Cu  E° = +0.34 V

• Zn²⁺ + 2e⁻ → Zn  E° = –0.76 V

Zinc has a much more negative value → zinc prefers to lose electrons → zinc is a good reducing agent.

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🔌 Building a Cell

When you connect two half-cells:

1. The more positive half-equation runs as reduction.

2. The more negative runs in reverse (oxidation).

3. Electrons flow from negative → positive.

4. The salt bridge completes the circuit.

Cell potential is calculated by:

$$E^\circ_{cell} = E^\circ_{reduction} - E^\circ_{oxidation}$$

If E°cell is positive → the reaction is feasible.

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🧠 The Big Ideas Students Must Master

At Hemel Private Tuition, I find students struggle with three key ideas:

1️⃣ Do NOT flip the sign unless reversing the equation.

The data book values are all written as reductions.

2️⃣ Never multiply E° values.

Even if you multiply the half-equation to balance electrons.

3️⃣ E° tells you about feasibility, NOT rate.

A reaction can be feasible but painfully slow.

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📊 Why This Topic Matters

Electrode potentials link directly to:

• Electrochemical cells

• Batteries

• Corrosion

• Disproportionation

• Transition metal chemistry

• Predicting reaction direction

It’s one of those topics that pulls inorganic chemistry together beautifully.

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🎥 How We Teach It

In the lab studio we:

• Build real electrochemical cells

• Measure voltages directly

• Compare results with data book values

• Use visual diagrams and animated redox flow

When students see the electrons physically moving through a wire, the abstraction disappears.

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🔎 OCR-Style Practice Question

A student mixes Fe²⁺(aq) with Ag⁺(aq).

Given:

• Ag⁺ + e⁻ → Ag  E° = +0.80 V

• Fe³⁺ + e⁻ → Fe²⁺  E° = +0.77 V

1. Predict whether the reaction is feasible.

2. Write the full ionic equation.

3. Calculate E°cell.