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5.1.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Standard Electrode Potential — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 43 Unit quiz

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Part 1 of 5What E⦵ Means

5.1.2 Standard Electrode Potential
Definition
  • The standard electrode potential is the voltage of a half-cell connected to a standard hydrogen electrode under standard conditions.
Conditions
  • Ion concentrations of 1.00 mol dm⁻³, gas pressure of 100 kPa and a temperature of 298 K.
Written as reduction
  • Half-equations are written with electrons on the left, for example Cu²⁺ + 2e⁻ ⇌ Cu with E⦵ = +0.34 V.
Reference
  • The standard hydrogen electrode is given E⦵ = 0.00 V so every other half-cell can be measured against it.

Part 2 of 5Reading the Scale

Reading the Scale
More positive
  • The species on the left is a stronger oxidising agent and is more easily reduced.
More negative
  • The species on the right is a stronger reducing agent and is more easily oxidised.
Metal ions
  • Zn²⁺/Zn has E⦵ = -0.76 V, so zinc is a good reducing agent, whereas Ag⁺/Ag at +0.80 V makes Ag⁺ a good oxidising agent.
Not scaled
  • Multiplying a half-equation does not change E⦵ because potential is independent of amount.
Half-equationE⦵ / V
Zn²⁺ + 2e⁻ ⇌ Zn-0.76
Cu²⁺ + 2e⁻ ⇌ Cu+0.34
Fe³⁺ + e⁻ ⇌ Fe²⁺+0.77
Ag⁺ + e⁻ ⇌ Ag+0.80
Cl₂ + 2e⁻ ⇌ 2Cl⁻+1.36
MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O+1.51

Part 3 of 5Using E⦵ Values

1
Identify the two half-equations and write them as reductions with their E⦵ values.
2
The half-cell with the more positive E⦵ is reduced, so it is the positive electrode, and the other half-equation is reversed to oxidation.
3
Calculate E⦵cell = E⦵(more positive) - E⦵(less positive), which gives a positive voltage for a feasible reaction.
4
Write the overall equation by adding the reduction and the reversed oxidation, balancing electrons first.
5
State that a positive E⦵cell predicts the reaction is feasible under standard conditions, though it does not guarantee a fast rate.

Part 4 of 5Worked Example

Question Calculate E⦵cell for a cell made from Zn²⁺/Zn and Cu²⁺/Cu and write the cell reaction.

  1. The more positive half-cell is Cu²⁺/Cu (+0.34 V), so copper(II) is reduced.
  2. E⦵cell = +0.34 - (-0.76) = +1.10 V.
  3. Zinc is oxidisedZn → Zn²⁺ + 2e⁻.

Answer

  • E⦵cell = +1.10 V;
    • Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Part 5 of 5Exam Checklist & Tips

5.1.2 – Key Points for the Exam

  • Standard electrode potential Voltage of a half-cell relative to the standard hydrogen electrode under standard conditions.
  • E⦵cell Difference between the two standard electrode potentials, positive minus negative.
  • Oxidising agent A species with a more positive E⦵ tends to be reduced.
  • Feasible A reaction with positive E⦵cell, thermodynamically able to occur.

Common Mistakes

  • Changing the sign of E⦵ when a half-equation is multiplied.
  • Subtracting in the wrong order so the cell voltage is negative.
  • Quoting E⦵ without units or without a sign.
  • Assuming that a positive E⦵cell means the reaction is fast.

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