Standard electrode potentialsEdexcel A-Level Chemistry: Revision notes
Section 1
Redox as electron transfer and oxidation number change
Oxidation is loss of electrons and reduction is gain of electrons (OIL RIG). The same changes are seen as a rise (oxidation) or fall (reduction) in oxidation number.
This applies to s-block (2Na + Cl₂ → 2NaCl: Na 0 → +1 oxidised, Cl 0 → −1 reduced), p-block (Cl₂, S, N in NO₃⁻) and d-block elements (Fe²⁺ → Fe³⁺, MnO₄⁻ → Mn²⁺, Cr₂O₇²⁻ → Cr³⁺).
Rules: free elements 0; simple ion = its charge; O is −2 (not in peroxides); H is +1 (not in metal hydrides); sum = overall charge.
Gaining electrons is reduction even though the charge on the species becomes more negative. Check the direction of the oxidation number change, not the sign.
Section 2
Standard electrode potential, E°
The standard electrode potential, E°, of a half-cell is its e.m.f. measured relative to the standard hydrogen electrode (SHE) under standard conditions: 298 K, 100 kPa for gases and 1.00 mol dm⁻³ for ions in solution.
It is written for the reduction half-equation, e.g. Cu²⁺ + 2e⁻ ⇌ Cu, E° = +0.34 V. A more positive E° means a greater tendency for the species on the left to be reduced.
Section 3
The standard hydrogen electrode
The SHE consists of hydrogen gas at 100 kPa bubbled over a platinum electrode (coated in platinum black) dipping in 1.00 mol dm⁻³ H⁺(aq) at 298 K. Half-equation: 2H⁺(aq) + 2e⁻ ⇌ H₂(g), E° = 0.00 V by definition.
A reference electrode is needed because the potential of a single half-cell cannot be measured on its own, only a potential difference between two half-cells. Platinum is inert, conducts and provides a surface for the equilibrium.
Section 4
Measuring E° of metals, non-metals and ions (Core Practical 10)
Metal in its ion (Zn²⁺/Zn): metal electrode in 1.00 mol dm⁻³ solution of its ions.
Non-metal (Cl₂/Cl⁻): inert platinum electrode, chlorine gas at 100 kPa bubbled over it, in 1.00 mol dm⁻³ chloride ions.
Two ions of the same element (Fe³⁺/Fe²⁺): inert platinum electrode in a solution containing both ions at 1.00 mol dm⁻³.
Always: connect to the SHE with a salt bridge (filter paper soaked in potassium nitrate solution) and a high-resistance voltmeter (so negligible current flows) at 298 K. The reading is E° of the half-cell.
Choose an unreactive salt bridge electrolyte such as potassium nitrate. Say 'platinum' whenever there is no solid metal in the half-cell.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Standard electrode potentials
- A teacher demonstrates three redox changes. In the first, sodium burns in chlorine: 2Na + Cl₂ → 2NaCl. In the second, orange acidified potassium dichromate(VI) turns green as it is reduced. In the third, purple acidified potassium manganate(VII) is decolourised when it is added to iron(II) sulfate solution.Use oxidation numbers and electron transfer to identify the species oxidised and the species reduced when sodium burns in chlorine.2 marks
- To compare the tendency of different species to gain electrons, chemists measure electrode potentials against a single reference. The reference used is the standard hydrogen electrode, which is given an electrode potential of exactly 0.00 V.Explain why a reference electrode is needed to measure the standard electrode potential of a half-cell.2 marks
- In Core Practical 10, a student investigates electrochemical cells by measuring standard electrode potentials. Each half-cell is connected to a standard hydrogen electrode so that a voltmeter can give the electrode potential. The student has zinc metal, zinc sulfate solution, iron(II) sulfate and iron(III) sulfate solutions, a platinum electrode, filter paper and potassium nitrate solution.Describe how the student sets up the zinc half-cell and connects it to the standard hydrogen electrode so that the standard electrode potential of Zn²⁺/Zn can be measured.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).