Standard electrode potentials and the hydrogen electrodeEdexcel International A Level Chemistry: Revision notes

Oxidation is loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number (OIL RIG). Oxidation numbers are assigned by rules: uncombined elements are 0; the oxidation numbers in a neutral compound sum to 0 and in an ion sum to the charge; hydrogen is +1 (−1 in metal hydrides); oxygen is −2 (−1 in peroxides); fluorine is always −1.

This applies across the blocks:

• s-block: Mg → Mg²⁺ (0 to +2), Na → Na⁺ (0 to +1)
• p-block: Cl₂ → 2Cl⁻ (0 to −1); sulfur in SO₄²⁻ is +6
• d-block: Mn in MnO₄⁻ is +7 and Cr in Cr₂O₇²⁻ is +6; Fe²⁺ → Fe³⁺ is oxidation

An oxidising agent is reduced (it takes electrons); a reducing agent is oxidised (it gives electrons). In Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O, chromium falls from +6 to +3 (reduced) and iron rises from +2 to +3 (oxidised).

A half-cell is a metal or inert electrode in contact with a species and its ion, for example Cu²⁺(aq) | Cu(s). The two forms are linked by an equilibrium, written as a reduction: Cu²⁺ + 2e⁻ ⇌ Cu.

The standard electrode potential, E°, of a half-cell is the potential difference (emf) measured when the half-cell is connected to a standard hydrogen electrode under standard conditions: 298 K, gases at 100 kPa, and all ions at 1.00 mol dm⁻³.

A more positive E° means the species on the left is a stronger oxidising agent (it gains electrons more readily). A more negative E° means the species on the right is a stronger reducing agent.

Only a potential difference can be measured, never the potential of one half-cell on its own. A reference electrode gives a fixed point to compare against. By convention the standard hydrogen electrode (SHE) is assigned E° = 0.00 V.

The SHE is the half-cell 2H⁺(aq) + 2e⁻ ⇌ H₂(g):

• hydrogen gas at 100 kPa bubbled over the electrode
• 1.00 mol dm⁻³ H⁺(aq), e.g. hydrochloric or sulfuric acid
• 298 K
• a platinum electrode coated with finely divided platinum black

Platinum is inert and conducts electrons; platinum black has a large surface area that adsorbs hydrogen so the equilibrium is set up quickly.

The half-cell is joined to the SHE by a salt bridge (filter paper soaked in potassium nitrate solution), and a high-resistance voltmeter is connected between the electrodes so almost no current flows and the concentrations stay constant.

• Metal in its own ions (Cu²⁺|Cu, Zn²⁺|Zn): the metal is the electrode, dipping into 1.00 mol dm⁻³ of its ions.
• Non-metal and its ion (Cl₂|Cl⁻): a platinum electrode in 1.00 mol dm⁻³ Cl⁻(aq) with chlorine gas at 100 kPa bubbled over it.
• Two ions of the same element (Fe³⁺|Fe²⁺): a platinum electrode in a solution containing both ions, each at 1.00 mol dm⁻³.

The voltmeter reading is E°. The sign shows the polarity: if the half-cell is the positive terminal the value is positive.

In Core Practical 12 you build cells from half-cells of metals in 1.00 mol dm⁻³ solutions of their ions and measure the cell voltage.

• Clean each metal strip with emery paper to remove the oxide layer.
• Use fresh solutions of known concentration at room temperature (about 298 K).
• Join the half-cells with a salt bridge of filter paper soaked in potassium nitrate solution, using a fresh strip for each pair.
• Read the high-resistance voltmeter and record the polarity.
• Compare the readings with values calculated from E° data. Small differences arise from non-standard conditions, impure or oxidised surfaces, and the voltmeter drawing current.