Electrode potentials and electrochemical cells (A Level) flashcards

Topic 11: Electrode potentials and electrochemical cells

Standard electrode potential, E⦵

The EMF of a half-cell measured against the standard hydrogen electrode at 298 K, 100 kPa and 1.00 mol dm⁻³ solutions of ions.

Salt bridge

Usually filter paper soaked in saturated KNO₃(aq). Ions move through it to balance charge. It does not carry electrons.

The standard hydrogen electrode

Platinum electrode; H₂ gas at 100 kPa; 1.00 mol dm⁻³ H⁺(aq); 298 K. E⦵ = 0.00 V by definition. Pt(s) | H₂(g) | H⁺(aq)

Conventional cell representation: what goes on the left and the right?

Left: the negative electrode, written as an oxidation, e.g. Zn(s) | Zn²⁺(aq). Right: the positive electrode, written as a reduction, e.g. Ag⁺(aq) | Ag(s). || is the salt bridge.

EMF of a cell from electrode potentials

EMF = E⦵(right) − E⦵(left) = E⦵(positive) − E⦵(negative). Do not multiply E⦵ when a half-equation is scaled.

EMF of Zn | Zn²⁺ || Cu²⁺ | Cu (E⦵ Zn²⁺/Zn −0.76 V, Cu²⁺/Cu +0.34 V)

+0.34 − (−0.76) = +1.10 V

How do you decide whether a redox reaction is feasible from E⦵ values?

The more positive half-equation goes forwards (reduction) and the other goes backwards. It is feasible if the oxidising agent’s E⦵ is more positive than the reducing agent’s.

Raising [Cu²⁺] in a Cu²⁺/Cu half-cell: effect on its electrode potential

The equilibrium Cu²⁺ + 2e⁻ ⇌ Cu shifts right, so E becomes more positive.

Alkaline hydrogen–oxygen fuel cell: electrode half-equations

Positive: O₂ + 2H₂O + 4e⁻ → 4OH⁻ Negative: H₂ + 2OH⁻ → 2H₂O + 2e⁻ Overall 2H₂ + O₂ → 2H₂O; EMF +1.23 V.

Required practical 8: why a high-resistance voltmeter?

So that negligible current flows and the reading is the EMF.