Standard electrode potentials can be listed as an electrochemical series.

The idea

List every half-equation as a reduction, order them by E⦵, and you have the electrochemical series. The most positive values sit at one end: the species on the left of those equations, such as F₂ at +2.87 V and MnO₄⁻ in acid at +1.51 V, are the strongest oxidising agents, because they take electrons most readily. The most negative values sit at the other end, and the species on the right of those, such as lithium and potassium metal, are the strongest reducing agents. The rule that follows is simple: an oxidising agent will oxidise the reduced species of any half-equation with a less positive E⦵. That is the basis of every feasibility prediction. The series tells you what is energetically possible, not what is fast.

What earns the marks

• Identifying the species on the left of the most positive half-equation as the strongest oxidising agent.
• Identifying the species on the right of the most negative half-equation as the strongest reducing agent.
• Stating that the reaction is feasible when the oxidising agent comes from the more positive half-equation.
• Using a positive overall EMF as the test of feasibility.

Common mistakes and how to put them right

• Ignoring the sign, so −0.76 V is treated as larger than −0.44 V.

• Reading the most negative value as belonging to the strongest oxidising agent.

• Assuming a positive EMF guarantees an observable reaction, when the rate may be immeasurably slow.

• Applying standard values to a mixture that is nowhere near 1.00 mol dm⁻³ and still calling the prediction certain.

Exam tips

• Mark which half-equation runs backwards before you do any arithmetic; that single decision fixes the sign of the EMF.
• If a predicted reaction is not observed, the mark is usually for saying the activation energy is high, so the rate is very slow.