Edexcel International A Level Chemistry

Topic 16: Redox Equilibria

Redox reactions move electrons, and the voltage of a cell measures how strongly they are pushed. This topic turns that voltage into predictions, titration results and fuel cells.

The one idea

You already know that oxidation is loss of electrons and reduction is gain. This topic puts a number on how strongly a species wants to gain electrons. That number is the standard electrode potential, $E^\ominus$, measured for each half-cell against one fixed reference, the hydrogen electrode.

Once you have the numbers, almost everything in the topic is the same move:

1. Look up two values of $E^\ominus$.
2. Subtract: $E^\ominus_\text{cell}$ = more positive − more negative.
3. Read the sign. Positive means the reaction is thermodynamically feasible, and the larger the value, the larger the equilibrium constant K.

The rest of the topic tests that rule against the real world: the practical cells, the limits of the prediction, redox titrations, and fuel cells.

How to use these notes

• Read the pages in order the first time. Pages 1 and 2 are the foundation: oxidation numbers and half-equations.
• Pages 3 to 8 are about cells and electrode potentials, including Core Practical 12. Pages 9 to 12 use the values to predict reactions.
• Pages 13 to 17 are about redox titrations (Core Practicals 13a and 13b) and the uncertainty of measurements. Pages 18 and 19 cover fuel cells.
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Relative atomic masses used here are from the Edexcel IAL periodic table, to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, S 32.1, K 39.1, Fe 55.8, Cu 63.5, I 126.9. Standard conditions are 298 K, 100 kPa and 1.00 mol dm⁻³. The standard electrode potentials in the worked examples are the values used throughout the topic; your exam gives its own data, so always use the values printed in the paper.

Pages

1. Oxidation and reduction

   Oxidation is loss of electrons and an increase in oxidation number; reduction is the opposite. Oxidation numbers let you follow the electrons in any compound, in the s-, p- and d-block alike.

   14 min 16.1

2. Building redox equations

   Write each half-equation in four steps, multiply so the electrons match, add, and cancel. The same method gives every redox equation in this topic.

   14 min 16.1

3. From a reaction to a cell

   Split a redox reaction into two half-cells and the electrons have to travel through a wire, so you can measure a voltage. Standard electrode potential is that voltage for one half-cell, under fixed conditions.

   13 min 16.216.3(i)16.3(ii)16.3(iii)

4. The standard hydrogen electrode and half-cells

   Single electrodes cannot be measured, so every half-cell is compared with the standard hydrogen electrode, defined as 0.00 V. The half-cell you build depends on whether the couple contains a solid metal.

   13 min 16.416.5(i)16.5(ii)

5. The electrochemical series

   Standard electrode potentials are standard reduction potentials. Listed in order, they rank oxidising and reducing agents and are the data you use for every calculation that follows.

   11 min 16.13

6. Cell emf and cell diagrams

   Subtract the less positive electrode potential from the more positive one to get the standard emf of a cell, and write the cell in the conventional shorthand with the negative electrode on the left.

   15 min 16.716.8

7. Core Practical 12: electrochemical cells

   Build cells from pairs of half-cells, measure their voltages with a high-resistance voltmeter, and compare them with the values calculated from standard electrode potentials.

   13 min 16.6

8. Why the conditions matter

   Every electrode potential comes from an equilibrium, so concentration, pressure and temperature all change it. E equals the standard value only under standard conditions.

   10 min 16.9

9. Predicting feasibility

   A reaction is thermodynamically feasible under standard conditions if its $E^\ominus_\text{cell}$ is positive. Find the species reduced, the species oxidised, subtract, and state the verdict.

   15 min 16.10

10. Cell emf, entropy and the equilibrium constant

    A positive cell emf means a positive total entropy change, which is why the reaction is feasible. The emf is directly proportional to the total entropy change and to ln K, so it also tells you how far the reaction goes.

    11 min 16.11

11. Limits of predictions from standard potentials

    A positive emf says a reaction can happen, not that it will happen quickly, and the value only applies under standard conditions. Learn the two limits and the evidence for each.

    11 min 16.12

12. Disproportionation

    In disproportionation one species is oxidised and reduced at the same time. Test it with two standard electrode potentials that both involve that species.

    12 min 16.14

13. Redox titration reactions and ratios

    Know the equation and the mole ratio of each titration reaction. When iodine is made first from an oxidising agent and excess iodide, chain the ratios together to find how much thiosulfate it needs.

    14 min 16.15

14. Core Practical 13a: iron(II) with manganate(VII)

    Titrate iron(II) ions against potassium manganate(VII) in acid. The manganate(VII) is its own indicator, and the end point is the first permanent pale pink.

    13 min 16.17(i)16.15

15. Core Practical 13b: sodium thiosulfate and iodine

    Use a known amount of iodine, made from potassium iodate(V) and excess iodide, to find the exact concentration of a sodium thiosulfate solution. Starch is added near the end, and the end point is blue-black to colourless.

    13 min 16.17(ii)16.15

16. Redox titration calculations

    Structured questions give you the steps; unstructured ones make you plan them. Both follow the same route, from moles of titrant to a mass or percentage, with the mole ratio doing the work in the middle.

    15 min 16.15

17. Uncertainty and validity

    Every measurement has an uncertainty. Turn each into a percentage, add them, and compare the total with how far your result is from the expected value to decide whether the result is valid.

    14 min 16.16

18. Fuel cells

    A fuel cell turns the energy released when a fuel reacts with oxygen into a voltage. Fuel and oxygen are supplied continuously, so it does not run flat.

    10 min 16.18

19. The hydrogen-oxygen fuel cell

    Hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode. The electrode equations differ in acidic and alkaline electrolytes, but the overall reaction and the emf are the same.

    12 min 16.19

Also in this topic: Topic 16 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.