Using electrode potentials to make predictions

Feasibility

  • With electrode potentials it is possible to predict whether or not a specific redox reaction can take place

  • To do this you need to compare the E0 for the two half equations

  • Can a solution of Cu2+ ions oxidise Pb(s) to Pb2+(aq)?

  • Write out both half equations and look up the E0 values

    • Cu2+(aq) + 2e- ⇌ Cu(s) E0 = +0.34V (forwards, reduced)

    • Pb2+(aq) + 2e- ⇌ Pb(s) E0 = -0.11V (backwards, oxidised)

  • Decide which will be reduction and oxidation in the usual way

  • Yes, a solution of Cu2+ ions oxidise Pb(s)

  • Work out the E0 of the cell

    • 0.34 - - 0.11 = +0.45V

Limitations to predictions

Limitation 1:

  • Predictions only tell us if a reaction is feasibly, they do not refer to the rate of reaction

  • A reaction may be feasible but too slow to measure

  • In other words it may have a very high activation energy

Limitation 2:

  • Our predictions are based on E0 values, these require standard conditions to be met

  • If the reaction is done under non-standard conditions then the reaction may not occur as predicted

Non-standard conditions

  • Under non-standard conditions the electrode potential (E) for the half equation will not be the same as the standard electrode potential (E0)

  • Half equations are written as equilibria

  • If the non-standard conditions would push the equilibrium to the right, then the E value will be less positive than E0.

    • E.g. Fe3+(aq) + e- ⇌ Fe2+(aq) E0 = +0.77V

    • If the concentration of Fe3+(aq) is higher than 1.00M then the electrode potential (E) will be more positive than E0.

  • If the E0 values are within 0.3V of each then under non-standard conditions the reaction may not go as predicted

  • The E values will differ from E0 and it may well be that the different reaction is now more positive

  • Consider the following equations

    • MnO2 + 4H+ + 2e- ⇌ 2H2O + Mn2+ E0 = +1.23V

    • Cl2 + 2e- ⇌ 2Cl- E0 = +1.36V

  • Using the E0 you would expect the first reaction to go backwards and the second forwards

  • This reaction is usually done with concentrated HCl

  • The first reaction will be pushed to the right as conc. of H+ is higher than 1.00M

    • MnO2 + 4H+ + 2e- ⇌ 2H2O + Mn2+ E0 = +1.33V

  • The second reaction will be pushed to the left as conc. of Cl- is higher than 1.00M

    • Cl2 + 2e- ⇌ 2Cl- E0 = +1.26V

  • Reactions go opposite way