Chemistry, Third edition

112 7 · OXIDATION AND REDUCTION

Predicting whether reactions can occur

Standard electrode potentials (E°values) may be used to decide whether redox re-

actions are allowed to occur. The key point to remember here is that the half-

reaction having the more positive electrode potential occurs as a reduction, while the

half-reaction having the more negative electrode potential occurs as an oxidation.

To help you apply this easily:

Write down both half-reactions, with the half-reaction possessing the more negative

(or least positive)E° value at the top. Then draw anticlockwise arrowsto predict

whether the overall reaction can occur.

Example 7.3

Will hydrogen gas reduce Fe^3 (aq) to Fe^2 (aq)?

Answer

Write down both half-reactions, with the half-reaction possessing the more

negative (or least positive) potential at the top:

2H(aq)2eH 2 (g) E°= 0.00 V

Fe^3 (aq)eFe^2 (aq) E°=0.77 V

Draw anticlockwise arrows:

2H(aq)2eH 2 (g) E°= 0.00 V

Fe^3 (aq)eFe^2 (aq) E°=0.77 V

The direction in which the arrows point show that H 2 changes to Hand that Fe^3 

changes to Fe^2 . The predicted overall reaction is therefore:

H 2 (g)2Fe^3 (aq)2Fe^2 (aq)2H(aq)

Comments

The conclusion that we can draw from this is that hydrogen gas is capable of

reducing Fe^3 (aq) to Fe^2 (aq) at room temperature.

Note, however, that we cannot predict how fast (or slow!) the reaction will be. The

reaction may be so slow that nothing will appear to happen.

In addition, our predictions only apply to solutions, since E°values cannot be

obtained for dry gases or solids.

Example 7.4

Will bromine water (Br 2 (aq)) react with an aqueous solution of

potassium chloride (Cl(aq))?

Answer

Following the same procedure as above: