Non-Standard Conditions (Cambridge (CIE) A Level Chemistry): Revision Note

Effect of Concentration on Electrode Potential

• Changes in temperature and concentration of aqueous ions will affect the standard electrode potential (E^ꝋ) of a half-cell

• Under these non-standard conditions, E is used as a symbol for the electrode potential instead of E^ꝋ 

Increasing the concentration of the species on the left

• If the concentration of the species on the left is increased, the position of equilibrium will shift to the right

• This means that the species on the left gets more easily reduced

• The E value becomes more positive (or less negative)

• Let’s look at the half-cell below as an example

Zn²⁺ (aq) + 2e^- ⇌ Zn (s)        E^ꝋ = -0.76 V

• If the concentration of Zn²⁺ (species on the left) is increased, the equilibrium position will shift to the right

• The species on the left (Zn²⁺) will get more easily reduced

• Therefore, the E value becomes less negative and will change too, for example, -0.50 V instead

• This principle can also be applied to a half-cell with a positive E^ꝋ value such as:

Fe³⁺ (aq) + e^- ⇌ Fe²⁺ (aq)        E^ꝋ = +0.77 V

• If the concentration of Fe³⁺ (species on the left) is increased, the equilibrium position will shift to the right

• The species on the left (Fe³⁺) will get more easily reduced

• Therefore, the E value becomes more positive and will change too, for example, +0.89 V instead

Increasing the concentration of species on the right

• If the concentration of the species on the right is increased, the position of equilibrium will shift to the left

• This means that the species on the left get less easily reduced

• The E value becomes less positive (or more negative)

• Let’s look again at the half-cell below

Zn²⁺ (aq) + 2e^- ⇌ Zn (s)        E^ꝋ = -0.76 V

• If the concentration of Zn (species on the right) is increased, the equilibrium position will shift to the left

• The species on the left (Zn²⁺) will get less easily reduced

• Therefore, the E value becomes more negative and will change too, for example, -0.82 V instead

• This principle can, again, also be applied to a half-cell with a positive E^ꝋ value:

Fe³⁺ (aq) + e^- ⇌ Fe²⁺ (aq)        E^ꝋ = +0.77 V

• If the concentration of Fe²⁺ (species on the right) is increased, the equilibrium position will shift to the left

• The species on the left (Fe³⁺) will get less easily reduced

• Therefore, the E value becomes less positive and will change too, for example, +0.56 V instead

Effect of concentration on the electrode potential

Summarising the effects of changing the concentration of reactants and products