5.4.4 Non-Standard Conditions

• 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 gets 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