Question

How can I use the Nernst equation to calculate the free energy change for oxidation reduction reactions?

Answer 1

There are five steps for calculating the free energy change for an oxidation-reduction reaction. You use the Nernst equation at Step 4.

EXAMPLE

Calculate the free energy change for the following electrochemical cell at 25.0 °C.

Zn(s) | Zn²⁺ (1.50 mol/L) || Cu²⁺ (0.250 mol/L) | Cu(s)

Solution

Step 1. Calculate the standard cell potential, `E°_"cell"`

Zn → Zn²⁺ + 2 e⁻; `E_"ox"^°` = + 0.762 V

Cu²⁺ + 2 e⁻ → Cu; `E_"red"^°` = + 0.339 V

Zn + Cu²⁺ → Zn²⁺ + Cu; `E_"cell"^°` = + 1.101 V

Step 2. Determine the number of moles of electrons `n` transferred

`n` = 2 mol of electrons.

Step 3. Calculate the reaction quotient, `Q`

`Q = (["Zn"²⁺])/(["Cu"²⁺]) = (1.50"mol/L")/(0.250"mol/L")` = 6.00

Step 4. Use the Nernst equation to calculate the nonstandard cell potential, `E_"cell"`.

`E_"cell" = E_"cell"^° - (RT)/(nF)lnQ` = 1.101 V - `(8.314"J·K⁻¹" × 298.2"K")/(2"mol" × 96485"J· V⁻¹mol⁻¹") × ln6.00` =

1.101 V – 0.0230 V = 1.078 V

Step 5. Calculate the free energy.

`ΔG = -nFE_"cell"` = -2 mol × 96 485 J·V⁻¹mol⁻¹ × 1.078 V = -208 000 J = -208.0 kJ