Nernst Equation

• This equation was named after a German physicist Walther Nernst.

• The Nernst Equation empowers the assurance of cell potential under non-standard conditions and relates the measured cell potential to the reaction quotient and permits the exact measurement of equilibrium constants.
• Let us consider an electrochemical reaction of the following type:

aA +bB  –> cC + dD

• Nernst equation for this can be written as follows:

• If the circuit in Daniel cell is closed:

Zn(s) + Cu²⁺ (aq) →Zn²⁺ (aq) + Cu(s)

Problem:

Calculate the emf of the cell in which the following reaction takes place: Ni_(s) + 2Ag⁺ (0.002 M) → Ni²⁺ (0.160 M) + 2Ag_(s). Given that E^ø_cell = 1.05 V.

Electrochemical Cell and Gibbs Energy of the Reaction

• Electrical work done (1 second) = Electrical potentialX Total charge passed
• Passing the charges reversibly through the galvanic cell results in maximum work.
• Reversible work done by galvanic cell = Decrease in Gibbs energy
• Let E = Emf of the cell

           nF = Amount of charge passed

        Δ_rG = Gibbs energy of the reaction

        Δ_rG = -nFE_cell

  For the reaction,

           Zn(s) + Cu²⁺ (aq) –> Zn²⁺ (aq) + Cu(s)

            [Δ_rG = -2FE_cell  ]

But when the equation becomes

2Zn(s) + 2Cu²⁺ (aq) –> 2Zn²⁺ (aq) + 2Cu(s)

  [Δ_rG = -4FE_cell  ]

Problem:

The cell in which the following reactions occurs: