Gibbs Energy & Standard Cell Potential (HL) (DP IB Chemistry): Revision Note

Gibbs Energy & Standard Cell Potential

• Previously we have seen the concept and term free energy, ΔG^θ

  • Free energy is a measure of the available energy to do useful work

  • It takes into account the entropy change, ΔS^θ and the enthalpy change, ΔH^θ, of a reaction

• For reactions to be spontaneous, the free energy change must be negative

• We have also seen that to calculate a cell potential using standard electrode potentials:

EMF = E_RHS – E_LHS

• This is not an arbitrary arrangement of the terms

• Convention places the more negative half cell on the left in the cell diagram

  • This means that the voltmeter gives a positive reading, indicating a spontaneous reaction

  • In lab experiments, you may be told to “swap the terminals" if the voltmeter reading is negative

• In electrochemical cells:

  • A spontaneous reaction occurs when the combination of half cells produces a positive voltage

  • This means that the more negative electrode pushes electrons to the more positive electrode

• With digital multimeters, a negative sign simply indicates reversed terminals

• With analogue voltmeters, the terminals must be correctly connected for the needle to move

• From this, we can conclude:

  • If ΔE^θ is positive, the reaction is spontaneous as written

  • If ΔE^θ is negative, the forward reaction is non-spontaneous but the reverse reaction will be spontaneous

• This links directly to the equation:

ΔG^θ = -nFE^θ

• Where:

  • n = number of electrons transferred

  • F = the Faraday constant, 9.65 x 10⁴ C mol^-1

• At equilibrium, there is no free energy change (ΔG^θ  = 0) and the cell potential is also zero (E^θ = 0)

  • This happens when the reactants in a voltaic cell are used up and electron flow stops

Summary of free energy and cell potential relationships

• The relationship between free energy, electrode potential and spontaneity is:

  • If ΔG^θ is negative and E^θ is positive:

    • The reaction is spontaneous

  • If ΔG^θ is positive and E^θ is negative:

    • The forward reaction is non-spontaneous

    • The reverse reaction is spontaneous

  • If ΔG^θ = 0 and E^θ = 0, the system is at equilibrium

    • There is no net electron flow