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A LevelChemistryCIERevision Notes24. Electrochemistry (AL)24.2 Standard Electrode Potentials E⦵, Standard Cell Potentials E⦵cell & the Nernst EquationStandard Electrode Potentials: Free Energy Change

Standard Electrode Potentials: Free Energy Change (CIE A Level Chemistry)

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Calculating Free Energy Change Using Standard Electrode Potentials

  • The standard free energy change can be calculated using the standard cell potential of an electrochemical cell

ΔGꝋ = - n x Ecellꝋ x F

    • ΔGꝋ = standard Gibbs free energy
    • n = number of electrons transferred in the reaction
    • Ecellꝋ = standard cell potential (V)
    • F = Faraday constant (96 500 C mol-1)

Worked example

Calculating the standard Gibbs free energy change

Calculate the standard Gibbs free energy change for the following electrochemical cell:

2Fe3+ (aq) + Cu2+ (aq) rightwards harpoon over leftwards harpoon 2Fe2+ (aq) + Cu (s)

Answer

  • Step 1: Determine the two half-equations and their Eꝋ using the Data booklet:

Fe3+ (aq) + e- ⇌ Fe2+ (aq)        Eꝋ = +0.77 V

Cu2+ (aq) + 2e- ⇌ Cu (s)       Eꝋ = +0.34 V 

  • Step 2: Calculate the Ecellꝋ
    • Ecellꝋ = Eredꝋ - Eoxꝋ
    • Ecellꝋ  = (+0.77) - (+0.34)
    • Ecellꝋ  = +0.43 V

  • Step 3: Determine the number of electrons transferred in the reaction
    • The Cu2+/Cu has a smaller Eꝋ value which means that it gets oxidised
    • It transfers two electrons to  two Fe3+ ions
    • Each Fe3+ ion accepts one electron so the total number of electrons transferred is two

  • Step 4: Substitute the values in for the standard Gibbs free energy equation
    • ΔGꝋ = - n x Ecellꝋ x F
    • ΔGꝋ  = -2 x (+0.43) x 96 500
    • ΔGꝋ = -82 990 J mol-1 = -83 kJ mol-1 

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    Richard

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