Galvanic Cell

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2 common types of qns:

• predict feasibility of redox rxn
• predict change in E° at non-standard conditions using Le Chatelier’s Principle (LCP)

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Predict feasibility of Redox Reaction

 

1. Identify redox active species; check if one of them can only be oxidized or reduced.
2. Determine the possible oxidation/ reduction reactions (use the electrochemical table if necessary)
3. Find electrode potentials for these half equations
4. Calculate E^o_cell = E^o_red – E^o_ox

E^o_cell → feasible

E^o_cell → not feasible

Example: Show using calculations if a redox reaction between H2O2 and I– is feasible.   1. redox active species: H2O2 and I–; I– can only be oxidised. 2. Since I– can only be oxidized, H2O2 must be reduced for a redox rxn to occur i.e.  I– → I2; H2O2 → H2O (check data booklet if you cannot recall what H2O2 is reduced to) 3. EoI2/I–  = –0.54 V; EoH2O2/H2O  = +1.77 V 4. Eocell  = +1.77 – 0.54 = +1.23 V > 0     (feasible)

 

Predict change in E° at non-standard conditions using LCP.

 

1. Identify which species is affected/ check which half eqn is affected.
2. Identify whether equilibrium will shift right or left.

    

   Shift right: E value becomes more +/ less –

   Shift left: E value becomes less +/ more –

3. Determine change in E_cell. Try substituting an arbitrary no. if you have problems.

 

Example: Predict how Ecell will change for the following reaction when pH2 is increased: 2NO3– + 2H+ + H2 → 2NO2 + 2H2O   1. Half eqn affected: 2H2O + 2e– → H2 + 2OH– 2. Equilibrium shifts left: E value becomes less +/ more – i.e. < –0.83 V (subst. an arbitrary more – value e.g. –0.93 V) 3. Ecell = Ered – Eox = +0.81 – (–0.93) i.e. Ecell becomes more +