How Changes to Half Cell Affect Ecell

Let's consider the following electrochemical cell:

Pt(s) | Fe2+(aq) , Fe3+(aq) || Ag+(aq) | Ag(s)

Fe2+ is oxidised to Fe3+ hence the iron half cell is the anode.

Ag+ is reduced to Ag so the silver half cell is the cathode.

We want to predict what happens to cell potential (Ecell) when changes are made to a half cell.

When the change shifts the position of equilibrium (POE) for the reversible electrode, E value will be affected since electrode potential measures the tendency for forward reaction (reduction).

When POE shifts right, forward reaction is favoured hence E value increases.

When POE shifts left, forward reaction is disfavoured hence E value decreases.

Remember to use the reduction half equation when considering shifts in POE as E values are based on reduction half equations in the Data Booklet.

230529 shift in poe

Let's consider the following 3 changes:

1. Solid NaCl added to silver half cell

When NaCl is added, silver cations will be precipitated as AgCl(s).

Concentration of Ag+ will decrease and POE for Ag+ + e- ↔ Ag will shift left.

Forward reaction is disfavoured hence E value for Ag half cell will decrease.

Since Ag half cell is the cathode, a decrease in Ereduction will cause Ecell to decrease.

230529 add nacl

2. Dipping the silver electrode deeper into the solution

Changing the surface area or shape of electrode in contact with the solution has no effect on E value.

Hence there will not be any changes to Ecell.

230529 dip electrode

3. Solid FeCl2 added to iron half cell

When FeCl2 is added, concentration of Fe2+ increases.

POE for Fe3+ + e- ↔ Fe2+ will shift left, forward reaction disfavoured and E value for Fe half cell will decrease.

Since Fe half cell is the anode, a decrease in Eoxidation will cause Ecell to increase.

230529 add fecl2

Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore

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