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 (E_cell) 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.

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 E_reduction will cause E_cell to decrease.

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 E_cell.

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 E_oxidation will cause E_cell to increase.

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

Back to other previous Physical Chemistry Video Lessons.

Found this A Level Chemistry video useful?

This free chemistry video lesson is brought to you by Chemistry Guru, Singapore's top JC Chemistry tuition centre since 2010.

Please like this video and share it with your friends!

Join my 18,000 subscribers on my YouTube Channel for new A Level Chemistry video lessons every week.

Check out other A Level Chemistry Video Lessons here!

Need an experienced tutor to make Chemistry simpler for you?

Do consider signing up for my JC Chemistry Tuition classes at Bishan or on-demand video lessons!