How Concentration Affects Electrolysis Product

When the electrolyte is concentrated, the products of electrolysis can be affected.

This applies to concentrated ions with small electrode potential difference between the species to be discharged at the electrode.

Let's use the electrolysis of brine or concentrated NaCl as an example.

230612 conc nacl

1. Reduction at Cathode

Species migrating to cathode will be Na+ and water.

Since reduction occurs at the cathode, we need to find both species on the left hand side of half equation in the Data Booklet.

230612 deduce reduction

The E value for Na+ is much more negative than E value for H2O.

This means it's much harder to reduce Na+ as compared to water, even if Na+ is concentrated.

Hence water will be reduced at the cathode.

230612 reduction

2. Oxidation at Anode

Species migrating to anode are Cl- and water.

We need to find both species on the right hand side of half equation since oxidation occurs at the anode.

230612 deduce oxidation

The E value for Cl- is very close to E value for H2O.

This means the ease of oxidation for both species are similar, E value factor is less significant and concentration is the dominant factor.

Since Cl- is concentrated, it will be oxidised at the anode.

230612 oxidation

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

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