10.09.3 Aqueous Electrolysis

Syllabus

- The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved.
- At the negative electrode (cathode), Hydrogen is produced if the metal is more reactive than Hydrogen.
- At the positive electrode (anode), Oxygen is produced unless the solution contains Halide ions when the halogen is produced.
- This happens because in the aqueous solution water molecules break down producing Hydrogen ions and Hydroxide ions that are discharged.

Students should be able to predict the products of the electrolysis of aqueous solutions containing a single ionic compound.

Required practical 3: investigate what happens when aqueous solutions are electrolysed using inert electrodes. This should be an investigation involving developing a hypothesis.

What does this mean?

A Choice of Ions

Whenever you dissolve an ionic solid there are more than two ions available to release because water provides both Hydrogen ions (H+) and Hydroxide (OH-) ions.

H2O --> H+(aq) + OH- (aq)

But only one Anion and one Cation can be released.

So how can you predict which one?

Rule 1. Cations

The least reactive positive ion is always released.

In practice this means that Hydrogen ions will always be released as Hydrogen unless the dissolved substance contains Copper or Silver.

Rule 2. Anions

The ion containing the fewest atoms is released

(DON’T QUOTE THIS IN AN EXAM!!)

So Cl-, Br-, and I- would be released instead of OH-.

But OH- would be released instead of SO42- or NO3- etc.

The electrolysis of Brine (Salty Water)

Cations

Salt provides Na+ ions, Water Provides H+ ions.

H+ ions are less reactive and so are released.

2H+ + 2e- --> H2

This is Reduction - gain of electrons (OILRIG)

Hydrogen can be tested for with the Pop Test

Anions

Salt provides Cl- ions, Water Provides OH- ions.

Cl- ions are less complicated than OH- and so are released.

2Cl- --> Cl2 + 2e-

This is Oxidation - loss of electrons (OILRIG)

Chlorine can be tested for by holding damp blue Litmus paper in it (it should go red and then white)

When the OH- ion is released.

In some electrolysis experiments the dissolved Anion will be more complicated than OH- .

So, the OH- ion will be released.

We see a gas being formed which, if we collect it, relights a glowing splint.

It must be Oxygen but OH- ions can't just turn into Oxygen.

What happens to the H atoms in the OH- ions?

They become water, which we don't see because there's so much water around

4OH- --> O2 + H2O + 4e-

Predicting the products of Aqueous Electrolysis.

Remember the rules!

Cation

least reactive released ( in practice, Hydrogen unless the solution contains Copper or Silver)

Anion

In Practice Chloride, Bromide, Iodide or Fluoride are less complicated than OH- ions and are released.
Sulphate, Nitrate and other complicated ions are never released. So the OH- ions form water