C4 - Electrolysis in Aqeaous Solutions

What does aqueous mean?

Dissolved in water.

What ions are formed when water ionises?

Hydrogen ions (H+) and hydroxide ions (OH-).

Which ions must you consider in electrolysis of aqueous solutions?

The ions from the dissolved compound + H+ and OH- ions from water.

Example: What ions are present in copper sulfate solution (CuSO4 aq)?

Cu2+, SO42-, H+, OH-.

At which electrode do positive ions go?

The cathode (negative electrode).

At which electrode do negative ions go?

The anode (positive electrode).

Rule for deciding which ion is reduced at the cathode in aqueous solutions

If the metal is more reactive than hydrogen, hydrogen is produced. If the metal is less reactive than hydrogen, the metal is produced.

Which ion is discharged at the cathode in copper sulfate solution?

Cu2+ (because copper is less reactive than hydrogen).

What product is formed at the cathode in CuSO4 solution?

Copper metal.

Which product is usually formed at the anode in aqueous solutions?

Oxygen gas (from hydroxide ions).

What is the exception to oxygen being formed at the anode?

If the solution contains halide ions (covered in the next lesson).

Why are inert electrodes used in electrolysis?

So the electrodes do not react with the products.

Give an example of an inert electrode.

Platinum (very unreactive metal).

Is the reaction at the cathode oxidation or reduction?

Reduction (gain of electrons).

Is the reaction at the anode oxidation or reduction?

Oxidation (loss of electrons).

Half equation at the cathode for Cu2+ in CuSO4 solution (Higher only).

Cu2+ + 2e- → Cu.

Half equation at the anode for OH- ions (Higher only, version 1).

4OH- → O2 + 2H2O + 4e-.

Alternative half equation for OH- ions at the anode (Higher only, version 2).

4OH- - 4e- → O2 + 2H2O.

Why are there two versions of the OH- half equation at the anode?

Both show the same reaction: hydroxide ions lose electrons to form oxygen and water.