Describe electrolysis of aqueous solutions, predict the products using reactivity, and explain the part played by water at the electrodes

What this dot point is asking

The syllabus wants you to describe the electrolysis of aqueous solutions (an ionic compound dissolved in water), to predict the products using the reactivity series, and to explain the part water plays at the electrodes. The new idea here is that the water itself adds extra ions, so the product is not always the metal or non-metal from the dissolved salt.

The answer

Why solutions are different from melts

When a salt is dissolved in water, the solution contains the ions from the salt plus a few ions from the water. This means there is a choice of ions at each electrode, so you cannot simply assume the metal and the non-metal of the salt will form. The water can give hydrogen at the cathode or oxygen at the anode instead.

The rule at the cathode

At the cathode (negative), the choice is between the metal and hydrogen (from water):

• if the metal is more reactive than hydrogen, then hydrogen forms (the metal stays in solution),
• if the metal is less reactive than hydrogen, then the metal is deposited.

So copper (below hydrogen) is deposited, but sodium (above hydrogen) is not; hydrogen forms instead.

The rule at the anode

At the anode (positive), the simple rule for N(A) level is:

• if the solution contains a halide (chloride, bromide or iodide) in reasonable amount, the halogen (chlorine, bromine or iodine) forms,
• otherwise (for example with a sulfate or nitrate), oxygen forms from the water.

Putting it together

Electrolysing dilute copper(II) sulfate gives copper at the cathode (copper is below hydrogen) and oxygen at the anode (sulfate, so oxygen from water). Electrolysing sodium chloride solution gives hydrogen at the cathode (sodium is above hydrogen) and chlorine at the anode (a halide).

Examples in context

Example 1. Purifying copper. Copper is purified by electrolysing copper(II) sulfate solution with copper electrodes. Pure copper builds up on the cathode because copper is less reactive than hydrogen, so it is deposited, a direct use of the cathode rule in industry.

Example 2. Making chlorine for water treatment. Electrolysing concentrated sodium chloride solution produces chlorine at the anode, which is used to disinfect drinking water and swimming pools. The halide rule explains why chlorine, not oxygen, is the useful product here.

Try this

Q1. State which gas forms at the cathode when dilute sodium chloride solution is electrolysed, and why. [2 marks]

• Cue. Hydrogen forms, because sodium is more reactive than hydrogen, so hydrogen is produced instead of the metal.

Q2. Dilute copper(II) sulfate solution is electrolysed. State the products at the cathode and the anode. [2 marks]

• Cue. Copper forms at the cathode (copper is below hydrogen); oxygen forms at the anode (sulfate, so oxygen from water).

Q3. Explain why electrolysing a salt solution can give different products from electrolysing the molten salt. [2 marks]

• Cue. The water in the solution adds extra ions, giving a choice at each electrode, so hydrogen or oxygen from the water may form instead of the salt's elements.