Electrolysis of solutions

Electrolysis of dissolved ionic compounds

Water molecules break down to form hydrogen and hydroxide ions:

H2O → 2H+ + OH-

Therefore, an electrolyte formed by dissolving an ionic compound contains:

  • hydrogen ions (H+) from the water, and positive ions from the compound
  • hydroxide ions (OH-) from the water, and negative ions from the compound

The ions compete at each electrode to gain or lose electrons.

At the cathode

Whether hydrogen or a metal is produced at the cathode depends on the position of the metal in the metal reactivity series:

  • the metal is produced at the cathode if it is less reactive than hydrogen
  • hydrogen is produced at the cathode if the metal is more reactive than hydrogen
The relative reactivity of selected elements from most to least: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, tin, lead, hydrogen, copper, silver, gold, platinum.The reactivity series of metals - carbon and hydrogen are non-metals, shown for comparison
Question

Predict the product formed at the negative electrode during the electrolysis of sodium chloride solution.

Hydrogen will be produced because sodium is more reactive than hydrogen.

Question

Predict the product formed at the negative electrode during the electrolysis of copper chloride solution.

Copper will be produced because copper is less reactive than hydrogen.

At the anode

Oxygen is produced (from hydroxide ions), unless halide ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (chlorine, bromine or iodine).

The table summarises the product formed at the anode during the electrolysis of different electrolytes in solution.

Negative ionElement given off at anode
Chloride, Cl-Chlorine, Cl2
Bromide, Br-Bromine, Br2
Iodide, I-Iodine, I2
Sulfate, SO42-Oxygen, O2
Nitrate, NO3-Oxygen, O2
Question

Predict the product formed at the positive electrode during the electrolysis of concentrated sodium chloride solution.

Chlorine will be produced.

Question

Predict the product formed at the positive electrode during the electrolysis of concentrated sodium sulfate solution.

Oxygen will be produced.

Testing for gases

Electrolysis of solutions can produce hydrogen, oxygen and chlorine. It is easy to test for these gases in the laboratory.

GasHow to test for the gasPositive result
ChlorinePut damp blue litmus paper into the test tube of gasTurns red then white
HydrogenHold a burning splint near to the neck of the test tubeSqueaky pop sound
OxygenPut a glowing splint into the test tube of gasRelights the splint

Half equations during electrolysis - Higher

During the electrolysis of aqueous solutions, positive ions from the unreactive metal or the hydrogen ion (from the water) are attracted to the negative electrode (the cathode) and they gain electrons to form neutral atoms. For example:

Cu2+ + 2e- → Cu

2H+ + 2e- → H2

Meanwhile, negative ions are attracted to the positive electrode (the anode) and they lose electrons to form neutral atoms or molecules. The ions discharged at the anode will either be halide ions (chloride, bromide or iodide) or they will be hydroxide ions:

2Cl- → Cl2 + 2e-

2Br- → Br2 + 2e-

2I- → I2 + 2e-

4OH- → 2H2O + O2 + 4e-