no Introduction

The electrodes dipping into the electrolyte are called the anode (positive electrode (+)) and the cathode (negative electrode (-)). Substances form at the electrodes as a result of the decomposition of the solution.

The animation below illustrates the process of electrolysis taking place in an aqueous copper chloride solution. The full chemical explanation will be given to you later on in this lesson.

Electrolysis of copper chloride solution
Click on the Next button to view the next frame.

The positive, metal ion (cation) is attracted to the cathode. The metal ion gains electron(s) at the electrode and becomes a metal which deposits on the electrode. The negative, non-metal ion (anion) is attracted to the anode. At this electrode, the non-metal ion gives up electrons and the non-metal product forms here.

Example

In the case of molten lead bromide, the following reactions occur:

At the cathode:
Pb2+ + 2e¯no Pb
Lead metal deposits here.
At the anode:
2Br¯no Br2 + 2e¯

Brown bromine vapour forms here.

no Electrolysis of aqueous solutions

Reactions carried out in aqueous solution provide an added complication. Water ionises to produce hydrogen (H+) ions and hydroxide (OH¯) ions. These are often displaced from solution to give hydrogen gas and oxygen gas.

The substance formed at the cathode is usually hydrogen, unless the solution contains copper or silver ions, in which case, copper or silver will form at the cathode. This is because these metals are less reactive than hydrogen and are lower down in the reactivity series. Reactive ions stay in solution and do not form atoms because they are more stable than the atoms.

The substance formed at the anode is usually oxygen, unless the solution contains chloride, bromide or iodide ions. In this case, chlorine, bromine or iodine will form at the anode, and may dissolve in the solution. This is because they are less reactive than oxygen.

As explained above, reactivity plays an important role in determining which electrolysis products are formed at the electrodes in aqueous solutions. In general:

  • Hydrogen will be generated at the cathode unless the metal cation is less reactive than hydrogen.
  • Oxygen will be generated at the anode unless the non-metal anion is less reactive than oxygen.

The reactions below represent the cathode and anode reactions when hydrogen and oxygen are produced during electrolysis of aqueous solutions:

cathode

4 H +(aq)+ 4 e-

2H2(g)

anode

4 OH-(aq)

O2 (g) + 2H2O (l) + 4e-

full reaction

4 H+(aq) + 4 OH-(aq)

2H2 (g) + O2 (g) + 2H2O (l)

The first two equations shown in this table are known as half-reactions. The overall chemical reaction must consist of what happens at the anode and the cathode combined. The reaction must also balance, with the number of electrons being given off at the anode equal to the number of electrons taken up at the cathode.

no Table of products formed

 Compound electrolysed Product at cathode Product at anode
molten sodium chloride

sodium (Na)

2Na+ + 2e-no 2Na

chlorine (Cl2)

2Cl-no Cl2 + 2e-

molten lead bromide

lead (Pb)

Pb2+ + 2e-noPb2+ + 2e-no Pb

bromine (Br2)

2Br-no Br2 + 2e-

aqueous potassium iodide

hydrogen (H2)

2H2O + 2e-no H2 + 2OH-

iodine(I2)

2I-no I2 + 2e-

aqueous sodium chloride

hydrogen (H2)

2H2O + 2e-no H2 + 2OH-

chlorine(Cl2)

2Cl-no Cl2 + 2e-

aqueous copper sulphate

copper (Cu)

2Cu2+ + 4e-no 2Cu

oxygen (O2)

2H2O no O2 + 4H+ + 4e-

hydrochloric acid

hydrogen (H2)

2H+ + 2e-no H2

chlorine (Cl2)

2Cl-no Cl2 + 2e-

molten calcium chloride

calcium (Ca)

Ca2+ + 2e-no Ca

chlorine (Cl2)

2Cl-no Cl2 + 2e-

Notice the following patterns in the table:

  • an element is produced at each electrode
  • a metal or hydrogen forms at the cathode
  • a non-metal or oxygen forms at the anode
no no Test yourself

Electrolysis of molten lead bromide
Drag the correct label to the appropriate place.

 



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