C10 Half Equations

Question 1

What does a half-equation show?

Answer 1

A half-equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e-. A half-equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.

Question 2

Positive metal ions

Answer 2

When positive metal ions (cations) arrive at the negative electrode (the cathode), they gain electrons to form neutral metal atoms. This is called reduction. For example:
Pb2+ + 2e- → Pb

Question 3

Negative non-metal ions

Answer 3

When negative non-metal ions (anions) arrive at the positive electrode (the anode), they lose electrons to form neutral atoms or molecules. This is called oxidation. For example:
2Br- → Br2 + 2e-

Question 4

Writing half equations: al3+

Answer 4

Cations go to the cathode. They need to gain enough electrons to make them neutral. So an Al3+ ion needs to gain three electrons:
Al3+ + 3e- → Al

Question 5

Writing half equations: chlorine

Answer 5

Half-equations for non-metal anions are more difficult to balance. For example, chloride ions make chlorine gas. Most non-metal elements formed in electrolysis are diatomic molecules (eg Cl2). For example:
Cl- → Cl2
Add in two electrons to balance the charge so that both sides have the same charge. The two electrons need to go on the right-hand side, so that both sides have an overall charge of -2. For example:
2Cl- → Cl2 + 2e-