Oxidation and reduction in electrolysis - Higher
Half equations
A half equationAn equation, involving ions and electrons, that describes the process happening at an electrode. is used to represent the reaction that happens at an electrodeA conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. during electrolysisThe decomposition (breakdown) of a compound using an electric current.. It shows what happens when ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. gain or lose electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. In half equations:
- electrons are shown as e-
- the numbers of atomThe smallest part of an element that can exist. of each elementA substance made of one type of atom only. must be the same on both sides
- the total chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge. on each side must be the same (usually zero)
Cathode reactions
Positively charged ions gain electrons at the cathodeThe negative electrode during electrolysis.. These are half equations for some reactions at the cathode:
Na+ + e- → Na
Pb2+ + 2e- → Pb
2H+ + 2e- → H2
Example
Balance the half equation for the formation of aluminium during electrolysis: Al3+ + e- → Al.
The balanced half equation is: Al3+ + 3e- → Al (because three negatively charged electrons are needed to balance the three positive charges on the aluminium ion).
Question
Write a balanced half equation for the formation of calcium from a calcium ion, Ca2+.
Ca2+ + 2e- → Ca
Anode reactions
Negatively charged ions lose electrons at the anodeThe positive electrode during electrolysis.. These are half equations for some reactions at the anode:
2Cl- → Cl2 + 2e-
2O2- → O2 + 4e-
Example
Write a balanced half equation for the formation of oxygen from hydoxide ions.
4OH- → 2H2O + O2 + 4e-
Question
Write a balanced half equation for the formation of bromine, Br2, from bromide ions, Br-.
2Br- → Br2 + 2e-
Oxidation and reduction
oxidationThe gain of oxygen, or loss of electrons, by a substance during a chemical reaction. and reductionThe loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. can be described in terms of electrons:
- oxidation is the loss of electrons
- reduction is the gain of electrons
This means that:
- reduction happens at the cathode
- oxidation happens at the anode
This can be seen by looking at the half equations above.
It helps to remember OIL RIG - Oxidation Is Loss of electrons, Reduction Is Gain of electrons.