A half equation describes what happens at one electrode
Definition
Half equation
An equation showing the electrons gained or lost by the species reacting at one electrode.
- Electrolysis runs two different reactions at once, one at each electrode.
- A half equation separates them, describing one electrode on its own.
- The reacting ion goes on one side and the product on the other.
- Electrons are then added to whichever side makes the total charge balance.
- They sit on the left when the ion gains them and on the right when it loses them.
Key Idea
- One equation per electrode, because the two reactions are different.
- Electrons are the currency, gained at one electrode and lost at the other.
Writing and balancing a half equation
- Identify the ion that reacts at that electrode and the product it forms.
- Balance the atoms first, using whole numbers.
- Count the total charge on each side.
- Add electrons until those two totals are equal.
- In Pb2++2e−→Pb\text{Pb}^{2+} + 2\text{e}^{-} \rightarrow \text{Pb}Pb2++2e−→Pb the left side is 2+2+2+ and 2−2-2− together, and the right side is zero.
- In 2Br−→Br2+2e−2\text{Br}^{-} \rightarrow \text{Br}_2 + 2\text{e}^{-}2Br−→Br2+2e− the left side is 2−2-2− and so is the right.
- Check the atoms and the charge before accepting any half equation.
Example
- Cathode: Cu2++2e−→Cu\text{Cu}^{2+} + 2\text{e}^{-} \rightarrow \text{Cu}Cu2++2e−→Cu, atoms balanced and charge zero on both sides.
- Anode: 2Cl−→Cl2+2e−2\text{Cl}^{-} \rightarrow \text{Cl}_2 + 2\text{e}^{-}2Cl−→Cl2+2e−, atoms balanced and charge 2−2-2− on both sides.
Oxidation is loss of electrons and reduction is gain
Definition
Oxidation
The loss of electrons by a substance.
Definition
Reduction
The gain of electrons by a substance.
- A substance that loses electrons has been oxidised.
- A substance that gains electrons has been reduced.
- OIL RIG is the usual reminder: oxidation is loss, reduction is gain.
- In a reduction half equation the electrons appear among the reactants, on the left.
- In an oxidation half equation the electrons appear among the products, on the right.
- In electrolysis these definitions are about electrons rather than about oxygen.
Common Mistake
- Oxidation here is loss of electrons, not gain of oxygen.
- Reduction here is gain of electrons, not loss of oxygen.
Reduction at the cathode, oxidation at the anode
Definition
Cathode
The negative electrode, which attracts positive ions.
Definition
Anode
The positive electrode, which attracts negative ions.
- Positive ions travel to the cathode and gain electrons there.
- Gaining electrons is reduction, so reduction happens at the cathode.
- Negative ions travel to the anode and lose electrons there.
- Losing electrons is oxidation, so oxidation happens at the anode.
- That pairing holds even though the cathode is the negative electrode and the anode the positive one.
- Molten lead bromide shows both at once: Pb2++2e−→Pb\text{Pb}^{2+} + 2\text{e}^{-} \rightarrow \text{Pb}Pb2++2e−→Pb at the cathode and 2Br−→Br2+2e−2\text{Br}^{-} \rightarrow \text{Br}_2 + 2\text{e}^{-}2Br−→Br2+2e− at the anode.
Exam technique
- Labelling each half equation cathode or anode is what makes the oxidation and reduction clear.
- The electrode charge says where an ion goes; the electron transfer says whether it was oxidised or reduced.
- An unbalanced charge means the wrong number of electrons, so that check comes before anything else.
Self review
- What does a half equation show?
- At which electrode does reduction happen?
- What does oxidation mean in terms of electrons?
- Write the cathode half equation for Pb2+\text{Pb}^{2+}Pb2+.
- Write the anode half equation for Br−\text{Br}^{-}Br−.
