Oxidation is loss and reduction is gain of electrons
Definition
Oxidation (electron transfer)
The loss of electrons by an atom, ion or molecule in a chemical reaction.
Definition
Reduction (electron transfer)
The gain of electrons by an atom, ion or molecule in a chemical reaction.
- The memory aid OIL RIG stands for oxidation is loss, reduction is gain, where the loss and gain refer to electrons.
- When a magnesium atom forms a magnesium ion, it loses two electrons, so it is oxidised: Mg→Mg2++2e−\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-Mg→Mg2++2e−
- When a copper(II) ion forms a copper atom, it gains two electrons, so it is reduced: Cu2++2e−→Cu\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}Cu2++2e−→Cu
- Losing negative electrons makes a species more positive, while gaining electrons makes it less positive or more negative.
- These definitions extend the oxygen definitions, so reactions with no oxygen at all can still be redox reactions.
Common Mistake
Do not mix up the two definitions, because oxidation means gaining oxygen but losing electrons.
Redox reactions transfer electrons from one species to another
Definition
Redox reaction
A reaction in which one substance is oxidised while another is reduced.
- When magnesium is added to copper(II) sulfate solution, a brown coating of copper forms and the blue colour fades:
Mg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}Mg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)
- Each magnesium atom transfers two electrons to a copper(II) ion.
- Magnesium atoms lose electrons, so magnesium is oxidised.
- Copper(II) ions gain electrons, so the copper(II) ions are reduced.
- The sulfate ions do not change, so they are spectator ions and are left out of the equation.
- The number of electrons lost always equals the number gained, because electrons are not created or destroyed.
Note
Magnesium is called the reducing agent because it gives electrons to the copper(II) ions, so the reducing agent is always the species that is oxidised.
Comparing charges shows which species lost or gained electrons
Definition
Species
Any atom, molecule or ion that takes part in a chemical reaction.
- Write the charge on each species on both sides of the equation, counting an uncharged atom or molecule as zero.
- A charge that becomes more positive, such as 000 to 2+2+2+ or 1−1-1− to 000, shows electron loss and oxidation.
- A charge that becomes less positive, such as 2+2+2+ to 000 or 000 to 1−1-1−, shows electron gain and reduction.
- In Cl2+2Br−→2Cl−+Br2\text{Cl}_2 + 2\text{Br}^- \rightarrow 2\text{Cl}^- + \text{Br}_2Cl2+2Br−→2Cl−+Br2, each chlorine atom goes from 000 to 1−1-1−, so chlorine is reduced.
- Each bromide ion goes from 1−1-1− to 000, so the bromide ions are oxidised to bromine.
Common Mistake
Do not write that bromine is oxidised, because the species that loses electrons is the bromide ion and bromine is the product.
Electron definitions explain burning and electrode reactions
- When magnesium burns, 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO, each magnesium atom loses two electrons to become Mg2+\text{Mg}^{2+}Mg2+.
- Each oxygen atom gains two electrons to become O2−\text{O}^{2-}O2−, so oxygen is reduced while magnesium is oxidised.
- Here the two definitions agree, because magnesium gains oxygen and also loses electrons.
- In electrolysis, positive ions gain electrons at the cathode, so they are reduced.
- Negative ions lose electrons at the anode, so they are oxidised.
Exam technique
- When a question asks which species is oxidised, name the atom, ion or molecule from the equation and state how many electrons it loses.
- Give the reduced species in the same way, for example 'copper(II) ions are reduced because each gains two electrons'.
Self review
- What does OIL RIG stand for?
- In Zn+Cu2+→Zn2++Cu\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}Zn+Cu2+→Zn2++Cu, which species is oxidised?
- Why must the number of electrons lost in a redox reaction equal the number gained?
- In Cl2+2I−→2Cl−+I2\text{Cl}_2 + 2\text{I}^- \rightarrow 2\text{Cl}^- + \text{I}_2Cl2+2I−→2Cl−+I2, which species is reduced?
- When magnesium burns in oxygen, which species gains electrons?
