Redox Reactions

REDOX REACTIONS

Many chemical reactions are redox reactions, where reduction and oxidation occur at the same time. Traditionally, oxidation was gain of `oxygen`, such as complete combustion of carbon (e.g. coal) to form carbon dioxide.

C(s) + O₂(g) → CO2(g)

Reduction was the opposite, `loss of oxygen` in this case zinc oxide is being reduced (loss of oxygen) and the magnesium metal oxidised (gain of oxygen):

Mg(s)+ ZnO(s) → MgO(s) + Zn(s)

More formally, the definition is:

• OXIDATION is LOSS of electrons

• REDUCTION is GAIN of electrons

Shown as ionic half-equations, it is easy to see what`s going on:

Mg(s) → Mg²⁺(s) +2e^-: OXIDATION (loss of electrons)
Zn²⁺(s) + 2e^- → Zn(s): REDUCTION (gain of electrons)

Oxidation and reduction always occur in the same equation.


There is a very useful acronym: O I L R I G



At A Level, oxidation is INCREASE in oxidation number (Magnesium from 0 to +2) reduction is DECREASE in oxidation number (Zn from +2 to 0). What is it for the first reaction? Carbon from 0 to +4 (oxidation) and oxygen from 0 to -2 (reduction). REDOX reactions occur all over the place, in theory and experiments, such as metal displacement reactions or a redox titration [MnO₄^- (aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺ (aq) manganese(VII) is reduced to Mn(II) and Fe(II) oxidised to Fe(III)].

Note: acid-base titrations are not redox reactions.

Reducing agents get OXIDISED and oxidising agents get REDUCED. So it is really important to be clear about the reducing agent (itself oxidised) and the other species gets reduced and oxidising agents (itself reduced) oxidisise the other species (hence their names!). In the following equation, what is the oxidising agent?

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Answer: hydrochloric acid (more specifically H⁺(aq)), as it oxidises Mg to Mg²⁺.

A combustion of hydrocarbons redox reactions?

Yes. For example:

CH₄(g)+ 2O₂(g) → CO₂(g) + 2H₂O(g)

It is therefore the oxidation of the hydrocarbon. In this example, the oxidation number of carbon changes from -4 to +4 (oxidation) and oxygen from 0 to -2 (reduction).