2.15 understand these displacement reactions as redox reactions.

By looking at the equation from Section 2.14 written as an ionic equation, we can see these are redox reactions.

Cl2(aq) + 2K+(aq) + 2I-(aq) → 2K+(aq) + 2Cl-(aq) + I2(aq)

We can see here that the potassium ions have not been involved in the reaction at all.  We call these “spectator ions” and we can leave them out of the equation:

Cl2(aq) + 2I-(aq) → 2Cl-(aq) + I2(aq)

Cl2 molecules becoming Cl- ions involves electron gain, so the halogen is reduced in these reactions.

I- ions becoming I2 molecules involves electron loss, so the halide ion is oxidised in these reactions.

Exam tip!

Always remember to call the elements and their ions by the correct names or use the correct formulae and symbols.  The examiners are VERY fussy!  They won’t let you have a mark if you write “iodine or iodine ions are oxidised” in the above example.  You have to write “iodide ions are oxidised”.

·        The ion names end in –IDE

·        The element names end in –INE