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The Periodic Table

The Halogens - Displacement Reactions.

The relative reactivity of the halogens, as described in group trends,
can be shown by displacement reactions.
These are similar to the metal displacement reactions
seen on page 5 of the reactivity series.

For example,
Bromine gas bubbled through a solution of potassium iodide in water
will displace (take the place of) the less reactive iodine,
forming iodine and potassium bromide.

bromine + potassium iodide     potassium bromide   +   iodine.
Br_2(g)    +     2KI_(aq)                      2KBr_(aq)      +        I_2(s)

Similarly, chlorine will displace less reactive halogens.
Chlorine will displace both bromine and iodine from the appropriate salt
(see extraction of bromine from sea water).

chlorine + potassium iodide     potassium chloride   +   iodine.
Cl_2(g)    +     2KI_(aq)                        2KCl_(aq)      +        I_2(s)

The equations can be written in terms of ions (called ionic equations).
For example, the last equation can be written as

Cl_2(g)   +    2I^-_(aq)                 2Cl^-_(aq)    +    I_2(s)

Potassium iodide, on the left, exists as
potassium ions (K⁺) and iodide ions (I^-)
and
potassium chloride, on the right, exists as
potassium ions (K⁺) and chloride ions (Cl^-).

Potassium ions (or other metal ions) can be left out of the ionic equation
because they do not take part in the reaction.
They are called 'spectator ions', as though they just sit back and watch!

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