Reactivity Trends and Displacement Reactions

Why the halogens react

The Group 7 elements are the halogens. Their atoms all have seven electrons in the outer shell, so they have similar chemical properties. An atom can reach a full outer shell by gaining one electron:

$Cl + e^- \rightarrow Cl^-$

The product is a halide ion with a charge. Because elemental halogens occur as diatomic molecules, one halogen molecule gains two electrons altogether:

$Cl_2 + 2e^- \rightarrow 2Cl^-$

Halogen reactivity therefore depends on how readily their atoms attract and gain electrons.

Why reactivity decreases down Group 7

The order of reactivity is:

$F_2 > Cl_2 > Br_2 > I_2$

Going down the group, each element has an additional occupied electron shell. Its outer shell is farther from the nucleus, and the extra inner electrons provide more shielding.

Although nuclear charge also increases, the greater distance and shielding mean that the nucleus attracts an incoming electron less strongly. It therefore becomes harder for an atom to gain the electron needed to complete its outer shell. Reactivity consequently decreases down Group 7.

This gives a useful prediction: an element below iodine would be expected to be less reactive than iodine, following the group trend.

What happens in a halogen displacement reaction

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. In such a solution, the salt has separated into ions. For example, potassium bromide solution contains $K^+$ and $Br^-$ ions.

When chlorine water is added, chlorine atoms gain electrons from bromide ions. Chlorine becomes chloride ions while bromide ions form bromine molecules:

$Cl_2(aq) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(aq)$

The more reactive chlorine has replaced, or displaced, bromine from potassium bromide. The potassium ions remain unchanged; they are spectator ions. The same reaction can therefore be represented by the ionic equation:

$Cl_2(aq) + 2Br^-(aq) \rightarrow 2Cl^-(aq) + Br_2(aq)$

In general, if $X_2$ is more reactive than $Y_2$:

$X_2 + 2Y^- \rightarrow 2X^- + Y_2$

Predicting which mixtures react

Use the order chlorine $>$ bromine $>$ iodine:

• chlorine displaces bromine from bromide solutions and iodine from iodide solutions;
• bromine displaces iodine from iodide solutions, but cannot displace chlorine from chloride solutions;
• iodine cannot displace chlorine or bromine from chloride or bromide solutions.

For example:

$Cl_2(aq) + 2KI(aq) \rightarrow 2KCl(aq) + I_2(aq)$

$Br_2(aq) + 2KI(aq) \rightarrow 2KBr(aq) + I_2(aq)$

There is no reaction when iodine solution is mixed with potassium bromide because iodine is less reactive than bromine.

Using observations as evidence

Displacement can be detected because the newly formed halogen has a characteristic colour. In aqueous solution, chlorine is pale yellow-green, bromine is yellow-orange and iodine is brown. For instance, adding chlorine water to potassium bromide solution produces the yellow-orange colour of bromine.

A classroom comparison can place chlorine water, bromine water and iodine solution against chloride, bromide and iodide solutions on a spotting tile. Clean pipettes are needed to avoid contamination, and eye protection must be worn. Chlorine and bromine solutions and their fumes require particular care, so exposure should be limited and disposal must follow the teacher’s instructions.