Reactivity of Halogens

Halogens are known for their high reactivity. However, the reactivity of halogens decreases as you move down the group.

Fluorine is the most reactive halogen, followed by chlorine, bromine and so on. This means that fluorine is more likely to react and gain an electron than chlorine, followed by bromine, and so on.

The reason for this trend in reactivity can be explained by two main factors:

  • Firstly, as you move down the group, each element adds another shell. This means that the outer shells are further away from the protons, which carry a positive charge. Electrons are strongly attracted to the positive charge from the protons in the nucleus. Therefore, as the distance between the outer shells and the nucleus increases, the electrons are less strongly attracted.
  • The second factor that affects the reactivity of halogens is shielding. This occurs when the inner electrons block some of the positive charge from the outer electrons. This reduction in positive charge makes it more difficult for the atom to attract and gain an electron to complete its outer shell.

So, as we move down group 7, the greater distance between the outer shells and the nucleus, combined with the increased shielding, results in a decrease in reactivity.

Displacement Reactions

When a more reactive halogen is added to a solution of a less reactive halogen, a displacement reaction takes place. In this reaction, the more reactive halogen takes the place of the less reactive halogen. This is because the more reactive halogen is better able to attract electrons from the less reactive halogen, effectively displacing it.

  • This is called a halogen displacement reaction

For example, if we add chlorine to a solution of potassium bromide, a displacement reaction occurs:

Chlorine + Potassium bromide → Potassium chloride + Bromine

Cl2(aq) + 2KBr(aq)2KCl(aq) + Br2(aq)

Here, since chlorine is more reactive than iodine, it replaces the iodine in the potassium iodide (KI) solution. This reaction forms potassium chloride (KCl) and bromine (Br2). Due to the bromine, the solution will turn brown.

Similarly, if we add chlorine to a solution of potassium iodide, the following reaction occurs:

Chlorine + Potassium Iodide → Potassium chloride + Iodine

Cl2(aq) + 2KI(aq)2KCl(aq) + I2(aq)

Here, chlorine is more reactive than iodine, so it takes the place of the iodine in the potassium iodide (KI) solution. The reaction forms potassium chloride (KCl) and iodine (I2).

However, if we add a less reactive halogen to a solution containing a more reactive halide ion, no reaction will occur. For example, if we add iodine to a solution of potassium bromide, no reaction will occur because iodine is less reactive than bromine.

Ionic equations – Higher

The halogen displacement reactions are a type of redox reaction, in which the halogens gain electrons and become reduced. Meanwhile, the halide ions lose electrons and become oxidised. These reactions involve a transfer of electrons from one substance to another, which forms new compounds.

For example, let’s look at the reaction between bromine and potassium iodide:

Bromine + Potassium iodide → Potassium bromide + Iodine

Br2 + 2KI → 2KBr + I2

The reaction can be represented by the ionic equation:

Br2 + 2I → I2 + 2Br

Bromine gains electrons and becomes reduced to form bromide ions present in potassium bromide. Meanwhile, the iodide ions lose electrons and become oxidised, forming iodine.

Another example is the reaction between chlorine and iodide ions, in which chlorine gains electrons and becomes reduced to form chloride ions. Meanwhile, the iodide ions lose electrons and become oxidised to form iodine. The ionic equation for this reaction is:

Cl2 + 2I 2Cl + I2

Ionic equations clearly represent the behaviour of ions in solution during a chemical reaction, showcasing the charged particles involved.