Group 7 (VII) displacement reactions - Higher tier

In a displacement reaction, a more reactive halogen (Cl2, Br2, I2) is added to a halide solution. The more reactive halogen pushes out and replaces the less reactive halogen.

Chlorine is more reactive than the iodine in potassium iodide solution:

Word equation:chlorine + potassium iodide → iodine + potassium chloride
Symbol equation:Cl2(g) + 2KI(aq) → I2(aq) + 2KCl(aq)
Ionic equation:Cl2(g) + 2I-(aq) → I2(aq) + 2Cl-(aq)

Observations: the colourless solution changes to brown

Chlorine is also more reactive than the bromine in potassium bromide solution:

Word equation:chlorine + potassium bromide → bromine + potassium chloride
Symbol equation:Cl2(g) + 2KBr(aq) → Br2(aq) + 2KCl(aq)
Ionic equation:Cl2(g) + 2Br-(aq) → Br2(aq) + 2Cl-(aq)

Observations: the colourless solution changes to orange

When a Group 7 element reacts, it gains one electron to form a negative ion with a stable electronic configuration – a full outer shell.

As we move down the column of Group 7 elements, their reactivity decreases. In each, the outer shell is further from the nucleus and the incoming electron is not as easily attracted to it.

Half equations can be used to represent these reactions (higher tier):

Chloride ions from a chlorine molecule:

Cl2 +2e- → 2Cl-

A chloride ion from a chlorine atom:

Cl +e- → Cl-