Displacement reactions Halogens and halides can be made to fight for an electron!

Imagine some potassium bromide. Its an ionic solid which dissolves in water to form a colourless solution. It contains bromide ions Br- They are ions because they have an extra electron! If we add some chlorine, the 2 non metals fight for the extra electron. In this case chlorine wins. It is smaller so it can attract the electron more strongly.

2Br- + Cl2  Br2 + 2Cl- or overall: 2KBr + Cl2  2KCl + Br2

Because the chlorine is more reactive it can displace the bromide What would you observe? A clear colourless solution would turn orange Why? Because bromine is made, and it forms an orange solution.

Fluorine is most reactive Chlorine Bromine Iodine Remember why?

Cl-(aq) Br-(aq) I-(aq) Cl2 Br2 I2 Colorless / no reaction   Cl-(aq) Br-(aq) I-(aq) Cl2 Colorless / no reaction Turns red due to formation of bromine turns brown due to formation of iodine Br2 no reaction I2

Its hard to tell the difference between bromine in water and iodine in water. To make it a bit easier, we sometimes shake the solution with a hydrocarbon solvent (e.g.cyclohexane) Iodine makes a violet solution and bromine makes an orange solution.

Identifying halides You learned this at IGCSE! Silver halides are insoluble. If you add silver nitrate to a halide in solution, you form an insoluble precipitate. The colour of the precipitate can be used to identify the halide.

Fluoride – no precipitate Chloride – White precipitate which turns purple/grey in light Bromide - Cream coloured precipitate Iodide – yellow precipitate