Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Displacement rule
A more reactive halogen displaces a less reactive halide ion from solution. Chlorine displaces bromide and iodide; bromine displaces iodide; iodine displaces neither chloride nor bromide.
Example equations
Cl2(aq) + 2KBr(aq) -> 2KCl(aq) + Br2(aq). The net ionic equation is Cl2 + 2Br- -> 2Cl- + Br2. Chlorine gains electrons and is reduced; bromide loses electrons and is oxidised.
Observations
The product halogen may colour the aqueous solution or an organic solvent layer if one is used in a described experiment. Because colours can overlap, conclusions should use the full pattern and any supplied colour reference.
Deducing order
If X2 displaces Y- but Y2 cannot displace X-, X is more reactive than Y. Build a comparison table rather than judging from one isolated result.
Practical or data skill
Use a displacement matrix to deduce the reactivity order of three labelled halogens and write ionic equations.
Examination tip
The more reactive halogen molecule is reduced to its halide ion.
Review questions and suggested answers
Question 1
Can bromine displace chloride?
Suggested answer
No.
Question 2
Write the ionic equation for chlorine with bromide.
Suggested answer
Cl2 + 2Br- -> 2Cl- + Br2.
Question 3
Which is more reactive, bromine or iodine?
Suggested answer
Bromine.
