6.3 Halogen displacement reactions

Question 1

What is a displacement reaction?

Answer 1

When a more reactive element displaces (kicks out) a less reactive element from an aqueous solution of its salt

Question 2

Give an example of a halogen displacement reaction

Answer 2

Cl is more reactive than bromine, so:

Cl2 (g) + 2KBr (aq) —> Br2 (aq) + 2KCl (aq)

… note that a less reactive halogen witll NOT displace a more reactive halogen, e.g. bromine will not displace Cl from NaCa solution

Question 3

What is a redox reaction?

Answer 3

A reaction where REDUCTION and OXIDATION both occur

Reduction = gaining electron(s)

Oxidation = losing electron(s)

Question 4

Write an ionic equation to show what happens when chlorine is added to a bromine salt in solution

Answer 4

Cl2 + 2Br- —> Br2 + 2Cl-

Chlorine is reduced here and bromine is oxidised

Question 5

What happens visually when Cl is added to (i) KBr and (ii) KI

Answer 5

When Cl added to KBr, orange solution is formed (from the displaced bromine)

WHen Cl added to KI, brown solution is formed (from the displaced iodine)

Similar visuals are seen for other displacement reactions (e.. Br added to KI will form brown solution due to displaced iodine)

Question 6

Describe how you would conduct a halogen displacement reaction

Answer 6

1. Measure some halide salt solution in a test tube
2. Add a few drops of a differen halogen and shake tube gently
3. If there is a colour change then a displacement reaction has happened (if no colour change, then no displacement reaction has occurrec)