Key Concepts: Halogens are diatomic molecules. Colour and state change down the group as molecular size increases. Reactivity decreases down the group — a more reactive halogen displaces a less reactive one from its salt solution.
Section A — Colours, States and Trends
1. Complete the table for the halogens at room temperature. [8]
| Halogen |
Formula |
State |
Colour |
| Fluorine |
|
|
|
| Chlorine |
|
|
|
| Bromine |
|
|
|
| Iodine |
|
|
|
2. Describe the trend in colour going down Group 7. [1]
3. Describe the trend in physical state and boiling point going down Group 7. [2]
Section B — Explaining the Trends
4. Explain why melting and boiling points increase down Group 7. [3]
5. Predict the state and colour of astatine at room temperature. Give a reason for each prediction. [3]
Section C — Reactivity and Displacement
6. Put chlorine, bromine and iodine in order of decreasing reactivity. [1]
7. Define a displacement reaction in terms of halogens. [1]
8. For each reaction below, predict the products (or state no reaction). [3]
a) Chlorine + potassium bromide solution
b) Bromine + potassium iodide solution
c) Iodine + potassium chloride solution
9. Describe the colour changes you would observe in reactions (a) and (b) in Question 8. [3]
Section D — Ionic Equations
10. Write the ionic equation for chlorine displacing bromide ions. [2]
11. Explain why iodine cannot displace bromide ions from potassium bromide solution. [2]
Total marks: 29
Mark Scheme
1. F₂: gas, pale yellow; Cl₂: gas, yellow-green; Br₂: liquid, red-brown; I₂: solid, grey/black (purple vapour) [8 — 2 per row]
2. Colours get darker/deeper down the group [1]
3. State changes from gas → liquid → solid going down; melting and boiling points increase [2]
4. Larger molecules have more electrons; stronger London/intermolecular (van der Waals) forces; more energy required to overcome these forces [3]
5. Solid — states increase from gas to liquid to solid so astatine will be solid; very dark/black — colours get darker down the group [3]
6. Cl₂ > Br₂ > I₂ [1]
7. A more reactive halogen displaces a less reactive halogen from a solution of its salt [1]
8. a) Potassium chloride + bromine (reaction occurs); b) Potassium bromide + iodine (reaction occurs); c) No reaction [3]
9. a) Solution turns orange/brown — bromine is formed; b) Solution turns dark brown — iodine is formed [3]
10. Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ [2]
11. Iodine is less reactive than bromine; it cannot oxidise bromide ions to bromine; a less reactive halogen cannot displace a more reactive one [2]