Trends and Reactions of Halogens
Group 17 elements, the halogens, exhibit clear, predictable patterns in their physical properties and chemical reactivity based on their atomic structure. Understanding these trends is crucial for predicting the outcomes of reactions, particularly halogen displacement.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Halogens (Fluorine, Chlorine, Bromine, Iodine) exist as diatomic molecules (F2, Cl2, etc.) and have 7 outer shell electrons.
- Physical properties trend: As you descend the group, boiling and melting points increase. This is because the molecules become larger with more electrons, leading to stronger intermolecular van der Waals forces that require more energy to overcome.
- At room temperature, this trend results in F2 and Cl2 being gases, Br2 being a liquid, and I2 being a solid.
- Chemical reactivity trend: Reactivity decreases as you descend the group. Fluorine is the most reactive, and iodine is the least reactive.
- The decrease in reactivity is because atoms get larger down the group. The outer shell is further from the nucleus and more shielded, making it harder for the nucleus to attract an eighth electron to form a halide ion (X-).
- A more reactive halogen will displace a less reactive halide from its aqueous salt solution, providing direct evidence for the reactivity trend.
Diagram
Formulae
X2(aq) + 2Y-(aq) → 2X-(aq) + Y2(aq) To represent a halogen displacement reaction, which only occurs if halogen X is more reactive (higher up Group 17) than halogen Y.
Definitions
- Displacement Reaction (Halogens)
- A redox reaction where a more reactive halogen element oxidises a less reactive halide ion in an aqueous solution, taking its place and forming the less reactive halogen element.
Worked example
An aqueous solution of sodium bromide is mixed with chlorine water. Describe the expected observation and provide the ionic equation for the reaction.
- 1
First, identify the positions of chlorine and bromine in Group 17.
Chlorine is above bromine.
- 2
Recall the reactivity trend for halogens:
reactivity decreases down the group.
This means chlorine is more reactive than bromine.
- 3
Since chlorine (Cl2) is more reactive than bromine, it can displace the bromide ions (Br-) from the sodium bromide solution.
- 4
Predict the observation.
The initially colourless sodium bromide solution will change colour as bromine (Br2) is formed.
Aqueous bromine is orange or yellow-brown.
- 5
Construct the ionic equation.
Chlorine molecules react with bromide ions.
Chlorine atoms gain an electron each to become chloride ions (Cl-), and bromide ions lose an electron each to form bromine molecules (Br2).
The sodium ions are spectator ions and are not included.
- 6
The balanced ionic equation is:
Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq).
Answer: The colourless solution will turn orange. The ionic equation is Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq).
Common mistakes
- ×Forgetting the condition for displacement: A halogen only displaces a *less reactive* halide below it in the group. For example, iodine solution added to potassium chloride will cause no reaction.
- ×Confusing reactivity trends: Halogen reactivity *decreases* down the group, whereas alkali metal (Group 1) reactivity *increases*. Don't mix them up.
- ×Mistaking the states: The reaction involves a halogen *element* (e.g., Cl2) reacting with a *halide ion* in solution (e.g., Br-). Students sometimes write the halide reacting with another halide.
- ×Incorrectly identifying colours: The colours of the halogens in aqueous solution (Cl2 pale green/colourless, Br2 orange, I2 brown) are key to describing observations correctly.
No-calculator tips
- ✓Use the periodic table as a 'reactivity ladder' for halogens. An element can only 'kick out' an ion below it on the ladder.
- ✓To remember the physical states at room temperature, think of them getting 'heavier' and more solid down the group: Gas (F2, Cl2) → Liquid (Br2) → Solid (I2).
- ✓The reason for reactivity is about electron *attraction*. A smaller atom has its nucleus closer to the outside, giving it a stronger pull on a new electron.