group 17 halogens
Overview
# Group 17: Halogens - Cambridge A-Level Chemistry Summary ## Key Learning Outcomes Students must understand the physical and chemical properties of the halogens (F₂, Cl₂, Br₂, I₂), including their decreasing oxidising ability down the group, displacement reactions, and reactions with alkali to form halides and halates. The trend in volatility, colour intensity, and atomic radius must be explained using electronic structure and bonding principles. Halide ion reactions, including identification tests with silver nitrate and their reducing power trend (I⁻ > Br⁻ > Cl⁻), are essential topics. ## Exam Relevance This topic frequently appears in Paper 2 (structured questions) testing displacement reactions, redox equations, and trend explanations, as well as Paper 4 (practical) for halide identification. Questions typically require students
Core Concepts & Theory
Group 17 elements (the halogens) include fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂), and astatine (At₂). They exist as diatomic molecules with seven electrons in their outer shell, making them highly reactive oxidising agents.
Physical Properties Trend: Down the group, atomic radius increases, electronegativity decreases (F: 4.0 → I: 2.5), and melting/boiling points increase due to stronger van der Waals forces between larger molecules. At room temperature: F₂ and Cl₂ are gases, Br₂ is a volatile liquid, I₂ is a solid.
Oxidising Power: Decreases down the group (F₂ > Cl₂ > Br₂ > I₂). A halogen higher in the group can displace one below from its halide solution:
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Disproportionation Reactions: In alkali, chlorine undergoes disproportionation where it is simultaneously oxidised and reduced:
Cl₂(aq) + 2OH⁻(aq) → Cl⁻(aq) + ClO⁻(aq) + H₂O(l) (cold dilute)
3Cl₂(aq) + 6OH⁻(aq) → 5Cl⁻(aq) + ClO₃⁻(aq) + 3H₂O(l) (hot concentrated)
Halide Ion Tests: Add dilute HNO₃ and AgNO₃(aq). Silver halide precipitates form:
- AgCl: white (soluble in dilute NH₃)
- AgBr: cream (soluble in concentrated NH₃)
- AgI: yellow (insoluble in NH₃)
Key Cambridge Definition: Oxidising agent = a species that accepts electrons and is itself reduced.
Detailed Explanation with Real-World Examples
Water Disinfection Analogy: Think of chlorine as a 'molecular security guard' at swimming pools. Cl₂ reacts with water forming chloric(I) acid (HClO), a powerful disinfectant:
Cl₂(g) + H₂O(l) ⇌ HClO(aq) + HCl(aq)
HClO destroys bacteria by oxidising their cell walls—this is why pool water smells of chlorine!
Displacement Reactions in Nature: Seaweed manufacturing is a real-world application. When chlorine gas is bubbled through seawater containing bromide ions, bromine is displaced and extracted commercially. This demonstrates the oxidising power trend: stronger oxidisers displace weaker ones, like a 'chemical hierarchy'.
Hydrogen Halide Formation: When halogens react with hydrogen:
H₂(g) + X₂ → 2HX(g)
The vigour decreases down the group (F₂ explosive even in dark; I₂ requires catalyst). Think of it as fluorine being 'desperate' for electrons due to high electronegativity.
Halide Salts in Medicine: Silver halides are used in traditional photography—AgBr darkens on light exposure. Iodised salt (KI) prevents thyroid deficiency, showing how essential halides are biologically.
Acid Strength Pattern: HF is a weak acid (forms H-bonds, partially dissociates) while HCl, HBr, and HI are strong acids. Imagine H-F bonds as 'sticky' due to high electronegativity, making H⁺ release difficult. Down the group, bonds weaken (larger anions = less orbital overlap), so HI dissociates most readily, like a 'loosely held handshake'.
Worked Examples & Step-by-Step Solutions
**Example 1: Displacement Reaction** *Question:* Chlorine water is added to aqueous potassium bromide. Write an ionic equation and explain the colour change observed. [3 marks] *Solution:* **Step 1:** Identify the halogen and halide: Cl₂ (higher in group) displaces Br⁻ **Step 2:** Write the ioni...
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Key Concepts
- Halogens: The elements in Group 17 of the Periodic Table (Fluorine, Chlorine, Bromine, Iodine, Astatine).
- Diatomic Molecules: Halogens exist as X₂ molecules in their elemental state (e.g., Cl₂).
- Electronegativity: The power of an atom to attract the electron pair in a covalent bond.
- Oxidising Agent: A substance that causes another substance to be oxidised (loses electrons) while itself being reduced (gains electrons).
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Exam Tips
- →Memorise the colours and physical states of the halogens at room temperature, and how they change down the group.
- →Understand and be able to explain the trends in reactivity (oxidising power) and physical properties (melting/boiling points, volatility) down Group 17, linking them to atomic structure and intermolecular forces.
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