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OCR A Level Chemistry A

3.1.3
The Halogens

Section 3.1.3 covers the halogens as diatomic molecules and the trend in their boiling points, the s²p⁵ outer shell and the formation of 1− ions, the trend in reactivity shown by displacement reactions and explained by atomic radius and shielding, disproportionation in the reactions of chlorine with water and with cold dilute sodium hydroxide, the benefits and risks of chlorine in water treatment, and the precipitation reactions of the halide ions with silver ions followed by ammonia.

Exam Paper
Paper 1 & 3
H432/01 and H432/03
Specification Points
3.1.3 (a) – (g)
7 learning outcomes
Topic Parts
4 pages
Revision notes available
Exam Board
OCR A
H432 (2015 onwards)

Revision Notes

Work through 3.1.3 The Halogens in a structured sequence.

F₂ to I₂
Halogen
Trends
1
Available

Trends in the Halogens

The trends in Group 17: states, colours, boiling points, electronegativity, reactivity and predictions for fluorine and astatine.

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Displacement
Oxidising
Agents
2
Available

Halogens as Oxidising Agents

The halogens as oxidising agents: displacement reactions, colours in water and cyclohexane, reactions with metals and hydrogen, and the reactivity trend.

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Water and bleach
Chlorine
Reactions
3
Available

Disproportionation Reactions of Chlorine

The disproportionation of chlorine with water and with cold and hot alkali, bleach, water treatment and analogous halogen reactions.

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Cl⁻, Br⁻, I⁻
Halide
Ions
4
Available

Halide Ions: Reducing Agents and Tests

Halide ions: reducing power with concentrated sulfuric acid, the acidified silver nitrate test and ammonia, and the hydrogen halides with ammonia and water.

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Specification Coverage

3.1.3 The Halogens – OCR A Level Chemistry A

The following OCR A learning outcomes state what candidates should be able to do. Wording is taken from the OCR A Level Chemistry A (H432) specification.

3.1.3 The halogens

3.1.3(a)
existence of halogens as diatomic molecules and explanation of the trend in the boiling points of Cl₂, Br₂ and I₂ in terms of induced dipole–dipole interactions (London forces)
3.1.3(b)
the outer shell s²p⁵ electron configuration and the gaining of one electron in many redox reactions to form 1− ions
3.1.3(c)
the trend in reactivity of the halogens Cl₂, Br₂ and I₂, illustrated by reaction with other halide ions
3.1.3(d)
explanation of the trend in reactivity from the decreasing ease of forming 1− ions, in terms of attraction, atomic radius and electron shielding
3.1.3(e)
explanation of the term disproportionation as oxidation and reduction of the same element, illustrated by:
i
the reaction of chlorine with water as used in water treatment
ii
the reaction of chlorine with cold, dilute aqueous sodium hydroxide, as used to form bleach
iii
reactions analogous to those specified in (i) and (ii)
3.1.3(f)
the benefits of chlorine use in water treatment (killing bacteria) contrasted with associated risks (hazards of toxic chlorine gas and possible risks from formation of chlorinated hydrocarbons)
3.1.3(g)
the precipitation reactions, including ionic equations, of the aqueous anions Cl⁻, Br⁻ and I⁻ with aqueous silver ions, followed by aqueous ammonia, and their use as a test for different halide ions