Chemistry › Group 2 and Group 7
Group 2 and Group 7
Unit C13 · 4 lessons.
Down Group 2 the metals react more vigorously with water, the hydroxides become more soluble and the sulfates less soluble, which gives the barium chloride test for a sulfate. Down Group 7 the halogens become weaker oxidising agents and the halide ions stronger reducing agents, which explains the displacement reactions, the silver nitrate test and the reactions with concentrated sulfuric acid. Chlorine in water ends the unit.
A trend in one property is easily written as a trend in its opposite. Reactivity of the halogens decreases down the group, but the reducing power of the halide ions increases, and an explanation of either needs the size of the atom or ion and its shielding, not only the word reactive.
AQA Papers 1 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1 and 2 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.2.2, 3.2.3.1, 3.2.3.2 · OCR A 3.1.2, 3.1.4, 3.1.3 · Edexcel 4.1 to 4.15 · CAIE 10.1, 11.1, 11.2, 11.3, 11.4
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Group 2: trends down the group and reactions with water
The Group 2 elements beryllium to barium, their electron configurations and the +2 oxidation state, the trends in atomic radius, first ionisation energy and melting point with the magnesium anomaly, reactivity down the group, the reactions of magnesium, calcium, strontium and barium with water, magnesium with steam, the reactions with oxygen, chlorine and dilute hydrochloric and sulfuric acids, the oxides with water and the rising pH of the solutions, the oxides as bases, and the extraction of titanium from TiCl₄ with magnesium as the reducing agent.
- Group 2 compounds: solubility, uses and the sulfate test
The solubility of the Group 2 hydroxides increasing down the group and of the sulfates decreasing, magnesium hydroxide as an antacid, calcium hydroxide on acid soil, calcium oxide and calcium carbonate in flue-gas desulfurisation, barium sulfate as a barium meal, the thermal decomposition of calcium carbonate, the test for sulfate ions with acidified barium chloride and why the acid is added, the tests that identify Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺, flame colours for Group 2 and for lithium, sodium and potassium, the thermal decomposition of the Group 2 carbonates and nitrates and the rise in their stability down the group explained by polarising power, the Group 1 carbonates and nitrates compared, and the procedure for comparing how easily they decompose.
- Group 7: trends down the group and displacement reactions
The halogens fluorine to iodine, their states and colours, the trend in boiling point explained by van der Waals forces, the trends in atomic radius and electronegativity, the halogens as oxidising agents and the decrease in oxidising power down the group, displacement reactions of chlorine, bromine and iodine with halide solutions, the colours in water and in cyclohexane, ionic equations and half-equations, the bond enthalpies of the halogens and the weak F–F bond, the halogens with hydrogen, the thermal stability of the hydrogen halides, and chlorine in water treatment.
- Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water
The halide ions as reducing agents and the increase in reducing ability down the group, the reactions of solid sodium halides with concentrated sulfuric acid with equations and observations, the silver nitrate test with the colours of the precipitates and their solubility in ammonia, why the test is acidified with nitric acid, identifying a halide from the mass of its silver halide precipitate, the hydrogen halides dissolving in water as acids and giving white smoke with ammonia, chlorine with water as disproportionation, chlorine with water in sunlight, chlorine with cold dilute sodium hydroxide to make bleach, the benefits and risks of chlorinating water, and the method of required practical 4 for identifying anions and cations.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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