How and why do the reactivity, solubility of hydroxides and solubility of sulfates change down Group 2?
The trend in atomic radius, first ionisation energy and melting point down Group 2. The reactions of Group 2 elements with water. The trend in solubility of the hydroxides and sulfates of Group 2 elements. Uses of magnesium in the extraction of titanium, of calcium hydroxide in agriculture, of barium sulfate in medicine and of Group 2 compounds in neutralising acidity.
A focused answer to the AQA A-Level Chemistry 3.2.2 specification points on Group 2. Covers the trends in atomic radius, ionisation energy and reactivity down the group, reactions with water, the opposite solubility trends of the hydroxides and sulfates, and the key uses of Group 2 compounds.
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What this dot point is asking
AQA wants you to describe the trends in atomic radius, ionisation energy and melting point down Group 2, the reactions of the metals with water, the opposite solubility trends of the hydroxides and the sulfates, and the named uses of Group 2 compounds.
Trends down the group
Down Group 2 the atomic radius increases (each element has an extra shell), the first ionisation energy decreases (more shielding and a larger radius, so outer electrons are held less tightly), and reactivity increases because the two outer electrons are lost more easily.
Melting point generally decreases down the group (with magnesium slightly out of line because of its different crystal packing) because the metal ions get larger while the charge stays at , so the delocalised electrons are spread over a larger ion and the metallic bonding weakens. First ionisation energy also falls down the group, which underlies the increase in reactivity: the elements react by losing their two outer electrons, and the easier that loss, the more reactive the metal.
Reaction with water
The metals react with water to form a hydroxide and hydrogen gas, and the reaction gets more vigorous down the group.
Magnesium reacts only very slowly with cold water but reacts readily with steam to give magnesium oxide and hydrogen:
Solubility trends (the two opposite trends)
The insolubility of is the basis of the test for sulfate ions: adding acidified barium chloride to a solution gives a white precipitate if sulfate is present.
The two opposite trends are explained by which energy term dominates as the cation grows. Solubility depends on the balance between the lattice enthalpy (holding the solid together) and the hydration enthalpies of the ions (favouring dissolving). For the hydroxides, the small hydroxide ion means the lattice enthalpy falls faster down the group than the hydration enthalpy of the metal ion, so dissolving becomes more favourable and solubility rises. For the sulfates, the large sulfate ion means the lattice enthalpy changes little down the group, while the hydration enthalpy of the metal ion falls markedly, so dissolving becomes less favourable and solubility drops. You are not required to calculate these at A-level, but the qualitative reasoning is examinable.
Uses of Group 2 compounds
Try this
Q1. Write an equation for the reaction of calcium with cold water. [2 marks]
- Cue. .
Q2. State and explain the trend in the solubility of Group 2 hydroxides. [2 marks]
- Cue. Solubility increases down the group; so is sparingly soluble but is more soluble.
Q3. Describe the test for sulfate ions. [2 marks]
- Cue. Add acidified barium chloride; a white precipitate of confirms sulfate.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksExplain why the reactivity of the Group 2 elements increases down the group.Show worked answer →
A 3-mark answer links reactivity to ionisation energy.
Group 2 reactions involve losing two outer electrons to form a 2+ ion. Down the group the atomic radius increases and there is more shielding, so the outer electrons are held less tightly and the first and second ionisation energies decrease. The electrons are therefore lost more easily, so reactivity increases down the group.
Markers reward larger radius and more shielding, lower ionisation energy, and easier electron loss.
AQA 20212 marksState why barium sulfate can be safely used as a barium meal even though barium ions are toxic.Show worked answer →
A 2-mark answer needs solubility plus the reason it matters.
Barium sulfate is insoluble in water (solubility of Group 2 sulfates decreases down the group, so is the least soluble). Because it does not dissolve, toxic barium ions are not released into the body, so it can be swallowed safely while still being opaque to X-rays.
Related dot points
- The classification of an element as s, p, d or f block according to its outer electron configuration. Trends in atomic radius and first ionisation energy across Period 3 and down a group, explained by nuclear charge, shielding and atomic radius. The trend in melting point across Period 2 and Period 3, explained by the structure and bonding of the elements.
A focused answer to the AQA A-Level Chemistry 3.2.1 specification points on periodicity. Covers s, p, d and f block classification, the trends in atomic radius and first ionisation energy across Period 3 and down a group, and the melting point trend across Periods 2 and 3 explained by structure and bonding.
- The trends in electronegativity and boiling point of the halogens. The trend in oxidising ability of the halogens down the group, including displacement reactions of halide ions in aqueous solution. The trend in reducing ability of the halide ions, including the reactions of solid sodium halides with concentrated sulfuric acid. The use of acidified silver nitrate to identify and distinguish halide ions, and the use of chlorine in water treatment.
A focused answer to the AQA A-Level Chemistry 3.2.3 specification points on Group 7. Covers the boiling point and electronegativity trends, the decrease in oxidising power down the group with displacement reactions, the increase in reducing power of the halide ions with concentrated sulfuric acid, the silver nitrate test and the use of chlorine in water treatment.
- The reactions of the Period 3 elements sodium and magnesium with water. The reactions of the Period 3 elements with oxygen to form oxides. The structure and bonding of the Period 3 oxides and the trends in their melting points. The reactions of the oxides with water and the acid-base nature of the resulting solutions. The behaviour of the oxides as acids or bases in their reactions with acids and bases.
A focused answer to the AQA A-Level Chemistry 3.2.4 specification points on Period 3 elements and their oxides. Covers the reactions of sodium and magnesium with water, the reactions with oxygen, the structure and bonding of the oxides, their melting point trend, their reactions with water, and the change from basic to amphoteric to acidic across the period.
- The definition of a transition metal in terms of an incomplete d sub-shell. The characteristic properties of transition metals: complex formation, coloured ions, variable oxidation states and catalytic activity. The shapes of complex ions and the meaning of coordination number and ligand. Stereoisomerism in complexes. Ligand substitution reactions and the chelate effect. The origin of colour in transition metal ions and its use in colorimetry. The role of transition metals as homogeneous and heterogeneous catalysts.
A focused answer to the AQA A-Level Chemistry 3.2.5 specification points on transition metals. Covers the d-sub-shell definition, complex ions, ligands and coordination number, the shapes and stereoisomerism of complexes, ligand substitution and the chelate effect, the origin of colour and colorimetry, variable oxidation states and homogeneous and heterogeneous catalysis.
- The acidity of metal-aqua ions in terms of the charge density of the metal ion and the polarisation of coordinated water. The reactions of metal-aqua ions with bases such as sodium hydroxide and ammonia, and with carbonate ions. The amphoteric character of the aluminium hydroxide complex. The use of these reactions to identify metal ions in solution by the colours and behaviour of the precipitates formed.
A focused answer to the AQA A-Level Chemistry 3.2.6 specification points on reactions of ions in aqueous solution. Covers the acidity of metal-aqua ions and the link to charge density, the reactions of 2+ and 3+ aqua ions with sodium hydroxide, ammonia and carbonate, the amphoteric behaviour of aluminium hydroxide, and how the precipitate colours identify metal ions.
Sources & how we know this
- AQA A-level Chemistry (7405) specification — AQA (2015)