OxfordAQA International A Level Chemistry 3.2.2 Group 2, the Alkaline Earth Metals Exam Questions

About OxfordAQA International A Level Chemistry 3.2.2 Group 2, the Alkaline Earth Metals

Group 2, the alkaline earth metals, is an inorganic chemistry topic examined in Unit 1. You need to explain the trends from magnesium to barium in atomic radius, first ionisation energy and melting point, and describe their reactions with water. You also need the trends in solubility of the hydroxides and sulfates, and uses of Group 2 compounds: magnesium in extracting titanium, magnesium hydroxide in medicine, calcium hydroxide in agriculture, calcium oxide or carbonate to remove sulfur dioxide from flue gases, and barium sulfate in medicine. The test for sulfate ions with acidified barium chloride is a regular exam question, including why the acid is needed.

What you need to know

• Physical trends Atomic radius increases down the group as electrons occupy more shells. First ionisation energy decreases because the outer electrons are further from the nucleus and more shielded. Melting points generally decrease because the larger ions have weaker attraction to delocalised electrons.
• Reactions with water Reactivity increases down the group. Magnesium reacts very slowly with liquid water but burns in steam to form magnesium oxide and hydrogen; calcium, strontium and barium react with cold water to form hydroxides and hydrogen.
• Solubility trends Hydroxides become more soluble down the group, so Mg(OH)2 is sparingly soluble. Sulfates become less soluble down the group, so BaSO4 is insoluble.
• Uses Magnesium extracts titanium from TiCl4. Mg(OH)2 is used as an antacid. Ca(OH)2 neutralises acid soils. CaO or CaCO3 removes SO2 from flue gases. BaSO4 is used in barium meals because it is insoluble and so not toxic.
• Sulfate test Add dilute hydrochloric or nitric acid, then barium chloride solution. A white precipitate of barium sulfate shows sulfate ions; the acid removes carbonate ions, which would otherwise also give a white precipitate.

Common mistakes in 3.2.2 Group 2, the Alkaline Earth Metals

• Getting the solubility trends the wrong way round. Hydroxides become more soluble down the group; sulfates become less soluble.
• Saying barium compounds are safe in barium meals. Soluble barium compounds are toxic; BaSO4 is safe only because it is insoluble.
• Leaving out the acid in the sulfate test or giving the wrong reason. Acid removes carbonate (and sulfite) ions that would give a false positive.
• Writing magnesium + steam gives magnesium hydroxide. It gives magnesium oxide and hydrogen.

Try three 3.2.2 Group 2, the Alkaline Earth Metals exam-style questions