CCEA A Level Chemistry 2.11 Group II Elements and Their Compounds Exam Questions

About CCEA A Level Chemistry 2.11 Group II Elements and Their Compounds

Section 2.11 is the inorganic chemistry of Unit AS 2. You need the trends in electronic configuration, atomic radius and first ionisation energy down Group II, and the reactions of the metals with oxygen, water and dilute acids, including measuring masses of solids and volumes of gas produced. You must describe the basic oxides and their reactions with water and acids, and CCEA names uses such as magnesium oxide in indigestion remedies, calcium carbonate in toothpaste, and quicklime and slaked lime in cement and concrete. Trends in thermal stability of carbonates and hydroxides are explained by cation size and charge. Solubility trends of sulfates and hydroxides complete the section.

What you need to know

• Physical trends Group II elements are in the s block with outer configuration ns2. Down the group, atomic radius increases and first ionisation energy decreases as the outer electrons are further from the nucleus and more shielded.
• Reactions Reactivity increases down the group. Magnesium reacts slowly with cold water but burns in steam: Mg + H2O gives MgO + H2. Calcium reacts with cold water: Ca + 2H2O gives Ca(OH)2 + H2. Metals react with oxygen to form oxides and with dilute acids to form salts and hydrogen.
• Oxides The oxides are basic: they react with water to form hydroxides (for example CaO + H2O gives Ca(OH)2) and with acids to form salts and water.
• Uses Magnesium oxide neutralises excess stomach acid in indigestion remedies; calcium carbonate is an abrasive in toothpaste; quicklime (CaO) and slaked lime (Ca(OH)2) are used in cement and concrete.
• Thermal stability Carbonates and hydroxides become more stable down the group, for example MgCO3 gives MgO + CO2 at a lower temperature than CaCO3. Smaller, more highly charged cations at the top polarise (distort) the anion more, weakening its bonds.
• Solubility Hydroxides become more soluble down the group; sulfates become less soluble (BaSO4 is insoluble, hence the barium chloride test for sulfate).

Common mistakes in 2.11 Group II Elements and Their Compounds

• Saying solubility of sulfates and hydroxides follows the same trend. Hydroxides increase and sulfates decrease in solubility down the group.
• Saying magnesium reacts vigorously with cold water. It reacts very slowly; it reacts readily with steam to give the oxide, not the hydroxide.
• Explaining thermal stability with ionisation energy. Use cation charge density and polarisation of the carbonate or hydroxide ion.
• Saying first ionisation energy increases down the group. It decreases because of greater distance and shielding.
• Forgetting that Group II oxides are basic. They neutralise acids, which is why MgO is used in indigestion remedies.

Try three 2.11 Group II Elements and Their Compounds exam-style questions