CIE A Level Chemistry Group 2 Exam Questions

About CIE A Level Chemistry Group 2

Group 2 in Cambridge International 9701 covers the elements magnesium to barium. At AS you describe the reactions of the elements with oxygen, water and dilute acids, the reactions of their oxides, hydroxides and carbonates, and the thermal decomposition of the nitrates and carbonates. You must state trends in reactivity, thermal stability and the solubility of hydroxides and sulfates, and know uses such as calcium hydroxide in agriculture. The A Level extension explains these trends: thermal stability using cation size and polarisation of the anion, and solubility using lattice energy and enthalpy of hydration. Examiners commonly ask for balanced equations, observations and explanations of the trends.

What you need to know

• Reactions with water Reactivity increases down the group as ionisation energies decrease. Mg reacts slowly with cold water but burns in steam to form MgO and H2, while Ca, Sr and Ba react with cold water to form the hydroxide and hydrogen.
• Reactions with oxygen and acids The metals burn in oxygen to form MO. Metals, oxides, hydroxides and carbonates react with dilute acids to form salts; with sulfuric acid, insoluble BaSO4 stops the reaction coating the surface.
• Thermal decomposition Carbonates decompose to oxide and CO2, and nitrates to oxide, NO2 and O2, for example 2Mg(NO3)2 → 2MgO + 4NO2 + O2. Thermal stability increases down the group.
• Solubility trends Hydroxides become more soluble down the group, so solutions become more alkaline. Sulfates become less soluble, and BaSO4 is insoluble, which is used in the test for sulfate ions.
• Uses Calcium hydroxide and calcium carbonate are used to neutralise acidic soils, and magnesium hydroxide is used in antacids.
• Explaining stability (A Level) Smaller cations with the same charge have a higher charge density and polarise (distort) the large carbonate or nitrate anion more, weakening the C-O or N-O bonds. So MgCO3 decomposes at a lower temperature than BaCO3.
• Explaining solubility (A Level) Solubility depends on the balance of lattice energy and hydration enthalpy. For sulfates, hydration enthalpy of the cation falls faster down the group than lattice energy, so enthalpy of solution becomes more endothermic.

Common mistakes in Group 2

• Saying thermal stability decreases down the group; it increases because larger cations polarise the anion less.
• Forgetting that magnesium with steam gives MgO, not Mg(OH)2.
• Mixing up the solubility trends; hydroxides get more soluble down the group while sulfates get less soluble.
• Leaving O2 out of the nitrate decomposition equation; both NO2 and O2 are produced.
• Explaining reactivity using electronegativity only; refer to atomic radius, shielding and the decreasing ionisation energies.

Try three Group 2 exam-style questions