Group 2 (A Level)

• The cation polarises the anion, never the reverse. And ΔHsol = −ΔHlatt + ΣΔHhyd: whichever term falls faster down the group decides the solubility trend.
• Radius up, charge the same. The cation polarises the anion. Which term falls faster decides the solubility.

• 27.1.1 I can describe — describe and explain qualitatively the trend in the thermal stability of the nitrates and carbonates, including the effect of ionic radius on the polarisation of the large anion
• 27.1.2 I can describe — describe and explain qualitatively the variation in solubility and of enthalpy change of solution, ΔH_sol, of the hydroxides and sulfates in terms of relative magnitudes of the enthalpy change of hydration and the lattice energy

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “The carbonate ion polarises the magnesium ion.” Correct The cation polarises the anion, never the reverse. The small Mg²⁺ ion distorts the electron cloud of the large CO₃²⁻ ion and weakens a C–O bond inside it.
• “ΔHsol = ΔHlatt + ΔHhyd.” Correct ΔHsol = −ΔHlatt + ΣΔHhyd. The lattice energy is defined for gaseous ions forming the solid; dissolving breaks the lattice, so its sign is reversed. And the hydration term is a sum over every ion in the formula.
• “Ba²⁺ has a bigger charge, so it polarises more.” Correct Every Group 2 ion is 2+. What changes is the radius, so what falls down the group is the charge density, and with it the polarising power.
• “Mg(OH)₂: ΣΔHhyd = ΔHhyd(Mg²⁺) + ΔHhyd(OH⁻).” Correct One Mg²⁺ and two OH⁻: ΣΔHhyd = ΔHhyd(Mg²⁺) + 2 × ΔHhyd(OH⁻).
• “The hydration enthalpy falls down the group, so every Group 2 compound gets less soluble.” Correct The lattice energy falls too. The trend depends on which term falls faster, and that depends on the size of the anion — which is why the hydroxides and the sulfates go opposite ways.
• “MgCO₃ decomposes most easily because magnesium is the most reactive metal.” Repair Magnesium is the least reactive metal of the four. The reason is the high polarising power of the small Mg²⁺ ion, which distorts the carbonate ion and weakens a C–O bond.
• “Ba²⁺ has a higher charge, so it polarises more.” Repair Every Group 2 ion is 2+. The difference is radius, so charge density falls down the group and Ba²⁺ polarises least.
• “The cation is polarised by the anion.” Repair The cation polarises the anion: it distorts the anion’s electron cloud.
• “Carbonates get more stable down the group because the lattice gets stronger.” Repair The syllabus’s explanation is polarisation of the anion by the cation. Name the bond that is weakened: a C–O bond (or an N–O bond in a nitrate).
• “ΔHsol = ΔHlatt + ΔHhyd.” Repair The lattice is broken, so its sign is reversed, and every ion is hydrated: ΔHsol = −ΔHlatt + ΣΔHhyd.
• “ΣΔHhyd for Sr(OH)₂ = −1480 + (−520).” Repair One Sr²⁺ and two OH⁻: −1480 + 2 × (−520) = −2520. ΣΔHhyd includes every ion in the formula.
• “Both trends are explained by the hydration enthalpy falling down the group.” Repair That alone would make both series less soluble. The hydroxides become more soluble because their lattice energy falls even faster than the hydration term.
• “The sulfate lattice energy changes a lot down the group.” Repair The large sulfate ion dominates r₊ + r₋, so the sulfate lattice energy changes only slightly. That is why the hydration term wins for the sulfates.
• “Use 1.00 g of each carbonate so the comparison is fair.” Repair Use the same amount in moles. The molar masses differ, so equal masses give different amounts of CO₂.
• “Lattice energy is positive because it is the energy needed to break the lattice.” Repair As defined in 23.1.1 it is the enthalpy change when the solid forms from its gaseous ions: exothermic, negative. Breaking the lattice is −ΔHlatt.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. When you reach topic 28, re-answer “what makes a cation strongly polarising?” for a small, highly charged transition-metal ion. Whenever you revisit topic 23, recompute one Group 2 ΔHsol and say which term decided its sign. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 27 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• There is no multiple-choice paper on A Level content, so Topic 27 is written out in full in Paper 4. Both outcomes are describe and explain: state the trend, then give the cause (ionic radius at constant charge), the mechanism (polarisation of the anion, or which enthalpy term falls faster) and the outcome. A trend stated without its cause answers only the describe half.
• The one calculation is ΔHsol = −ΔHlatt + ΣΔHhyd, with every ion in the formula counted. The Data section holds no lattice energies, hydration enthalpies or decomposition temperatures: a question supplies them. Relative atomic masses for a mass-loss calculation come from the Data section’s Periodic Table.
• Topic 27 supplies a natural Paper 5 plan: rank the carbonates by the time their CO₂ takes to turn limewater milky, with equal amounts in moles and a fixed tube position. Its quantitative twin is the syllabus’s gravimetric procedure, heating a solid in a crucible to constant mass.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.