Group 2

• Same equations, changing vigour: down Group 2 the metals react faster, the carbonates and nitrates get harder to decompose, the hydroxides dissolve more and the sulfates dissolve less.
• Cold water gives the hydroxide, steam gives the oxide. The sulfate coats the metal. Nitrates give oxide, NO₂ and O₂. Down the group: reactivity up, stability up, hydroxides more soluble, sulfates less.

• 10.1.1 I can describe — describe, and write equations for, the reactions of the elements with oxygen, water and dilute hydrochloric and sulfuric acids
• 10.1.2 I can describe — describe, and write equations for, the reactions of the oxides, hydroxides and carbonates with water and dilute hydrochloric and sulfuric acids
• 10.1.3 I can describe — describe, and write equations for, the thermal decomposition of the nitrates and carbonates, to include the trend in thermal stabilities
• 10.1.4 I can describe — describe, and make predictions from, the trends in physical and chemical properties of the elements involved in the reactions in 10.1.1 and the compounds involved in 10.1.2, 10.1.3 and 10.1.5
• 10.1.5 I can state — state the variation in the solubilities of the hydroxides and sulfates

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.

• “Barium is the most reactive, so it reacts most vigorously with dilute sulfuric acid.” Correct Calcium, strontium and barium start to react and then slow down or stop, because their sulfates are insoluble or only slightly soluble and form a coating on the metal that keeps the acid out. Only magnesium, whose sulfate is soluble, reacts freely. With dilute hydrochloric acid all four react completely, because every Group 2 chloride is soluble.
• “Mg(s) + 2H₂O(g) → Mg(OH)₂(s) + H₂(g) with steam.” Correct Steam gives the oxide: Mg(s) + H₂O(g) → MgO(s) + H₂(g). Cold water gives the hydroxide, very slowly.
• “2M(NO₃)₂(s) → 2M(NO₂)₂(s) + O₂(g).” Correct Group 2 nitrates decompose all the way to the oxide, brown nitrogen dioxide and oxygen: 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g).
• “The metals get more reactive down the group, so their carbonates get easier to decompose.” Correct Thermal stability increases down the group: magnesium carbonate decomposes most easily, barium carbonate least. The two trends are separate; state each on its own.
• “The hydroxides and the sulfates both get less soluble down the group.” Correct They run in opposite directions: hydroxide solubility increases, sulfate solubility decreases.
• “Thermal stability increases down the group because…” (followed by a paragraph of reasons) Correct At AS you state the thermal-stability and solubility trends and use them. Their explanations are topic 27. The one trend this topic explains is reactivity, with ionisation energy and radius.
• “Magnesium does not react with water.” Repair It reacts very slowly with cold water, and rapidly with steam to give MgO, not the hydroxide: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
• “Ca(s) + H₂O(l) → CaO(s) + H₂(g).” Repair With cold water the product is the hydroxide: Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g).
• “Barium reacts vigorously with dilute sulfuric acid because it is the most reactive.” Repair It starts vigorously and then stops, because insoluble BaSO₄ coats the metal.
• “Group 2 carbonates dissolve in water to give alkaline solutions.” Repair All four are insoluble in water; there is no reaction.
• “MCO₃ + H₂SO₄ → MSO₄ + H₂ + CO₂.” Repair An acid and a carbonate give a salt, water and carbon dioxide; hydrogen comes from a metal and an acid, not from a carbonate.
• “M(NO₃)₂ → MO + 2NO₂”, or “M(NO₃)₂ → M(NO₂)₂ + O₂.” Repair Group 2 nitrates give the oxide, NO₂ and O₂: 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g). The first version is not balanced for oxygen. The nitrite product is what Group 1 nitrates give, which is not in this topic.
• “Thermal stability decreases down the group because the metals get more reactive.” Repair Stability increases down the group; the two trends are separate, and the reason for this one is topic 27.
• “Reactivity increases down the group because the nuclear charge increases.” Repair The nuclear charge does increase, but it is outweighed by the extra shell and extra shielding; the ionisation energies fall, and that is why the electrons are lost more easily. A larger nuclear charge on its own would make them harder to remove.
• “Hydroxide solubility decreases down the group, like the sulfates.” Repair Hydroxides become more soluble, sulfates less; the two series run opposite ways.
• “Mg(OH)₂ is insoluble, so magnesium with water gives a neutral solution.” Repair Sparingly soluble is not insoluble: enough dissolves to make the solution weakly alkaline.
• “The sulfate test uses barium chloride; the acid is added to speed it up.” Repair The acid removes carbonate and sulfite, which would also give white precipitates with Ba²⁺ but whose barium salts dissolve in acid.
• “Radium is in Group 2, so it behaves like magnesium.” Repair It has the same kind of chemistry, but its properties are predicted by extrapolating from the nearest known element, barium, along each stated trend.
• “Use 1.00 g of each carbonate so the test is fair.” Repair Use equal amounts in moles, so each sample can release the same amount of CO₂. 1.00 g of MgCO₃ is 0.0119 mol but 1.00 g of BaCO₃ is only 0.00507 mol.

• 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.
• A reactive metal can look unreactive. Barium is the most reactive of the four, yet with dilute sulfuric acid it gives far less hydrogen than its reactivity would suggest. When a question sets a reactivity trend against an observation, check whether an insoluble product is getting in the way.
• The sulfate coating again. With dilute sulfuric acid, calcium, strontium and barium carbonates are soon coated with their own insoluble sulfate and the fizzing dies away, for example BaCO₃(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O(l) + CO₂(g). Hydrochloric acid gives no such trouble, because every Group 2 chloride is soluble. This is also why marble chips (calcium carbonate) are reacted with hydrochloric acid, not sulfuric acid, to make carbon dioxide.
• The errors, named properly. The largest random error is judging the moment the limewater first turns milky, an observer judgement of the same kind as the disappearing-cross experiment. Reduce it by comparing every tube against the same reference tube of fresh limewater, or by using a light sensor and data logger. The largest systematic error is the air in the delivery tube, which has to be pushed out before any CO₂ reaches the limewater and so adds a delay to every run; keep the tube short and use the same one each time. Using equal masses instead of equal amounts in moles would be a design flaw, not an error. “Human error” is never an acceptable answer.
• Interleave with the chapters that use this one. Topic 27 explains the thermal-stability and solubility trends you only stated here: when you reach it, re-answer “why does BaCO₃ need a higher temperature than MgCO₃?”. Topic 9 (periodicity) is where the group pattern comes from, and topic 11 (Group 17) is the second group case study: compare how its trends run with these. 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 10 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 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.
• A Paper 1 item on Group 2 can turn on one fact placed against a trend: which compound decomposes most easily, which hydroxide is most soluble, which gas a nitrate gives, which metal stops reacting with dilute sulfuric acid. Paper 2 asks you to describe a reaction and write its equation with state symbols, to explain the reactivity trend from ionisation energy and radius, and to predict the behaviour of strontium or radium from the trends.
• The calculations are topic 2 mole calculations in a Group 2 setting: the volume of gas from a heated nitrate or carbonate, the mass of oxide left, the mass of barium sulfate precipitated, and the identity of a carbonate from its mass loss. The ionisation energies (Data section, table 2), the molar gas volume and the Ar values are supplied; solubilities and decomposition temperatures are not, so they are only ever used as trends.
• Topic 10 supplies the gravimetric procedure (heating a carbonate to constant mass in a crucible, balance read to 0.01 g), a timed comparison of thermal stabilities with limewater, and the observational problem: the sulfate test and the NaOH(aq) results for Mg²⁺, Ca²⁺ and Ba²⁺ from the qualitative-analysis notes. A plan to compare the carbonates would need to control the amount in moles, the flame and the limewater, and to name the largest error.
• 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.