A-LevelChemistryGroup 29701/42 · May/June 2019 · Q29 Marks

Group 2 carbonates decompose when heated. Write an equation for the decomposition of the carbonate ion.

(a)Group 2 carbonates decompose when heated. Write an equation for the decomposition of the carbonate ion.[1]
(b)Describe and explain how the thermal stability of the Group 2 carbonates changes with increasing atomic number.[3]
(c)Lead(II) carbonate, PbCO₃, and zinc carbonate, ZnCO₃, decompose on heating in a similar way to calcium carbonate, CaCO₃. State relevant data from the Data Booklet and use it to predict the order of thermal stability of these three carbonates. data (most stable) > > (least stable)[2]
(d)Dolomite contains the double carbonate of calcium and magnesium, CaMg(CO₃)₂, and some impurities. When 0.642 g of a sample of dolomite reacts with an excess of hydrochloric acid, 125.0 cm³ of CO₂ is formed under room conditions. Calculate the percentage of CaMg(CO₃)₂ in the sample of dolomite. Show all your working. Assume that none of the impurities react with HCl. % of CaMg(CO₃)₂ in dolomite =[3]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] CO₃²⁻ → O²⁻ + CO₂
[1] Increases (with increasing atomic number/implied)
[1] cationic radius / ion size increases (down the group)
[1] less polarisation / distortion of anion/CO₃²⁻
[1] (Pb²⁺) 0.120 nm; (Ca²⁺) 0.099 nm; (Zn²⁺) 0.075 nm
[1] (most stable) PbCO₃ > CaCO₃ > ZnCO₃ (least stable) ECF from atomic radii
[1] amount of CO₂ = 125 / 24000 = 5.21 × 10⁻³ mol
[1] CaMg(CO₃)₂:CO₂ 1:2 amount of carbonate = 2.60(4) × 10⁻³ mol ECF
[1] mass of carbonate = 184(.4) × 2.60(4) × 10⁻³ = 0.480 g % of CaMg(CO₃)₂ = 100 × 0.480/0.642 = 74.8 % ECF

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