CAIE A-Level Chemistry 27.1 Group 2 Metals and Compounds

Practise explaining nitrate/carbonate stability and hydroxide/sulfate solubility trends using ionic energetics.

Exam points

link increasing cation radius down the group to lower charge density and weaker anion polarisation
explain increasing nitrate and carbonate thermal stability as the large anion is less distorted
compare lattice and hydration enthalpy changes to explain opposite hydroxide and sulfate solubility trends

Covered syllabus points

27.1.1—Trends: the trend in the thermal stability 27.1.2—Trends: the variation in solubility

[Maximum number: 2]

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.

KI slowly oxidises in air, forming $\mathrm{I}_{2}$ .

$reaction $1 \quad 4 \mathrm{KI}(\mathrm{s})+2 \mathrm{CO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{~K}_{2} \mathrm{CO}_{3}(\mathrm{~s})+2 \mathrm{I}_{2}(\mathrm{~s}) \quad \Delta H^{\ominus}=-203.4 \mathrm{~kJ} \mathrm{~mol}^{-1}$$

Table 1.2 shows some data relevant to this question.

Table 1.2

The Group 1 carbonates are much more thermally stable than the Group 2 carbonates. State and explain the trend in the thermal stability of the Group 2 carbonates.

increases (in thermal stability down the group)

AND (cat)ionic radius / ion size increases (down the group)

less polarisation of anion / $\ce{C-}$ O bond / distortion of carbonate ion / $\ce{CO3}$ ${ }^{2-}$

OR $\ce{C-}$ O is less weakened / stronger (down the group)

[Maximum number: 6]

(a)

Describe and explain how the solubility of the Group 2 sulfates varies down the group.

[ 4 ]

M1 solubility decreases down the group

M2 $\Delta H_{\text {latt }}$ and $\Delta H_{\text {hyd }}$ both become less exothermic / more endothermic

M3 $\Delta H_{\text {latt }}$ changes less (than $\Delta H_{\text {hyd }}$ as $\mathrm{SO}_{4}{ }^{2-}$ being larger than $\mathrm{M}^{2+}$ )

M4 $\Delta H_{\text {sol }}$ becomes less exothermic / less negative

(b)

The trend in the decomposition temperatures of Group 2 peroxides, $\mathrm{MO}_{2}$ , is similar to that of Group 2 carbonates.

Suggest which of barium peroxide, $\mathrm{BaO}_{2}$ , and calcium peroxide, $\mathrm{CaO}_{2}$ , will decompose at the lower temperature. Explain your answer.

[ 2 ]

M1 $\mathrm{CaO}_{2}$ and $\mathrm{Ca}^{2+}$ has a smaller ionic radii/ $\mathrm{Ca}^{2+}$ has a higher charge density

M2 anion/ $\mathrm{O}_{2}{ }^{2-}$ becomes more polarised / distorted

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Covered syllabus points

27.1 Trends in Group 2 metals and compounds question 1

27.1 Trends in Group 2 metals and compounds question 2

Question (a)

Question (b)