CAIE A Level Chemistry 27 Group 2 Trends

Practise explaining Group 2 thermal-stability and solubility trends through polarisation and enthalpy cycles, using ionic size and lattice or hydration effects to justify…

[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. Group 2 question 1

27. Group 2 question 2

Question (a)

Question (b)