CAIE A-Level Chemistry 27.1.1 Group 2 Nitrate and Carbonate Stability

Practise explaining why Group 2 nitrates and carbonates become more thermally stable down the group.

Exam points

state that nitrate and carbonate thermal stability increases from magnesium to barium
link increasing M2+ radius to decreasing charge density and polarising power down the group
explain that the large anion is less distorted, so more heat is required for decomposition

[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)

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27.1.1—Trends: the trend in the thermal stability question 1