The table gives the approximate temperatures at which the Group 2 carbonates begin to decompose on strong heating. Carbonate Approximate decomposition temperature / °C MgCO3 400 CaCO3 840 SrCO3 1210 BaCO3 1360 (a) Describe the trend in thermal stability of the Group 2 carbonates…

Answer: ((a)) Thermal stability of the Group 2 carbonates increases down the group, from magnesium to barium (the decomposition temperature increases down the group). ((b)) Down the group the cation radius increases, so its charge density (polarising power) decreases. A smaller, more strongly polarising cation such as Mg^2+ distorts the large carbonate ion more, weakening its C–O bonds and allowing it to decompose at a lower temperature; a larger,… ((c)) SrCO3(s) → SrO(s) + CO2(g) ((d)) 2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g); a brown (reddish-brown) gas, NO2, is produced.. Part (a): The trend in thermal stability Reading down the table, the decomposition temperature increases from MgCO3 (400 °C) to BaCO3 (1360 °C). So thermal stability of the Group 2 carbonates increases down the group. Part (b): Explaining…

AS Chemistry 9701: Thermal Stability of Group 2 Carbonates and Nitrates

Carbonate	Approximate decomposition temperature / $^\circ\text{C}$
$\text{MgCO}_3$	400
$\text{CaCO}_3$	840
$\text{SrCO}_3$	1210
$\text{BaCO}_3$	1360

Carbonate

Approximate decomposition temperature /

^\circ\text{C}

\text{MgCO}_3

400

\text{CaCO}_3

840

\text{SrCO}_3

1210

\text{BaCO}_3

1360

Part (a): The trend in thermal stability

Reading down the table, the decomposition temperature increases from $\text{MgCO}_3$ ( $400\,^\circ\text{C}$ ) to $\text{BaCO}_3$ ( $1360\,^\circ\text{C}$ ). So thermal stability of the Group 2 carbonates increases down the group.

Part (b): Explaining the trend using polarising power

Down Group 2, the ionic radius of the $\text{M}^{2+}$ cation increases (from $\text{Mg}^{2+}$ to $\text{Ba}^{2+}$ ), so its charge density (charge-to-radius ratio), or polarising power, decreases.

A small, highly-charge-dense cation such as $\text{Mg}^{2+}$ can get close to the large carbonate ion and pulls on its electron cloud strongly, distorting (polarising) it. This distortion weakens the C–O bonds within the carbonate ion, making it easier for the ion to break apart into $\text{CO}_2$ and an oxide ion, so $\text{MgCO}_3$ decomposes at a relatively low temperature.

A large, weakly polarising cation such as $\text{Ba}^{2+}$ distorts the carbonate ion far less, leaving its C–O bonds closer to their original strength. More thermal energy, a higher temperature, is therefore needed to break the carbonate ion apart, so $\text{BaCO}_3$ is more thermally stable and decomposes only at a much higher temperature.

Part (c): Thermal decomposition of strontium carbonate

Like all the Group 2 carbonates, strontium carbonate decomposes on heating to give the metal oxide and carbon dioxide gas:

$\text{SrCO}_3\text{(s)} \rightarrow \text{SrO(s)} + \text{CO}_2\text{(g)}$

Part (d): Thermal decomposition of magnesium nitrate

Group 2 nitrates decompose on strong heating to give the metal oxide, nitrogen dioxide and oxygen:

$2\text{Mg(NO}_3\text{)}_2\text{(s)} \rightarrow 2\text{MgO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$

Check the balance: left-hand side has $2$ Mg, $4$ N and $12$ O (each $\text{Mg(NO}_3)_2$ contains $6$ O, so $2 \times 6 = 12$ ). Right-hand side has $2$ Mg (from $2\text{MgO}$ ), $4$ N (from $4\text{NO}_2$ ) and $2 + 8 + 2 = 12$ O (from $2\text{MgO}$ , $4\text{NO}_2$ and $\text{O}_2$ respectively), consistent.

Observation: a brown (reddish-brown) gas, $\text{NO}_2$ , is evolved as the white solid decomposes.

Final answers

(a) Thermal stability increases down the group, from magnesium to barium.
(b) Decreasing charge density/polarising power of the cation down the group means the carbonate ion is distorted less, so a higher temperature is needed to decompose it. Thermal stability increases.
(c) $\text{SrCO}_3\text{(s)} \rightarrow \text{SrO(s)} + \text{CO}_2\text{(g)}$
(d) $2\text{Mg(NO}_3\text{)}_2\text{(s)} \rightarrow 2\text{MgO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$ ; a brown gas ( $\text{NO}_2$ ) is produced.