Thermal stability of nitrates and carbonates

In a nutshell

The thermal stability of nitrates and carbonates increases as you go down Groups $1$ and $2$ . Thermal stability can be tested by producing gases and testing for them, $NO_2$ is tested for nitrates and $CO_2$ is produced from carbonates.

Thermal stability of nitrates and carbonates

Thermal stability is based on how much heat a substance can withstand before undergoing thermal decomposition. The higher the thermal stability the higher the temperature required to decompose the compound.

Thermal stability down a group

Thermal stability increases as you go down a group. Carbonates and nitrates are large negative ions (anions). They can be made unstable by the positive ions (cations). The greater the distortion the more this destabilises the anion.

Chemistry; Inorganic chemistry and the periodic table; KS5 Year 12; Thermal stability of nitrates and carbonates

The greater the charge on the cation the more this can destabilise the anion. As well as this the smaller the cation, the higher the charge density and the more this can distort the anion. Going down the group would mean a lower charge density and more thermal stability.

Group 2 are less thermally stable than Group 1

A higher charge density makes the anion more unstable. Group $2$ metals have a $2+$ charge compared to a $1+$ charge for Group $1$ metals which will cause more significant destabilisation. Group $2$ carbonates and nitrates are less thermally stable than Group $1$ .

Chemistry; Inorganic chemistry and the periodic table; KS5 Year 12; Thermal stability of nitrates and carbonates

Testing for thermal stability

Nitrate decomposition can be measured by...

How long it takes until a glowing split can be relit or how much brown gas $(NO_2)$ is produced. This must be done in a fume cupboard because $(NO_2)$ is toxic.

Example

Decomposition of Group $1$ and Group $2$ nitrates.

$2NaNO_3\ \rightarrow\ 2NaNO_2\ +\ O_2 \\2Ca(NO_3)_2\ \rightarrow \ 2CaO\ +\ 4NO_2\ +\ O_2$

Metals such as potassium and sodium, combust to form nitrite $(NO_2^-)$ salts. Metals such as; calcium, magnesium, aluminium, zinc, iron, lead and copper decompose to form a metal oxide and release $NO_2$ and $O_2$ gas.

Carbonate decomposition can be measured by...

Carbon dioxide is produced when carbonates are decomposed. This can be measured by testing with lime water (saturated $NaOH$ solution). A positive result is that the solution turns cloudy.

Example

Decomposition of Group $1$ and Group $2$ carbonates.

$K_2CO_3\ \rightarrow\ K_2O\ +\ CO_2 \\MgCO_3 \ \rightarrow\ MgO\ +\ CO_2$