Group II - Thermal decomposition

Group II metal carbonates, nitrates and hydroxides are capable of decomposition to give the corresponding metal oxide and release CO₂, NO₂ and O₂, and H₂O respectively. The following are the equations.

MCO₃ (s) -> MO (s) + CO₂ (g)

M(NO₃)₂ (s) -> MO (s) + 2NO₂ (g) + ½O₂ (g)

M(OH)₂ (s) -> MO (s) + H₂O (g)

The ease of decomposition decreases as we go down the group. I will use the metal carbonates as an example to illustrate this trend. The reason used will be applicable to describe the ease of decomposition of Group II nitrates and hydroxides.

1. Charge density and Polarising power of cation.

The Group II metals share a common charge of +2, but down the group the ionic radius increases. Hence, the charge density of the cation decreases and its polarising power becomes weaker. Hence, the cation is less able to polarise the anion and more importantly is less able to distort the covalent bond found in the carbonate ion.

This distortion weakens the covalent bond and hence making the decomposition easier.

Interestingly, this reason is why Group I nitrates decompose to give MNO₂ and O₂ instead of forming MO, NO₂ and O₂ as products. Only LiNO₃ decomposes like a Group II nitrate. This is not surprising as Li and Mg have a diagonal relationship. Their small size and charge will lead to a high charge density which make Li⁺ more polarising than the other Group I cations.

2. Percentage change in lattice energy.

This reason is not often used as it requires some mathematical conceptualisation.

The decomposition of metal carbonates form metal oxides. The different between the lattice energy (LE) of the two ionic compounds is the sum of ionic radius. Since both compound have the same charges for the cation and that for the anions, we can conclude that the difference in lattice energy will determine whether the decomposition is more favourable or less.

A smaller cation will have a larger percentage difference in the two LE while the bigger cation will have a smaller difference. You can make use of the above picture to conceptualise this statement.

Therefore, down the group the difference in lattice energy becomes smaller and hence decomposition becomes more difficult.

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^{Article written by Kwok YL 2010.}

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