Group 1 and 2 Nitrates And Carbonates

The s-block elements The s-block elements all form salts when they react. All compounds contain M+ and M2+ cations. Various factors determine the stability of these (ionic) compounds. Main one is lattice energy – which is determined by two factors What are they?

Lattice energy trends One is distance between the centre of the cation and the anion. The other is charge on the cation.

General stability rule of thumb Stability of an ionic compound increases as: The charge on the ion increases Cation radius decreases

Q2. why would there be a difference in stability between oxides of potassium and barium? Q1. Compare the stability of a sodium salt with that of a rubidium salt

Thermal stability of nitrates and carbonates When talking stability, you don’t only think about stability of a salt in relation to its elements, but also in relation to other compounds that may be formed. For example: The carbonates (CO32-) anion may also decompose to form carbon dioxide and the oxide anion: E.g. CaCO3(s) → CaO(s) + CO2(g) The nitrate (NO3-) anion may decompose to form an oxide ion too, or the larger nitrate anion. E.g. 2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g) E.g. 2KNO3(s) → 2KNO2(s) + O2(g) http://www.chemguide.co.uk/inorganic/group2/thermstab.html

Lattice energy trends As you travel down the group 2, for example, the lattice energies of both carbonates and oxides becomes less negative. However, not at the same rate – the oxide value falls faster than that of the carbonate. The value of overall enthalpy change for decomposition reaction becomes more positive as you go down as a result:

Lattice energy trends Why do we think the lattice energy of the carbonates falls and oxides falls at different rates? Oxide ion is smaller (0.140 nm) than the carbonate ion. As you go down from Mg to Ca, the distance increases from 0.212 nm to 0.240 nm (13%). For the carbonate ion, distance would increase by same amount – as a percentage the distance increase will be less.

Thermal Stability of Nitrates Group 1 Nitrates (except Li): Decompose to form nitrites and oxygen: This is because NO2- ion is smaller than NO3- ion; so lattice of (Na+ NO2-) is more stable than of (Na+ NO3-). Considering the small size of the group 1 cations, this is sufficient to achieve thermal stability and avoid further decomposition to the oxide.

Thermal Stability of Nitrates Group 2 Nitrates (and Li): Decompose to form the metal oxide, nitrogen dioxide and oxygen: This is because the O2- ion is smaller and more highly charged than the NO3- ion. The smaller cations needing a much smaller anion than the nitrite one to give them stability.

Thermal Stability of Carbonates Carbonates of Group 1 are thermally stable: Exception is lithium carbonate which decomposes to give the oxide: All group 2 carbonates decompose to form stable oxides. E.g.

Group 2 carbonates Temperature at which groups 2 carbonates start to decompose increases down the group. Beryllium carbonate is so unstable, is doesn’t even exist at room temperature.

Thermal Stability of Hydroxides. These follow same pattern as carbonates and nitrates. The group 1 hydroxides are stable to quite high temperatures, with lithium hydroxide first to decompose at 650°C. Group 2 hydroxides all decompose to give the oxide and water:

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