Group 1 and 2

1. Nitrates And Carbonates Group 1 and 2

2. 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?

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

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

5. 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

6. 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

7. 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:

8. 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.

9. 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.

10. 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.

11. 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. Thermal Stability of Carbonates

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

13. 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:

14. Questions

15. Answers