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2.4.5Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Nitrates and Carbonates — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 30 of 53 Unit quiz

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Part 1 of 5Thermal Decomposition Products

2.4.5 Nitrates and Carbonates

Heating splits these salts into simpler substances, and the products depend on the group and the metal.

Group 2 carbonates
  • MCO₃(s) → MO(s) + CO₂(g), for example MgCO₃(s) → MgO(s) + CO₂(g).
Group 2 nitrates
  • 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g) gives a brown toxic gas, so do it in a fume cupboard.
Lithium
  • Li₂CO₃(s) → Li₂O(s) + CO₂(g) and 4LiNO₃(s) → 2Li₂O(s) + 4NO₂(g) + O₂(g), unlike the other Group 1 salts.
Other Group 1 carbonates
  • These do not decompose at Bunsen burner temperatures.
Other Group 1 nitrates
  • They form the nitrite and oxygen only: 2NaNO₃(s) → 2NaNO₂(s) + O₂(g).

Part 2 of 5Explaining the Trend

Thermal stability increases down both groups because of polarising power.

1
A small, highly charged metal cation attracts electron density in a neighbouring anion strongly.
2
This distorts the carbonate or nitrate ion, which is polarised, and weakens a bond within the anion.
3
Mg²⁺ has a high charge density, so MgCO₃ decomposes at a lower temperature than BaCO₃.
4
Down the group the cation radius increases and charge density falls, so there is less polarisation.
5
More heat is therefore needed to decompose the compound, so stability increases from Mg to Ba.

Part 3 of 5Identifying the Gases

Identifying the Gases
Carbon dioxide
  • This gas turns limewater cloudy because insoluble calcium carbonate forms in the solution.
Oxygen
  • Oxygen gas is identified because it relights a glowing splint held in the gas.
Nitrogen dioxide
  • This is a toxic brown gas that dissolves in water to give an acidic solution.
Safety
  • Decompose nitrates in a fume cupboard because nitrogen dioxide is toxic.

Part 4 of 5Worked Example

Question

  • 5.00 g of calcium carbonate (Mr = 100.1) is heated until it fully decomposes.
  • Calculate the mass of calcium oxide (Mr = 56.1) and the volume of CO₂ at room temperature and pressure (24.0 dm³ mol⁻¹).

  1. EquationCaCO₃(s) → CaO(s) + CO₂(g).
  2. Moles of CaCO₃ = 5.00 ÷ 100.1 = 0.04995 mol.
  3. Mass of CaO = 0.04995 × 56.1 = 2.80 g.
  4. Volume of CO₂ = 0.04995 × 24.0 = 1.20 dm³.

Answer 2.80 g of calcium oxide forms and 1.20 dm³ of carbon dioxide is released.

Part 5 of 5Exam Checklist & Tips

2.4.5 – Key Points for the Exam

  • Thermal decomposition The breakdown of a compound into simpler substances when heated.
  • Polarisation Distortion of the electron cloud of an anion by a nearby positive ion.
  • Charge density The charge of an ion relative to its size.
  • Thermal stability A measure of how resistant a compound is to decomposition on heating.

Common Mistakes

  • Stating that thermal stability decreases down the group;
    • for carbonates and nitrates of Groups 1 and 2 it increases.
  • Writing NO instead of NO₂ in nitrate decomposition equations, or forgetting the oxygen.
  • Forgetting that lithium is the exception in Group 1 because of its small size.
  • Not balancing the 4NO₂ and O₂ coefficients in the Group 2 nitrate equation.

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