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2.2.4Organic Chemistry

Group 1 and 2 Carbonates and Nitrates — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 42

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Part 1 of 5Carbonate Decomposition

2.2.4 Group 1 and 2 Carbonates and Nitrates
Group 2 carbonates
  • Decompose on heatingMCO₃(s) → MO(s) + CO₂(g).
Group 1 carbonates
  • Are stable to normal laboratory heating, except lithium carbonate: Li₂CO₃(s) → Li₂O(s) + CO₂(g).
Trend
  • Thermal stability increases down both groups, so more heat is needed to decompose barium carbonate than magnesium carbonate.
Test
  • Carbon dioxide released is identified because it turns limewater cloudy or milky.
Equation reminder
  • Group 2 carbonate decomposition equations all have the same shape, with a metal oxide and carbon dioxide as products.

Part 2 of 5Explaining Stability

Explaining Stability
Polarising power
  • A small cation with high charge density distorts the electron cloud of the carbonate ion.
Magnesium
  • Mg²⁺ is small and strongly polarising, so the carbonate decomposes easily.
Barium
  • Ba²⁺ is larger with lower charge density, polarises less and the carbonate is more stable.
Group 1
  • Cations have a smaller charge, so polarisation is weaker and carbonates are stable.
Lithium anomaly
  • Lithium is the smallest Group 1 ion, so its charge density is high and its compounds behave more like Group 2.
CarbonateEase of decomposition
MgCO₃Easiest
CaCO₃Intermediate
BaCO₃Hardest

Part 3 of 5Nitrate Decomposition

1
Heating Group 2 nitrates gives a metal oxide, nitrogen dioxide and oxygen.
2
The equation is 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g).
3
Brown fumes of NO₂ are seen and the oxygen relights a glowing splint.
4
Group 1 nitrates (except lithium) form the nitrite and oxygen: 2MNO₃(s) → 2MNO₂(s) + O₂(g).
5
Lithium nitrate behaves like Group 2: 4LiNO₃(s) → 2Li₂O(s) + 4NO₂(g) + O₂(g).

Part 4 of 5Exam Approach

Name the products
  • State the solid and the gases formed, with correct state symbols.
Use observations
  • Mention brown gas for nitrogen dioxide and relighting of a glowing splint for oxygen.
Explain with polarisation
  • Link stability to the charge density of the cation and its distorting effect on the anion.
Compare groups
  • Group 1 compounds are generally more stable than Group 2 because the ions carry a smaller charge.

Part 5 of 5Exam Checklist & Tips

2.2.4 – Key Points for the Exam

  • Thermal decomposition A compound breaking down when heated.
  • Polarising power Ability of a cation to distort the electron cloud of an anion.
  • Charge density Charge of an ion relative to its size.
  • Thermal stability Resistance to decomposition on heating.

Common Mistakes

  • Saying Group 1 nitrates give nitrogen dioxide, except for lithium.
  • Forgetting to balance the oxygen in the nitrate decomposition equations.
  • Explaining the trend by atomic size instead of ionic size and charge density.
  • Stating that stability decreases down the group.

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