Predict thermal stability of Group 2 carbonates and nitrates
Use anion polarisation to explain why carbonates and nitrates become more thermally stable down the group.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 27. Compare Group 2 ions at the same charge and distinguish lattice formation, lattice dissociation, hydration and solution enthalpy. Trend explanations are qualitative unless data are supplied. Written practice is self-checked
Before you begin
- Use AS Group 2 reactivity trends, ionic charge and radius, Hess cycles, and balanced equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does thermal stability of Group 2 carbonates change down the group?
What you will learn
- State the thermal-stability trend for Group 2 carbonates
- State the thermal-stability trend for Group 2 nitrates
- Explain the shared trend through cation size and anion polarisation
Thermal stability increases down the group
For Group 2 carbonates and nitrates, thermal stability increases from magnesium to barium. In a comparable heating experiment, a lower-group compound resists decomposition more strongly and requires a higher temperature to decompose.
Keep stability distinct from reaction speed or solubility. The syllabus asks for a qualitative trend and its explanation; use supplied experimental data when a question asks for a numerical comparison.
Link the trend to polarisation
Mg²⁺ is smaller than Ba²⁺ and has greater polarising power. It distorts CO₃²⁻ or NO₃⁻ more strongly, weakening the large anion and making decomposition easier. Ba²⁺ causes less distortion, so BaCO₃ and Ba(NO₃)₂ are more thermally stable.
This is an ionic polarisation model for a trend. Avoid saying that the carbonate or nitrate decomposes because the metal cation itself breaks apart.
Pause and explain
Why is Ba(NO₃)₂ more thermally stable than Mg(NO₃)₂?
Worked example
Which is more thermally stable, MgCO₃ or BaCO₃? Explain the comparison.
Show the worked solution
- Thermal stability of Group 2 carbonates increases down the group.
- Ba²⁺ is larger and less polarising than Mg²⁺.
- The carbonate ion is less distorted in the barium compound.
Answer BaCO₃ is more thermally stable. The larger Ba²⁺ ion has lower polarising power, so it distorts CO₃²⁻ less than Mg²⁺ does; the carbonate ion is therefore less destabilised and needs stronger heating to decompose.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict which needs stronger heating to decompose: CaCO₃ or SrCO₃.
Give me a hint
Move down Group 2 and apply the thermal-stability trend.
Compare my reasoning
- Sr is below Ca in Group 2.
- Thermal stability increases down the group.
- The more stable carbonate requires stronger heating.
SrCO₃ is more thermally stable and needs stronger heating to decompose than CaCO₃.
Look for these in your work
- Group position correct
- Trend applied
- Heating prediction follows stability
Explain why the nitrate thermal-stability trend is consistent with the carbonate trend.
Give me a hint
Both are large polyatomic anions paired with the same 2+ cation series.
Compare my reasoning
- Mg²⁺ is smaller and more polarising than Ba²⁺.
- Both NO₃⁻ and CO₃²⁻ can be distorted by the cation.
- Less distortion down the group makes decomposition less easy.
Both trends are consistent with decreasing cation polarising power down the group. The larger Group 2 cations distort NO₃⁻ and CO₃²⁻ less, so both sets of compounds become more thermally stable from Mg to Ba.
Look for these in your work
- Cation trend stated
- Both anions considered
- Thermal consequence explained
A student concludes that BaCO₃ is less thermally stable because it is less soluble in water. Identify the error and give the correct comparison with MgCO₃.
Give me a hint
Solubility and decomposition on heating are different properties.
Compare my reasoning
- Do not use aqueous solubility as a measure of thermal stability.
- Use the cation-radius/polarisation explanation.
- State which carbonate is more stable and why.
The inference confuses solubility with thermal stability. BaCO₃ is more thermally stable than MgCO₃ because larger Ba²⁺ polarises CO₃²⁻ less strongly, so greater heating is needed for decomposition.
Look for these in your work
- Properties distinguished
- Correct compound selected
- Reason linked to polarisation
Common mistakes
- Reversing the trend and saying stability decreases down the group
- Giving the trend without the cation–anion explanation
- Confusing thermal stability with solubility
- Claiming that larger Ba²⁺ has greater polarising power
What can you explain now?
How does thermal stability of Group 2 carbonates change down the group?
Compare with the explanation
It increases
The larger cations polarise the carbonate ion less strongly.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does a change in cation radius lead to the thermal-stability trend for large oxyanions?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.