Write thermal decomposition equations for Group 2 carbonates
Balance carbonate decomposition equations and describe the increase in thermal stability down the group.
Section 10 · Group 2
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Group trends are qualitative unless data are supplied. Show physical states and balance equations; distinguish reaction from dissolution and precipitation. This sequence does not replace supervised practical experience. Written practice is self-checked
Before you begin
- Use Group 2 formulas, ionic charges, state symbols, balanced equations and acid reaction patterns.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the products when a Group 2 carbonate decomposes on heating?
What you will learn
- Write carbonate-to-oxide decomposition equations
- Identify carbon dioxide as the gas product
- Compare carbonate thermal stability down Group 2
A metal carbonate forms its oxide and carbon dioxide on heating
The general equation is MCO₃(s) → MO(s) + CO₂(g). One metal, one carbon and three oxygen atoms appear on each side. For example, CaCO₃ → CaO + CO₂.
The thermal stability of Group 2 carbonates increases down the group. Therefore more heating is needed to decompose BaCO₃ than MgCO₃ under comparable conditions. The outcome asks learners to describe and predict the trend; do not add a mechanism beyond the AS requirement.
Use the trend to compare two samples, not to invent exact temperatures
If two carbonates are heated under the same conditions, the lower-group carbonate decomposes more readily; the higher-group carbonate is more thermally stable.
A qualitative trend cannot give a precise decomposition temperature or guarantee a particular time. Real observations depend on sample size, heating rate and apparatus.
Pause and explain
Which is more thermally stable: MgCO₃ or BaCO₃?
Worked example
Write the decomposition equation for barium carbonate and compare its stability with magnesium carbonate.
Show the worked solution
- A Group 2 carbonate forms the oxide and CO₂.
- BaCO₃ → BaO + CO₂ is balanced.
- Barium carbonate is more thermally stable than magnesium carbonate because stability increases down the group.
Answer BaCO₃(s) → BaO(s) + CO₂(g); BaCO₃ is more thermally stable than MgCO₃.
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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.
Write the balanced thermal decomposition equation for magnesium carbonate.
Give me a hint
Replace CO₃ with oxide plus carbon dioxide.
Compare my reasoning
- The oxide is MgO.
- The other product is CO₂.
- MgCO₃ → MgO + CO₂ already balances Mg, C and O.
MgCO₃(s) → MgO(s) + CO₂(g).
Look for these in your work
- Both products correct
- Equation balanced
- Gas state shown
A teacher heats CaCO₃ and BaCO₃ in identical conditions. Which is expected to decompose less readily?
Give me a hint
Use the thermal-stability trend down the group.
Compare my reasoning
- Barium is below calcium in Group 2.
- Thermal stability increases down the group.
- BaCO₃ is therefore expected to decompose less readily.
BaCO₃ is expected to decompose less readily because it is more thermally stable than CaCO₃.
Look for these in your work
- Group order correct
- Trend direction correct
- Prediction stated
An unknown Group 2 carbonate decomposes less readily than CaCO₃ under comparable conditions. What can you infer about its likely group position, and what cannot be inferred?
Give me a hint
Use stability as a positional clue, but avoid claiming an exact temperature or identity from one observation.
Compare my reasoning
- Greater thermal stability suggests a position below calcium.
- The trend supports a qualitative prediction.
- One observation does not identify the element or supply its decomposition temperature.
It is likely below calcium in Group 2, but the observation alone cannot identify it or give an exact decomposition temperature.
Look for these in your work
- Likely position inferred
- Trend linked
- Evidence limits stated
Common mistakes
- Writing CO instead of CO₂
- Keeping the carbonate ion intact in the oxide product
- Claiming stability decreases down the group
- Giving an exact temperature not supplied by the question
What can you explain now?
What are the products when a Group 2 carbonate decomposes on heating?
Compare with the explanation
The metal oxide and carbon dioxide
MCO₃ → MO + CO₂ conserves atoms.
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Make it stick
What products form from MCO₃ on heating, and how does thermal stability change from Mg to Ba?
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