Write and interpret Group 2 thermal-decomposition equations
Represent carbonate and nitrate decomposition with balanced equations and connect the products to the stability trend.
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
What are the typical products when a Group 2 carbonate is heated?
What you will learn
- Write the typical decomposition equation for a Group 2 carbonate
- Write the typical decomposition equation for a Group 2 nitrate
- Use equation evidence without overclaiming a specific experimental temperature
Carbonates form an oxide and carbon dioxide
The standard thermal decomposition pattern for a Group 2 carbonate is MCO₃(s) → MO(s) + CO₂(g), where M is a Group 2 metal. Check atoms and charge by writing the neutral solids and carbon dioxide explicitly.
The equation gives the reaction pattern, not a measured onset temperature. Thermal stability is compared qualitatively unless the question supplies data.
Nitrates form an oxide, nitrogen dioxide and oxygen
For a Group 2 nitrate, the balanced pattern is 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g). The coefficients matter: two nitrate formula units contain four nitrogen atoms and twelve oxygen atoms.
The equation predicts products under heating. In real laboratory work, nitrate decomposition can produce harmful nitrogen dioxide; this lesson is a paper-based chemistry model, not a practical instruction.
Pause and explain
Why is 2M(NO₃)₂ used in the balanced nitrate equation?
Worked example
Write balanced equations for heating MgCO₃ and Ca(NO₃)₂.
Show the worked solution
- For the carbonate, make one oxide and one carbon dioxide.
- For the nitrate, begin with two formula units.
- Balance metal, nitrogen and oxygen atoms.
Answer MgCO₃(s) → MgO(s) + CO₂(g). 2Ca(NO₃)₂(s) → 2CaO(s) + 4NO₂(g) + O₂(g).
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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.
Complete and balance the decomposition of SrCO₃(s).
Give me a hint
Use the carbonate pattern MCO₃ → MO + CO₂.
Compare my reasoning
- Put Sr in the oxide.
- Keep one carbon atom in CO₂.
- Check oxygen atoms on both sides.
SrCO₃(s) → SrO(s) + CO₂(g).
Look for these in your work
- Correct oxide formula
- Carbon dioxide included
- Atom count balanced
Write the balanced thermal decomposition equation for Ba(NO₃)₂(s).
Give me a hint
Use two nitrate formula units, oxide, nitrogen dioxide and oxygen.
Compare my reasoning
- Start with 2Ba(NO₃)₂.
- Form 2BaO and 4NO₂.
- Add O₂ and check oxygen atoms.
2Ba(NO₃)₂(s) → 2BaO(s) + 4NO₂(g) + O₂(g).
Look for these in your work
- Metal balanced
- Nitrogen balanced
- Oxygen balanced
A proposed equation is M(NO₃)₂(s) → MO(s) + 2NO₂(g). What product or coefficient is missing?
Give me a hint
Count oxygen atoms before and after for one formula unit or double the entire equation.
Compare my reasoning
- The left side has six oxygen atoms per M(NO₃)₂.
- The shown products contain five oxygen atoms.
- Add half O₂ for one unit, or double to use whole-number coefficients.
The equation needs ½O₂(g): M(NO₃)₂(s) → MO(s) + 2NO₂(g) + ½O₂(g). With whole-number coefficients, use 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
Look for these in your work
- Oxygen imbalance detected
- Missing oxygen product identified
- Equivalent balanced forms recognised
Common mistakes
- Forgetting the coefficient 2 before the nitrate
- Omitting oxygen gas from the nitrate decomposition pattern
- Writing a hydroxide instead of the oxide product
- Treating an equation as evidence for an exact temperature
What can you explain now?
What are the typical products when a Group 2 carbonate is heated?
Compare with the explanation
The metal oxide and carbon dioxide
MCO₃ decomposes to MO and CO₂.
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Make it stick
How can atom counting check the products and coefficients in a nitrate decomposition equation?
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.