Write thermal decomposition equations for Group 2 nitrates
Balance nitrate decomposition equations and use the stability trend to compare Group 2 nitrates.
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 metal-containing solid forms when a Group 2 nitrate decomposes?
What you will learn
- Identify the oxide, nitrogen dioxide and oxygen products
- Balance Group 2 nitrate decomposition
- Compare thermal stability down the group
A Group 2 nitrate forms an oxide and two gases
The general equation is 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g). The coefficients balance two metal atoms, four nitrogen atoms and twelve oxygen atoms on each side.
For example, 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂. Nitrogen dioxide is a harmful gas; the lesson teaches the equation and expected products, not an unsupervised heating procedure.
Thermal stability increases down Group 2
Group 2 nitrate stability increases from Mg(NO₃)₂ towards Ba(NO₃)₂. Under comparable heating, the lower-group nitrate decomposes more readily; the higher-group nitrate needs stronger heating.
The AS learning outcome requires equations and the stability trend. The detailed ionic-polarisation explanation is part of A Level Topic 27, so keep that mechanism outside this AS lesson unless a question explicitly supplies it.
Pause and explain
Which gas besides nitrogen dioxide is formed in Group 2 nitrate decomposition?
Worked example
Write the equation for calcium nitrate decomposing on heating and identify both gases.
Show the worked solution
- The metal-containing solid is CaO.
- Nitrate decomposition also forms NO₂ and O₂.
- Use coefficients 2, 2, 4 and 1 to balance the equation.
Answer 2Ca(NO₃)₂(s) → 2CaO(s) + 4NO₂(g) + O₂(g); the gases are nitrogen dioxide and oxygen.
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.
Balance the thermal decomposition equation for barium nitrate.
Give me a hint
Use BaO, NO₂ and O₂ as products, then balance in that order.
Compare my reasoning
- Start with Ba(NO₃)₂ → BaO + NO₂ + O₂.
- Use 2 barium nitrates to make 2 BaO and 4 NO₂.
- One O₂ balances the remaining oxygen atoms.
2Ba(NO₃)₂(s) → 2BaO(s) + 4NO₂(g) + O₂(g).
Look for these in your work
- Correct products
- Coefficients balance Ba and N
- Oxygen balanced
Compare the thermal stability of magnesium nitrate and strontium nitrate.
Give me a hint
Locate both metals in Group 2 and apply the down-group direction.
Compare my reasoning
- Strontium is below magnesium.
- Nitrate thermal stability increases down the group.
- Sr(NO₃)₂ is more thermally stable than Mg(NO₃)₂.
Sr(NO₃)₂ is more thermally stable than Mg(NO₃)₂; the nitrate stability increases down Group 2.
Look for these in your work
- Group order correct
- Trend direction correct
- Comparison explicit
A student writes M(NO₃)₂ → MO + 2NO₂ + ½O₂. Explain why an integer-coefficient equation is preferable and write it.
Give me a hint
Multiply every coefficient by two, then confirm atom counts.
Compare my reasoning
- A half coefficient is mathematically possible but standard equations use the smallest whole-number coefficients.
- Multiplying the equation by two gives two nitrate units and two oxide units.
- The integer equation is 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g).
Look for these in your work
- All coefficients scaled consistently
- Smallest whole-number form
- Atoms balanced
Common mistakes
- Forgetting O₂
- Writing NO instead of NO₂
- Using MNO₃ for a Group 2 nitrate
- Treating the AS trend as requiring the A Level mechanism
What can you explain now?
What metal-containing solid forms when a Group 2 nitrate decomposes?
Compare with the explanation
The metal oxide
The general product is MO.
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Make it stick
What are the products of nitrate decomposition, how do you balance the equation, and how does stability vary down the group?
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.