Compare Group 2 thermal stability and solubility data
Synthesize the two Topic 27 outcome groups in structured explanations and data-based comparisons.
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
Which explanation best matches increasing carbonate thermal stability down Group 2?
What you will learn
- Choose the correct trend for each anion and property
- Support a trend with a particle-level explanation
- Qualify conclusions drawn from enthalpy and solubility data
One cation-size change has different consequences
As Group 2 cation radius increases, polarising power and cation hydration strength decrease, while the lattice becomes easier to separate. These ideas connect the syllabus outcomes but do not predict every property in the same direction.
For thermal stability, weaker polarisation of carbonate and nitrate ions down the group makes the anions less destabilised. For dissolution, the balance between lattice separation and hydration differs for hydroxides and sulfates; hydroxide solubility increases, whereas sulfate solubility decreases.
Build a complete comparison from evidence
A strong response names the direction, compares the relevant species, explains the microscopic cause and cites the data given. When a table gives ΔHsol, use the stated sign convention and distinguish an enthalpy trend from an exact solubility measurement.
Do not transfer the sulfate trend to hydroxides or use a thermal-decomposition argument to explain solubility. Each explanation should match the property asked about.
Pause and explain
Which pair of solubility trends is correct?
Worked example
A table shows increasing nitrate decomposition temperature from Mg to Ba, rising hydroxide solubility, and falling sulfate solubility. Give a reasoned summary without treating these as one trend.
Show the worked solution
- State the thermal-stability direction and use polarisation.
- State each solubility direction separately.
- For solubility, refer to the balance of lattice and hydration terms.
Answer Nitrate thermal stability increases down Group 2 because larger M²⁺ ions polarise NO₃⁻ less strongly. Hydroxide solubility increases, while sulfate solubility decreases; these opposite solubility trends reflect different balances between the lattice-separation cost and ion hydration for the two anions.
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.
Complete the comparison: carbonate thermal stability __ down Group 2; hydroxide solubility __; sulfate solubility __.
Give me a hint
Recall each property separately before explaining it.
Compare my reasoning
- Thermal stability increases down the group.
- Hydroxide solubility increases.
- Sulfate solubility decreases.
Carbonate thermal stability increases; hydroxide solubility increases; sulfate solubility decreases down Group 2.
Look for these in your work
- Thermal trend correct
- Hydroxide trend correct
- Sulfate trend correct
A data table gives ΔHsol values for two hydroxides and two sulfates. Outline a safe method for comparing the data with solubility trends.
Give me a hint
Check units, formula ratios, the sign convention and the property being inferred.
Compare my reasoning
- Calculate signed sums only if component data are given.
- Compare hydroxide values with hydroxide values and sulfate with sulfate.
- State that ΔHsol supports an explanation but is not a direct solubility value.
Keep the two anion series separate, apply the stated lattice convention and stoichiometry, and compare signed ΔHsol values. Relate the enthalpy pattern to the known qualitative solubility trend while noting that ΔHsol alone does not give exact solubility.
Look for these in your work
- Data and convention checked
- Like compounds compared
- Inference limitation stated
A response explains falling sulfate solubility by saying ‘Ba²⁺ polarises SO₄²⁻ less, so the sulfate ion is more stable.’ Why is this not the appropriate syllabus explanation?
Give me a hint
Separate the thermal-stability outcome for large anions from the solution-enthalpy outcome.
Compare my reasoning
- Polarisation explains thermal stability of nitrates and carbonates in this topic.
- Sulfate solubility is explained by lattice and hydration contributions.
- Give the net solubility direction after comparing those terms.
The polarisation explanation is the specified model for nitrate and carbonate thermal stability. Sulfate solubility should be explained through the relative lattice and hydration contributions to ΔHsol; the net trend is decreasing solubility down the group.
Look for these in your work
- Outcome scope distinguished
- Correct enthalpy model selected
- Trend stated accurately
Common mistakes
- Writing that all Group 2 properties increase or decrease together
- Using polarisation as the explanation for sulfate solubility
- Forgetting to name the measured property
- Claiming enthalpy data alone prove exact solubility
What can you explain now?
Which explanation best matches increasing carbonate thermal stability down Group 2?
Compare with the explanation
Lower polarising power of the larger cation
Less distortion leaves the carbonate ion more thermally stable.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which physical model belongs to thermal stability, and which belongs to hydroxide/sulfate dissolution?
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.