Group 2 thermal stability of nitrates/carbonates and solubility trends (A2)

Chemistry· 9701 A2 Unit 10.3· 25 min read

1. The Polarisation Model (Fajans' Rules) for Group 2★★★☆☆⏱ 6 min

All Group 2 cations have a +2 charge, but ionic radius increases down the group from Be²⁺ to Ba²⁺. This means charge density, the ratio of charge to ionic volume, decreases down the group.

📘 Definition

Anion polarisation

A high charge density cation pulls electron density away from the anion, distorting its spherical shape and weakening covalent character in the ionic lattice

Example:

Small Be²⁺ strongly distorts the large CO₃²⁻ anion, while large Ba²⁺ causes almost no distortion

📐 Worked Example

Rank the following Group 2 cations in order of increasing polarising power: Sr²⁺, Mg²⁺, Ba²⁺, Ca²⁺

1
Step 1: Recall that polarising power is directly proportional to charge density, which for same-charge cations depends only on ionic radius
2
Step 2: List ionic radii down Group 2: Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺
3
Step 3: Lower radius = higher charge density = higher polarising power
4
Final order (increasing polarising power): Ba²⁺ < Sr²⁺ < Ca²⁺ < Mg²⁺

✓ Quick check

Test your understanding of polarisation fundamentals:

Which cation will cause the greatest distortion of a nitrate anion?
- Ca²⁺
- Be²⁺
- Ba²⁺
- Sr²⁺
Reveal answer
Be²⁺ —
Be²⁺ is the smallest Group 2 cation, so it has the highest charge density and strongest polarising power

2. Thermal Stability of Group 2 Carbonates★★★★☆⏱ 7 min

Group 2 carbonates decompose on heating to form a solid metal oxide and carbon dioxide gas. The general reaction is:

MCO_3(s) \rightarrow MO(s) + CO_2(g)

🔬 Derivation

Goal:

Explain why decomposition temperature increases down Group 2

Starting from:

Small top-group cations have high polarising power

1
High polarising power distorts the CO₃²⁻ anion, breaking the C-O bond in the carbonate ion more easily
2
Less energy is required to break this distorted bond, so lower temperatures trigger decomposition
3
Down the group, cation polarising power falls, so less anion distortion occurs, requiring higher temperatures to decompose the carbonate

Result:

Thermal stability of Group 2 carbonates increases down the group

📐 Worked Example

Write the balanced decomposition equation for calcium carbonate, and state the approximate temperature required for full decomposition

1
Step 1: Identify products: solid calcium oxide and carbon dioxide gas
2
Step 2: Balance atoms on both sides, no extra coefficients needed
3
$CaCO_3(s) \xrightarrow{>825^\circ C} CaO(s) + CO_2(g)$
4
Step 3: Note that calcium carbonate decomposes at ~825°C, which is higher than magnesium carbonate (~500°C) and lower than strontium carbonate (~1350°C)

3. Thermal Stability of Group 2 Nitrates★★★★☆⏱ 6 min

Group 2 nitrates follow the same stability trend as carbonates, but decompose to form a metal oxide, brown nitrogen dioxide gas, and oxygen gas. The general balanced reaction is:

2M(NO_3)_2(s) \rightarrow 2MO(s) + 4NO_2(g) + O_2(g)

📐 Worked Example

Compare the thermal stability of magnesium nitrate and barium nitrate, and justify your answer

1
Step 1: State the trend: Barium nitrate is more thermally stable than magnesium nitrate
2
Step 2: Justify: Mg²⁺ has a smaller ionic radius than Ba²⁺, so higher charge density and stronger polarising power
3
Step 3: Stronger polarisation distorts the large NO₃⁻ anion more, breaking internal N-O bonds at a lower temperature, making magnesium nitrate less stable

4. Group 2 Solubility Trends★★★★☆⏱ 6 min

Group 2 solubility follows two opposing patterns that examiners frequently test. For Group 2 hydroxides, solubility increases down the group, while for Group 2 sulfates, solubility decreases down the group.

📐 Worked Example

Which of the following compounds is the most soluble in water? Ba(OH)₂, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂

1
Step 1: Recall the solubility trend for Group 2 hydroxides: solubility increases down the group
2
Step 2: Identify the lowest Group 2 cation in the list: Ba²⁺
3
Step 3: Confirm: Barium hydroxide is the most soluble of the options, while magnesium hydroxide is almost insoluble

5. Common Pitfalls

Wrong move:

Stating that thermal stability increases down Group 2 because ionic bonds get stronger

Why:

The decomposition does not break the ionic bonds between M²⁺ and anion, it breaks internal covalent bonds inside the anion

Correct move:

Explicitly reference polarisation of the anion and weakening of internal anion covalent bonds for full marks

Wrong move:

Writing the decomposition of Group 2 nitrates to form a metal nitrate(III) and oxygen

Why:

This is the reaction for Group 1 nitrates, not Group 2, and will lose all marks for the equation

Correct move:

Use the 2:2:4:1 stoichiometry for Group 2 nitrate decomposition producing NO₂ gas

Wrong move:

Mixing up the solubility trends for hydroxides and sulfates

Why:

CIE awards 1 mark for correctly stating each trend, mixing them up loses both marks

Correct move:

Use the HIS mnemonic to confirm hydroxides increase, sulfates decrease down the group

Wrong move:

Including lithium or sodium in Group 2 trend comparisons

Why:

Group 1 cations have +1 charge, so their charge density values are not comparable to +2 Group 2 cations

Correct move:

Only reference other Group 2 cations when explaining Group 2 specific trends

Wrong move:

Stating that BeCO₃ is the most stable Group 2 carbonate

Why:

Beryllium cation has the highest polarising power, so BeCO₃ is unstable at room temperature, the least stable of the group

Correct move:

Note that BeCO₃ requires storage in a CO₂ atmosphere to prevent spontaneous decomposition

6. Quick Reference Cheatsheet

Compound Class	Trend Down Group 2	Key Explanation	Representative Example
Group 2 Carbonates	Thermal stability increases	Falling cation charge density reduces anion polarisation	BaCO₃ decomposes at >1400°C
Group 2 Nitrates	Thermal stability increases	Falling cation charge density reduces nitrate anion distortion	Mg(NO₃)₂ decomposes at ~330°C
Group 2 Hydroxides	Solubility increases	Hydration enthalpy falls slower than lattice enthalpy	Ba(OH)₂ is fully soluble in water
Group 2 Sulfates	Solubility decreases	Hydration enthalpy falls faster than lattice enthalpy	BaSO₄ is almost completely insoluble

What's Next

Mastering these Group 2 trends gives you a solid foundation for all periodicity questions in your A2 exam, which make up 8-12% of the total A2 inorganic chemistry marks. You will now be able to tackle practical analysis questions for identifying unknown Group 2 compounds via their decomposition temperatures and solubility behaviour, which are common in Paper 3 and Paper 5. These trend rationalisation skills also transfer directly to Group 17 redox potential trends and d-block compound stability explanations, so you will see the same polarisation and charge density logic reused across multiple inorganic topics.