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

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u10-group-2-thermal-stability-of/

This module explains Group 2 nitrate and carbonate thermal stability trends via the polarisation model, balanced decomposition reactions, and opposing solubility patterns for hydroxides and sulfates.

**Prerequisites:** [Ionic bonding and lattice energy fundamentals](https://www.owlsprep.com/study/cie-9701-u4-ionic-bonding-lattice-energy/); [Group 2 element physical properties overview](https://www.owlsprep.com/study/cie-9701-u10-group-2-intro-physical-properties/)

## Learning objectives

- Explain Group 2 nitrate and carbonate thermal stability trends using the polarisation model
- Write fully balanced decomposition equations for all common Group 2 carbonates and nitrates
- Rationalise the opposing solubility trends of Group 2 hydroxides and sulfates
- Predict properties of uncharacterised Group 2 compounds using established periodic trends

## The Polarisation Model (Fajans' Rules) for Group 2

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.

**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⁺

**Check your understanding**

Test your understanding of polarisation fundamentals:

1. Which cation will cause the greatest distortion of a nitrate anion?

   - Ca²⁺
   - Be²⁺
   - Ba²⁺
   - Sr²⁺

   *Why:* Be²⁺ is the smallest Group 2 cation, so it has the highest charge density and strongest polarising power

## Thermal Stability of Group 2 Carbonates

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

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

> **Exam Mark Note**
>
> Only beryllium carbonate is unstable at room temperature; all other Group 2 carbonates require heating to decompose, with decomposition temperature rising down the group

**Derivation:** 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

*Conclusion:* 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)

## Thermal Stability of Group 2 Nitrates

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:

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

**Exam command terms**

CIE exam questions use specific command terms for this topic:

- **Describe the trend** — State the direction of stability change down the group only, no explanation required

- **Explain the trend** — You must reference cation charge density, anion polarisation, and weakening of the anion internal bonds to get full marks

**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

## Group 2 Solubility Trends

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.

> **Trend Mnemonic**
>
> HIS (Hydroxides Increase, Sulfates) Decrease to remember the two opposing trends

**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

## Common pitfalls

- **Wrong:** Stating that thermal stability increases down Group 2 because ionic bonds get stronger
  - Why it fails: The decomposition does not break the ionic bonds between M²⁺ and anion, it breaks internal covalent bonds inside the anion
  - Correct: Explicitly reference polarisation of the anion and weakening of internal anion covalent bonds for full marks
- **Wrong:** Writing the decomposition of Group 2 nitrates to form a metal nitrate(III) and oxygen
  - Why it fails: This is the reaction for Group 1 nitrates, not Group 2, and will lose all marks for the equation
  - Correct: Use the 2:2:4:1 stoichiometry for Group 2 nitrate decomposition producing NO₂ gas
- **Wrong:** Mixing up the solubility trends for hydroxides and sulfates
  - Why it fails: CIE awards 1 mark for correctly stating each trend, mixing them up loses both marks
  - Correct: Use the HIS mnemonic to confirm hydroxides increase, sulfates decrease down the group
- **Wrong:** Including lithium or sodium in Group 2 trend comparisons
  - Why it fails: Group 1 cations have +1 charge, so their charge density values are not comparable to +2 Group 2 cations
  - Correct: Only reference other Group 2 cations when explaining Group 2 specific trends
- **Wrong:** Stating that BeCO₃ is the most stable Group 2 carbonate
  - Why it fails: Beryllium cation has the highest polarising power, so BeCO₃ is unstable at room temperature, the least stable of the group
  - Correct: Note that BeCO₃ requires storage in a CO₂ atmosphere to prevent spontaneous decomposition

## 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.

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