- describe and explain qualitatively the trend in the thermal stability of the nitrates and carbonates including the effect of ionic radius on the polarisation of the large anion
- describe and explain qualitatively the variation in solubility and of enthalpy change of solution, $\Delta H^{\ominus}_{\text{sol}}$, of the hydroxides and sulfates in terms of relative magnitudes of the enthalpy change of hydration and the lattice energy
Group 2
A-Level Chemistry · Topic 27
27.1
Thermal stability of the nitrates and carbonates
Syllabus
Source: Cambridge International syllabus
The thermal stability 热稳定性 of the Group 2 nitrates 硝酸盐 and carbonates 碳酸盐 increases down the group. Here is the reason, in terms of how the ions affect each other.
Heating limestone (calcium carbonate) in a kiln decomposes it to calcium oxide — a thermal decomposition
A small cation 阳离子 with a small ionic radius 离子半径 has a high charge density. It pulls on the electrons of the nearby anion and distorts its shape — this is polarisation 极化. Distorting the large anion 阴离子 (the carbonate or nitrate ion) weakens a bond inside it, so the compound breaks down more easily.
Going down the group, the cation gets larger. Its charge density drops, so it polarises the anion less. The anion is less distorted, so the compound is harder to break down — it is more thermally stable and needs a higher temperature to decompose.
A small cation has a high charge density, so it distorts (polarises) the large anion more — weakening it, so the compound is less stable
Worked example. Down Group 2 the hydroxides become more soluble while the sulfates become less soluble. Explain why the two trends run in opposite directions. Solubility is a contest between the lattice energy holding the solid together and the hydration energy rewarding the ions for dissolving. Both get less exothermic as the cation grows, so whichever falls faster decides the trend. With the small hydroxide ion, the lattice energy depends strongly on the cation's size, so it falls faster than the hydration energy: the lattice gets relatively easier to break and solubility rises. With the large sulfate ion, the lattice energy is already dominated by that big anion and barely changes down the group, while the cation's hydration energy still falls - so solubility falls. Name both energies and say which one falls faster; simply restating the trends earns no explanation marks.
Explore
Group 2 thermal stability ladder
Move down Group 2 and see why carbonates become harder to decompose.
Explore
Group 2 solubility lab
Classify Group 2 compounds by solubility trend.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| thermal stability | 热稳定性 | rè wěn dìng xìng |
| nitrate | 硝酸盐 | xiāo suān yán |
| carbonate | 碳酸盐 | tàn suān yán |
| cation | 阳离子 | yáng lí zi |
| ionic radius | 离子半径 | lí zi bàn jìng |
| polarisation | 极化 | jí huà |
| anion | 阴离子 | yīn lí zi |
27.1
Solubility of the hydroxides and sulfates
When an ionic solid dissolves, two energy changes compete. The enthalpy change of solution 溶解焓变 is the sum of them:
Down Group 2 the hydroxides get more soluble but the sulfates less
$$\Delta H^{\ominus}_{\text{sol}} = -\Delta H_{\text{latt}} + \Delta H_{\text{hyd}}$$
- you must first pull the lattice apart (this needs the lattice energy 晶格能).
- then water surrounds the ions (this releases the enthalpy change of hydration 水合焓变).
If the energy released on hydration roughly matches (or beats) the energy needed to break the lattice, the solid dissolves easily.
An insoluble metal hydroxide forms as a gelatinous precipitate when its lattice energy is too large to be repaid by hydration
Going down the group, both the lattice energy and the hydration enthalpy get smaller (less negative), because the cation is larger. But they shrink at different rates, and this explains the opposite trends:
Why the trends are opposite: the small OH$^-$ lattice energy falls a lot down the group (hydroxides dissolve more), but the large SO$_4^{2-}$ lattice energy barely changes (sulfates dissolve less)
| Compound | Trend in solubility 溶解度 down the group | Why |
|---|---|---|
| hydroxides 氢氧化物 | increases | the $\text{OH}^-$ ion is small, so the lattice energy falls a lot down the group; this change outweighs the fall in hydration energy |
| sulfates 硫酸盐 | decreases | the $\text{SO}_4^{2-}$ ion is large, so the lattice energy stays almost the same; the fall in hydration energy then dominates, so dissolving becomes less favourable |
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| enthalpy change of solution | 溶解焓变 | róng jiě hán biàn |
| lattice energy | 晶格能 | jīng gé néng |
| enthalpy change of hydration | 水合焓变 | shuǐ hé hán biàn |
| solubility | 溶解度 | róng jiě dù |
| hydroxide | 氢氧化物 | qīng yǎng huà wù |
| sulfate | 硫酸盐 | liú suān yán |
27.1
Exam tips
- Thermal stability of carbonates and nitrates increases down the group — larger cations polarise the anion less.
- Give the decomposition products: carbonates → oxide + $\text{CO}_2$; nitrates → oxide + $\text{NO}_2$ + $\text{O}_2$ (brown gas).
- Hydroxides get more soluble and sulfates less soluble down the group.
- Explain trends with cation charge density (polarising power), not just size.