Thermal stability of Group 2 compounds
Introduced| English | Meaning |
|---|---|
| thermal stability/ˈθɜːml stəˈbɪlɪti/ | The resistance of a substance or system to decomposition or other unwanted change when heated. |
| cation/ˈkætaɪən/ | An ion with a positive electric charge. |
| anion/ˈænaɪən/ | An ion with a negative electric charge. |
| polarisation/ˌpəʊləraɪˈzeɪʃn/ | The distortion of an ion’s electron cloud by a nearby charge, giving ionic bonds some covalent character. |
| charge density/tʃɑːdʒ ˈdensɪti/ | charge density |
Thermal stability 热稳定性 (Group 2)
- The thermal stability of Group 2 nitrates and carbonates increases down the group.
- The reason is how the cation 阳离子 distorts the anion 阴离子.
- A larger cation distorts less, so the compound is harder to break.
Group 2 thermal stability ladder
Move down Group 2 and see why carbonates become harder to decompose.
Going down Group 2, the thermal stability of the nitrates and carbonates:
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A larger cation polarises the anion less, so the compound is harder to decompose — more stable down the group.
Group 2 carbonates become more stable to heat going down the group.
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Larger cations distort the carbonate ion less.
Polarisation 极化
- A small cation has a high charge density 电荷密度. It pulls on the nearby anion's electrons and distorts its shape — polarisation.
- Distorting the large carbonate/nitrate anion weakens a bond inside it, so the compound decomposes more easily.
A small cation distorts the large anion's electron cloud, weakening it — polarisation
Heating limestone (calcium carbonate) in a kiln decomposes it to calcium oxide.
A small cation makes a carbonate decompose more easily because it:
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High charge density distorts the anion, weakening it so it breaks down at a lower temperature.
Match each thermal-stability idea.
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Each item links the term to its correct meaning.
Down the group
- The cation gets larger, so its charge density drops and it polarises the anion less.
- The anion is less distorted → the compound is harder to break down → more thermally stable, needing a higher temperature.
Many metal hydroxides are insoluble and form as a gelatinous precipitate.
A larger cation lower in the group:
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Bigger cation = lower charge density = less polarisation = more thermally stable.
A more thermally stable carbonate needs a higher temperature to decompose.
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Greater stability means more energy is needed to break it down.
Polarising power explains it
- A smaller, more charged cation polarises the carbonate ion more, so it decomposes more easily.
- Going down Group 2 the cation gets larger, so the carbonates become more thermally stable.
You've got it
- thermal stability increases down Group 2 (nitrates and carbonates)
- a small cation polarises (distorts) the anion → weakens it → decomposes easily
- a larger cation polarises less → more stable → needs a higher temperature