Thermal stability of Group 2 compounds
Thermal stability (Group 2)
- The thermal stability of Group 2 nitrates and carbonates increases down the group.
- The reason is how the cation distorts the anion.
- A larger cation distorts less, so the compound is harder to break.
Practice
Going down Group 2, the thermal stability of the nitrates and carbonates:
A larger cation polarises the anion less, so the compound is harder to decompose — more stable down the group.
Polarisation
- A small cation has a high charge density. 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.
Practice
A small cation makes a carbonate decompose more easily because it:
High charge density distorts the anion, weakening it so it breaks down at a lower temperature.
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.
Practice
A larger cation lower in the group:
Bigger cation = lower charge density = less polarisation = more thermally stable.
Practice
A more thermally stable carbonate needs a higher temperature to decompose.
Greater stability means more energy is needed to break it down.
You've got it
Key idea
- 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