Topic 5 of 6
Group 2 and Group 17 chemistry
Use electrode potentials and explain thermal stability.
A-Level 9476 (2026-2027)
Read reduction potentials in their written direction
Group 2 metals reduce other species; halogens oxidise them.
For M2+(aq) + 2e- ⇌ M(s), a more negative standard reduction potential means the metal is more readily oxidised in a suitable paired reaction. Group 2 metals are reducing agents because they lose electrons. The standard potentials become generally more negative from Mg towards Ba, supporting greater reducing power down the group.
For X2 + 2e- ⇌ 2X-(aq), a more positive standard reduction potential means the halogen is a stronger oxidising agent. The usual values are approximately +1.36 V for chlorine, +1.07 V for bromine and +0.54 V for iodine, with the standard physical states specified in the data. Oxidising strength therefore decreases Cl2 > Br2 > I2.
Worked example
Predict a displacement using E degrees
Use +1.36 V for Cl2/Cl- and +1.07 V for Br2/Br- to assess Cl2 + 2Br- → 2Cl- + Br2.
- Chlorine is reduced; bromide is oxidised.
- Ecell = E(reduction at cathode) - E(reduction for the reverse oxidation pair) = 1.36 - 1.07.
- Ecell = +0.29 V, so the written reaction is thermodynamically favourable under standard conditions.
Chlorine can oxidise bromide to bromine. Reversing the reaction gives -0.29 V and is not favourable under those standard conditions.
Two thermal trends need two different explanations
Carbonates depend on cation polarisation; hydrogen halides depend on their covalent bond strength.
Group 2 carbonates decompose on heating: MCO3(s) → MO(s) + CO2(g). Down Mg to Ba, the cation radius increases while its charge remains +2. Charge density and polarising power decrease. The larger cation distorts the large carbonate ion less, so the carbonate is harder to decompose and its thermal stability increases.
- Same charge, larger cation down the group
Mg2+ is smaller than Ba2+, so Mg2+ has greater charge density.
- Polarise the large anion
The smaller cation distorts the carbonate electron cloud more strongly, destabilising the carbonate relative to decomposition products.
- Predict the heating result
Magnesium carbonate decomposes more readily; barium carbonate requires stronger heating.
For hydrogen halides, the relevant reaction is 2HX(g) ⇌ H2(g) + X2(g). From HCl to HBr to HI, the halogen atom becomes larger, the H-X bond becomes longer and the bond energy decreases. Thermal stability therefore decreases in that order: HI decomposes most readily.
| Trend down the group | Key reasoning | Direction of stability |
|---|---|---|
| MgCO3 to BaCO3 | Lower cation charge density and less polarisation of carbonate. | Increases |
| HCl to HI | Longer, weaker H-X covalent bond. | Decreases |