Group 2

The single subtopic here is printed under "A Level subject content" in the 9701 syllabus and both its objectives carry the tier "A Level". None of it is AS, and it is a separate topic from the AS topic 10 of the same name, which covers the reactions of the elements themselves. It is examined on Paper 4. Paper 1 is the AS multiple-choice paper and the whole 9701 bank on this site comes from it, so no practice here is tagged to this topic.

This is the shortest topic in the A Level chemistry course, with two objectives. Both are explanations of trends, not lists of facts, and both come back to the same idea: the size and charge of the ions.

Syllabus points

27.1 Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

• 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, ΔH_sol, of the hydroxides and sulfates in terms of relative magnitudes of the enthalpy change of hydration and the lattice energy.

The one fact everything follows from

Going down Group 2 from magnesium to barium, the cation gains a shell of electrons, so the ionic radius increases:

Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺

The charge stays at 2+, so the charge density falls steadily down the group. Every trend in this topic is a consequence of that, and an answer that does not mention ionic radius is unlikely to earn much.

Thermal stability of the nitrates and carbonates

The trend. Thermal stability increases down the group. Magnesium carbonate decomposes at the lowest temperature and barium carbonate at the highest, and the same order holds for the nitrates.

The decomposition reactions are:

MgCO₃ → MgO + CO₂

2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂

so a carbonate gives the oxide and carbon dioxide, and a nitrate gives the oxide, brown nitrogen dioxide and oxygen. The brown gas is the usual practical clue that a nitrate is being decomposed.

The explanation, which is where the marks are. The cation attracts the electrons of the large anion towards itself, distorting the anion's electron cloud. This distortion is called polarisation.

1. A small cation with a high charge has a high charge density, so it polarises the anion strongly.
2. Polarisation draws electron density away from one of the carbon-to-oxygen or nitrogen-to-oxygen bonds within the anion, weakening it.
3. A weakened bond breaks at a lower temperature, so the compound is less thermally stable.

Down the group the cation gets larger, its charge density falls, it polarises the anion less, the bonds within the anion are less weakened, and the compound is more stable.

Two extensions worth being ready for. Group 1 carbonates are far more stable than the corresponding Group 2 ones, because a 1+ ion polarises much less than a 2+ ion of similar size. And lithium is the exception in Group 1, since Li⁺ is small enough to polarise appreciably, which is why lithium carbonate decomposes when the other Group 1 carbonates do not.

Solubility of the hydroxides and sulfates

The trends run in opposite directions, and remembering which is which is easier once you see why.

• Hydroxides become more soluble down the group. Mg(OH)₂ is almost insoluble, and Ba(OH)₂ is appreciably soluble. This is why calcium hydroxide solution is limewater but magnesium hydroxide is a suspension.
• Sulfates become less soluble down the group. MgSO₄ is very soluble and BaSO₄ is essentially insoluble, which is what makes barium sulfate the reagent for the sulfate test and safe to swallow as a barium meal.

The explanation, through the energy cycle

From topic 23, the enthalpy change of solution is the difference between two large quantities:

ΔH_sol = ΣΔH_hyd - ΔH_latt

A compound tends to be soluble when ΔH_sol is small or negative, so the question in each case is which of the two terms changes faster down the group.

Both become less exothermic down the group, because the cation is getting larger:

• The lattice energy becomes less exothermic, since the ions in the lattice are further apart.
• The hydration enthalpy of the cation becomes less exothermic, since water molecules cannot approach the centre of a larger ion so closely.

Which one falls faster depends on the size of the anion, and that is the whole argument.

Hydroxides: a small anion. OH⁻ is small, so the interionic distance in the lattice is dominated by the cation size, and the lattice energy is very sensitive to it. Going down the group the lattice energy decreases in magnitude more rapidly than the hydration enthalpy does, so ΔH_sol becomes more negative and solubility increases.

Sulfates: a large anion. SO₄²⁻ is large, so it already dominates the interionic distance, and changing the cation makes proportionally little difference to the lattice energy. Meanwhile the hydration enthalpy of the cation still falls appreciably, because hydration depends on the cation alone. So the hydration enthalpy decreases in magnitude more rapidly than the lattice energy, ΔH_sol becomes less negative, and solubility decreases.

Reduced to one sentence: whichever term changes faster decides the trend, and the size of the anion decides which term that is. A small anion makes the lattice energy the sensitive term; a large anion makes hydration the sensitive term.

Common mistakes

• Saying thermal stability decreases down the group. It increases.
• Explaining thermal stability by the strength of the bond between the cation and the anion, rather than by polarisation weakening a bond within the anion.
• Saying a larger cation polarises more. Polarising power depends on charge density, which falls as the ion grows.
• Forgetting that the nitrate decomposition also gives oxygen, or giving NO rather than NO₂ as the brown gas.
• Giving the same solubility trend for the hydroxides and the sulfates. They run in opposite directions.
• Explaining a solubility trend by referring to only one of the two energy terms. The trend is set by which changes faster, so both must be mentioned.
• Saying the hydration enthalpy of the anion changes down the group. The anion is the same throughout; only the cation changes.
• Writing "the lattice energy increases down the group" without saying whether that means a larger or a smaller magnitude, when the values are negative.