Group 2

Syllabus points

• Describe the reactions of Group 2 elements with oxygen, water and dilute acids.
• Describe the behaviour of Group 2 oxides and hydroxides with water and with dilute acid.
• Explain the trend in solubility of the Group 2 hydroxides and sulfates.
• Describe and explain the thermal decomposition of Group 2 carbonates and nitrates.
• Explain the trends using ionic radius and charge density.

The elements and the trend

Group 2 runs beryllium, magnesium, calcium, strontium, barium. Each has two electrons in its outer s sub-shell and loses both to form a 2+ ion.

Reactivity increases down the group, and the reason is the same one that runs through the whole topic: the outer electrons are further from the nucleus and better shielded, so the ionisation energies fall and the metal loses its electrons more readily.

That single trend explains almost everything here, so it is worth stating in full whenever a question asks why.

Reactions of the elements

With oxygen, they burn to the oxide:

2Mg + O₂ → 2MgO

Magnesium burns with a brilliant white flame. The vigour increases down the group.

With water, the trend is clear and often examined:

• Magnesium reacts very slowly with cold water, giving Mg(OH)₂ and hydrogen. With steam it reacts rapidly to give MgO and hydrogen instead, which is a different product and a common trap.
• Calcium reacts steadily with cold water, fizzing, to give Ca(OH)₂ and hydrogen.
• Barium reacts rapidly.

Ca + 2H₂O → Ca(OH)₂ + H₂

With dilute acids, all react to give a salt and hydrogen, faster down the group:

Mg + 2HCl → MgCl₂ + H₂

Calcium with sulfuric acid is a useful exception to remember: the calcium sulfate formed is only slightly soluble, coats the metal and the reaction slows and stops.

Oxides and hydroxides

The oxides are basic. With water they give hydroxides:

CaO + H₂O → Ca(OH)₂

With dilute acid they give a salt and water:

MgO + 2HCl → MgCl₂ + H₂O

Calcium hydroxide solution is limewater, and the test for carbon dioxide is that it turns limewater milky, because insoluble calcium carbonate is formed.

Solubility trends

Two trends run in opposite directions, and mixing them up is the most common error in the topic.

So Mg(OH)₂ is only slightly soluble while Ba(OH)₂ dissolves readily, whereas MgSO₄ is very soluble while BaSO₄ is essentially insoluble.

The insolubility of barium sulfate is the basis of the test for sulfate ions: add dilute hydrochloric acid, then barium chloride solution, and a white precipitate confirms sulfate. The acid is added first to remove carbonate, which would also give a white precipitate and a false positive.

Barium sulfate is also used as a "barium meal" in medical imaging. Barium ions are toxic, but the sulfate is so insoluble that almost none dissolves.

Thermal decomposition

Group 2 carbonates decompose on heating to the oxide and carbon dioxide:

CaCO₃ → CaO + CO₂

Group 2 nitrates decompose to the oxide, nitrogen dioxide and oxygen:

2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂

Brown fumes of NO₂ are the observation to expect.

The trend, and why

Thermal stability increases down the group, so the compounds need a higher temperature to decompose as you go down.

The explanation is polarisation, and the reasoning is worth setting out in full because it earns several marks:

• Going down the group the cation gets larger, while the charge stays 2+.
• So the charge density of the cation falls.
• A cation of lower charge density polarises the carbonate or nitrate anion less, distorting its electron cloud less.
• Less distortion means the bonds within the anion are weakened less, so more energy is needed to break them.

Magnesium carbonate therefore decomposes at a lower temperature than barium carbonate.

The same argument explains why Group 1 carbonates are more stable than Group 2 ones: a 1+ ion has a lower charge density than a 2+ ion of similar size, so it polarises less.

Flame colours

The Group 2 metals give characteristic flame colours, used to identify them:

Magnesium gives no colour. The colour arises because heat promotes an electron to a higher energy level, and light of a particular wavelength is emitted as it falls back.

Common mistakes

• Getting the two solubility trends the wrong way round. Hydroxides become more soluble down the group; sulfates become less.
• Saying magnesium reacts vigorously with cold water. It is very slow, and only fast with steam.
• Giving Mg(OH)₂ as the product with steam. Steam gives MgO, not the hydroxide.
• Explaining thermal stability by ionisation energy. The reason is polarisation by the cation.
• Forgetting to acidify before adding barium chloride in the sulfate test.
• Saying barium compounds are safe because barium sulfate is used medically. It is safe only because it is so insoluble.