Group 2 reactions and thermal decomposition

Last time

Before we start: last time, in Period 3 oxides and physical properties, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Chalk, limestone and fireworks

Limestone cliffs, chalk, the lime in cement, and the brilliant colours of fireworks all contain Group 2 elements. They share a pattern: each metal forms a two plus ion, and the properties change in a steady way down the group. The key to every trend is the size of that ion.

Group 2 basics

Group 2 elements have two outer electrons, in an s sub-shell. They lose both to form 2 plus ions. Ionic radius increases down the group, because each element has an extra shell. A large jump between the second and third ionisation energies of beryllium shows that the third electron comes from an inner shell: two outer electrons. The sum of the first two ionisation energies decreases down the group, so the metals get more reactive.

Reactions with water

Reactivity increases down the group. Magnesium reacts very slowly with cold water: Mg plus 2 H two O goes to Mg(OH) two plus H two. Calcium, strontium and barium react readily with cold water, giving an alkaline solution and hydrogen. For strontium: Sr solid plus 2 water liquid goes to Sr(OH) two aqueous plus H two gas. Magnesium burns in steam to give magnesium oxide and hydrogen. Barium makes barium hydroxide, the most soluble of the hydroxides: Ba plus 2 H two O goes to Ba(OH) two plus H two.

Solubility trends

The solubility of the hydroxides increases down the group. Magnesium hydroxide is almost insoluble; barium hydroxide is soluble. The solubility of the sulfates decreases down the group. Magnesium sulfate is very soluble; barium sulfate is insoluble. This gives a test: add dilute sulfuric acid or sodium sulfate to distinguish Mg nitrate from Ba nitrate. Barium gives a white precipitate.