1. Trends in chemical properties of group 2

2. Reactivity Group 2 are less reactive than group 1. Na vigorously reacts with water. But Ca reacts very slowly. The difference is due to the amount of energy required to form the metal ion.

3. Energy needed Li519 kj/mol Na494 K418 Rb402 Cs376 Energy needed Be2660 kj/mol Mg2189 Ca1735 Sr1614 Ba1468

4. The combined ionisation energies decrease down the group. This means that reactivity increases as it becomes progressively easier to form the M 2+ ion. Energy needed Be2660 kj/mol Mg2189 Ca1735 Sr1614 Ba1468

6. Reactions of magnesium with water. Magnesium reacts slowly with cold water; Mg + 2H 2 O → Mg(OH) 2 + H 2 Magnesium reacts more readily with steam to form its oxide; Mg + H 2 O (g) → MgO + H 2

7. Reactions of magnesium hydroxide Mg(OH) 2 neutralises acids. Mg(OH) 2 + 2HCl→ MgCl 2 + 2H 2 O

8. Treating indigestion. HCl is secreted in the stomach to aid digestion. Excess acid causes indigestion. “Milk of Magnesia” contains magnesium hydroxide so neutralises excess acid.

9. Reactions of Ca, Sr and Ba with water Calcium reacts rapidly with cold water to give hydrogen and a white percipitate of calcium hydroxide. Ca + 2H 2 O → Ca(OH) 2 + H 2 Strontium and barium react in the same way, but more vigorously.

10. Reactions of calcium hydroxide Ca(OH) 2 neutralises acids. Ca(OH) 2 + 2HCl→ CaCl 2 + 2H 2 O It is commonly known as “slaked lime” and is used by farmers to lower soil acidity. It is also used in some anti acids.

11. Relative solubilities of the hydroxides The solubilities of group 2 hydroxides increase from Mg to Ba. Magnesium hydroxide is almost insoluble whereas barium hydroxide readily dissolves. Compound Solubility mol/100g at 298K Mg(OH) 2 2 x 10 -5 Ca(OH) 2 1.5 x 10 -3 Sr(OH) 2 3.4 x 10 -3 Ba(OH) 2 1.5 x 10 -2

12. Relative solubilities of the sulphates Compound Solubility mol/100g at 298K MgSO 4 1.8 x 10 -1 CaSO 4 4.7 x 10 -3 SrSO 4 7.1 x 10 -5 BaSO 4 9.4 x 10 -7 The solubilities of group 2 sulphates decrease from Mg to Ba.

13. Testing for the presence of sulphate ions. Barium chloride is used as a reagent to test for the presence of sulphate ions. First dilute hydrochloric acid is added to remove any carbonate ions present. CO 3 2- + 2H + → CO 2 + H 2 O Otherwise white barium carbonate might be precipitated. BaCl 2 + SO 4 2- → BaSO 4 (s) + 2Cl - Barium sulphate forms a white ppt.

14. Barium meals Soft tissue normally lets through X rays. But barium is opaque, its heavy atom absorbs X rays. A “barium meal”, barium sulphate, can be administered to examine the gut. Although barium salts are highly toxic the sulphate is safe to use as it is insoluble.