WJEC Chemistry for AS: Student Bk

All react with oxygen and burn in air while caesium inflames spontaneously. The oxides are all basic, i.e. they react with acids to give salts as in CaO + 2HCl → CaCl 2 + H 2 O Remember that the Group 2 hydroxide formulae are M(OH) 2 since (OH) is −1, i.e. [O(−2) H(+1)]. While Group 1 salts are all soluble the reactions of Group 2 ions with hydroxide, carbonate and sulfate ions give a variety of results that must be known. Mg(OH) 2 is insoluble in water but solubility increases down the group; BaSO4 is insoluble and solubility increases up the group. Although you are not required to remember any numerical values, they should help understanding of the scale of changes. Hydroxide solubilities rise from 0.01 g dm –3 with Mg to 3.9 g dm –3 in Ba; sulfate solubilities fall down the group from 330 g dm –3 in Mg to 0.002 g dm –3 in Ba. All Group 2 carbonates are insoluble and all s-block nitrates soluble. The thermal stability of Group 2 hydroxides and carbonates increases down the group; for example magnesium carbonate decomposes at 400° as against 1300° for barium carbonate. Although not in the Specification, it is useful in practical work to know that all the nitrates and chlorides of s-block elements are soluble in water. Flame colours: All of the common elements of Groups 1 and 2 except Mg show characteristic flame colours that must be known and that are useful in qualitative analysis. You should be aware of the great importance of calcium carbonate in both living and inorganic systems and of calcium phosphate minerals in living bones and skeletons. Calcium and magnesium ions play a vital role in the biochemistry of living systems – chlorophyll, muscle operation, etc., and the carbonates exist in huge amounts in rocks – such as chalk, limestone and dolomite. The halogens These reactive, electronegative elements typically form anions having an oxidation state of −1 so that oxidation is the usual reaction as in: Na(0) + 1 2 Cl 2 (0) → Na + (1) + Cl − (−1) with the Na being oxidised and the oxidising chlorine being reduced from 0 to −1. The tendency to form anions decreases down the group from fluorine to iodine with fluorine being the most electronegative element. The melting temperatures of the elements increase down the group from gaseous fluorine to solid iodine owing to the increasing intermolecular forces holding the diatomic elements together in a liquid or solid. This increase is due to the increasing number of electrons in the molecules contributing to the induced dipole–induced dipole intermolecular force. The halogens react with most metals to form halides with the reactivity decreasing down the group from fluorine to iodine. A similar feature is shown in displacement reactions in which a halogen higher in the group displaces one lower in the group from a salt as in: Cl 2 + 2NaBr → Br 2 + 2NaCl This essentially reflects the decrease in oxidising power down the group with chlorine oxidising the bromide ion to bromine and being itself reduced to chloride: Cl 2 (0) + 2 Br − (−1) → 2Cl − (−1) + Br 2 (0) ▲ Flame tests ▲ CaCO 3 fossils Knowledge check State the result, if any of adding: (a) Iodine to a solution of potassium bromide. (b) Chlorine to aqueous sodium bromide. 41 Knowledge check Which of the following additions will give a precipitate? (a) Ba 2+ + OH – (b) Ca 2+ + CO 3 2– (c) Ba 2+ + SO 4 2– 40 1.6 The periodic table 59