Chapter 21 Main Groups Week 2 2009

Chapter 21 Main Groups Week 2 2009 - GROUP 13. (ns2np1)...

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GROUP 13 . (ns 2 np 1 ) Metallic character decreases moving right, and we find that boron (B) is not a metal = “metalloid”. B 2 O 3 is acidic (cf. Li 2 O is basic, gives OH - in water). Down group, metallic character increases - remainder are metals but oxides of Al and Ga are amphoteric (see below), while those of In and Tl are basic. Compare : B : B 2 O 3 (s) + 6 NaOH (aq) 2 Na 3 BO 3 (aq) + 3 H 2 O (l) acid base salt water (cf. HCl (g) + NaOH (aq) NaCl (aq) + H 2 O(l) ) in water, forms B(OH) 3 (or H 3 BO 3 ), known as boric acid. B(OH) 3 + 2 H 2 O B(OH) 4 - + H 3 O + pK a = 9.25 Al, Ga : oxides react with bases as above, but also with acids. They are “amphoteric” (= can behave as both acids or bases) Al 2 O 3 (s) + 6 NaOH 2 Na 3 AlO 3 (aq) + 3 H 2 O (l) acid base salt water Al 2 O 3 (s) + 3 H 2 SO 4 (l) Al 2 (SO 4 ) 3 (s) + 3 H 2 O (l) base acid salt water In, Tl : oxides are basic and react only with acids. In 2 O 3 (s) + 3 H 2 SO 4 (l) In 2 (SO 4 ) 3 (s) + 3 H 2 O (l) base acid salt water Bonding : B compounds are covalent, Al are sometimes ionic and sometimes covalent; ionic character increases down the group. 1
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Oxidation States All form +3, but Tl also +1. In general, when two oxidation states are possible, lower one becomes more important down the group, and its properties are more metal-like . Again, B (period 2) more different from the rest of group. For example, B forms many electron- deficient compounds: stable but nevertheless reactive to Lewis bases attain an octet e.g., BF 3 :F: B has only 6 e - in its outer (valence) :F:B:F: shell strong Lewis acid will accept electron pair from Lewis base e.g., BF 3 + :NH 3 F 3 B NH 3 this is source of acidity of B(OH) 3 + H 2 O B(OH) 4 - + H + Uses : Plentiful. B 2 O 3 used in production of borosilicate glass. B(OH) 3 (boric acid) used as disinfectant, eyewash, insecticide. Na 2 [B 4 O 5 (OH) 4 ] 8H 2 O used in washing powders. Boron hydrides B
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Chapter 21 Main Groups Week 2 2009 - GROUP 13. (ns2np1)...

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