Reaction with limited OH- and limited NH3 The bases OH- and ammonia when in limited amounts form the same hydroxide precipitates. They form in deprotonation acid base reactions M(OH)2 (H2O)4 (s) : Cu blue ppt, Fe (II) green ppt M(OH)3 (H2O)3 (s) : Fe(III) brown ppt, Al white ppt [Cu(H2O)6 ] 2+ (aq) + 2OH- (aq) Cu(H2O)4 (OH)2 (s) + 2H2O (l) [Al(H2O)6 ] 3+ (aq) + 3OH- (aq) Al(H2O)3 (OH)3 (s) + 3H2O (l) [Fe(H2O)6 ] 2+ (aq) + 2NH3 (aq) Fe(H2O)4 (OH)2 (s) + 2NH4 + (aq) [Fe(H2O)6 ] 3+ (aq) + 3NH3 (aq) Fe(H2O)3 (OH)3 (s) + 3NH4 + (aq) This process can happen step wise removing one proton at a time. Be able to write equations for this too. e.g. [Al(H2O)6 ] 3+ (aq) + OH- (aq) [Al(H2O)5 (OH)]2+ (aq) + H2O (l) Reaction with excess OHWith excess NaOH the Al hydroxide dissolves. Al becomes [Al(OH)4 ] – (aq) colourless solution. This hydroxides is classed as amphoteric because it reacts and dissolves in both acids and bases. Al(H2O)3 (OH)3 (s) + OH- (aq ) [Al(OH)4 ] – (aq) + 3H2O (l) Al(H2O)3 (OH)3 (s) + 3H+ (aq ) [Al(H2O)6 ] 3+ (aq) Reaction with excess NH3 With excess NH3 a ligand substitution reaction occurs with Cu and its precipitate dissolve This substitution is incomplete with Cu. Cu becomes [Cu(NH3 ) 4 (H2O)2 ] 2+ deep blue solution Cu(OH)2 (H2O)4(s) + 4NH3 (aq) [Cu(NH3 ) 4 (H2O)2 ] 2+ (aq) + 2H2O (l) + 2OH- (aq) [Al(H2O)5 (OH)]2+ (aq) + OH- (aq) [Al(H2O)4 (OH)2 ] + (aq) + H2O (l) Here the NH3 and OHions are acting as Bronsted-Lowry bases accepting a proton In this reactions NH3 is acting as a Lewis base donating an electron pair.
Reactions with Carbonate solution The 2+ ions react differently to the 3+ ions with carbonate solutions. The 2+ ions with carbonate solution results in MCO3 ppt being formed (Cu blue/green, Fe(II) green ) Cu2+ (aq) + CO3 2- (aq) CuCO3 (s) These are precipitation reactions The 3+ ions with carbonate solution form a M(OH)3 ppt and CO2 gas is evolved. MCO3 is formed with 2+ ions but M2 (CO3 )3 is not formed with 3+ ions. The difference is explained by the greater polarising power of the 3+ ion due to its higher charge density. Al forms white ppt of Al(OH)3 (H2O)3 + CO2 Fe(III) forms brown ppt of Fe(OH)3 (H2O)3 + CO2 These are classed as acidity reactions. Fe2+ (aq) + CO3 2- (aq) FeCO3 (s) 2[Fe(H2O)6 ] 3+ (aq) + 3CO3 2- (aq) 2Fe(OH)3 (H2O)3(s) +3CO2 + 3H2O(l) 2[Al (H2O)6 ] 3+ (aq) + 3CO3 2- (aq) 2Al(OH)3 (H2O)3(s) +3CO2 + 3H2O(l) [Cu(H2O)6 ] 2+ + CO3 2- CuCO3 + 6H2O [Fe(H2O)6 ] 2+ + CO3 2- FeCO3 + 6H2O
3.2.6 Reactions of ions in aqueous solution (A-level only)
Some metal hydroxides show amphoteric character by dissolving in both acids and bases (eg hydroxides of Al3+).
Students should be able to:
• describe and explain the simple test-tube reactions of: M2+(aq) ions, limited to M = Fe and Cu, and of M3+(aq) ions, limited to M = Al and Fe, with the bases OH– , NH3 and CO3 2–
2.6 Reactions Of Aqueous Ions Page 1 - 2
Oxford Textbook Pages : 380 - 381
CGP Revision Guide Pages : 125 - 127