- Strong Acids & Bases – - - Autoionization of Water - 1

 a strong acid ionizes completely in water to form hydrogen ions, percent ionization is greater than 99% (we assume 100% in calculations)  Example: HCl completely ionizes in water 100% HCl (g)  H + (aq) + Cl - (aq) H 2 O (l) 2

 Common strong acids: hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid  monoprotic acid is an acid that possesses only one ionizable (acidic) proton  diprotic acid possesses two or more ionizable protons 3

 a strong base (Arrhenius definition) dissociates completely in water to release hydroxide ions  all hydroxides of Group 1 elements are strong bases (only one mole of hydroxide ions are formed when one mole of Group 1 hydroxides are dissolved in water) 100% NaOH (s)  Na + (aq) + OH - (aq) H 2 O (l) 4

 these metal hydroxides are all highly soluble in water  all hydroxides of Group 2 elements are also bases when dissolved in water (two moles of hydroxide ions are formed when one mole of Group 2 hydroxides are dissolved in water) 100% Ba(OH) 2(s)  Ba + (aq) + 2OH - (aq) H 2 O (l)  these metal hydroxides are only slightly soluble in water (main reason used in medicine – milk of magnesia, Mg(OH) 2(aq) ) 5

 Experiments have revealed that some water molecules react with each other to produce hydronium, H 3 O + (aq), and hydroxide, OH - (aq), ions according to the following equation H 2 O (l) +H 2 O (l)  H 3 O + (aq) + OH - (aq)  conductivity is slight, an equilibrium is formed between hydronium and hydroxide ions, and water molecules are greatly favoured 6

 the collision that forms hydronium and hydroxide ions are very rare (need the right energy and orientation)  when this reaction does occur, it is called the autoionization of water (since water molecules ionize one another). 7

 chemists often omit the water molecule that carries the H+ ion for convenience H 2 O (l)  H + (aq) + OH - (aq)  water molecules do NOT spontaneously dissociate into H+ and OH- ions, but rather, they are formed by the ionization process in which a proton is transferred from one molecule to another 8

 water equilibrium also obeys equilibrium law K eq = [H + (aq) ][OH - (aq) ] [H 2 O (l) ] 9

 the concentration of water molecules in pure water and in dilute aqueous solutions is essentially CONSTANT and equal to 55.6 mol/L  therefore, a new constant, which incorporates both the constant value of [H 2 O (l) ] and the equilibrium constant, can be calculated called the ion product constant for water, K w 10 [H + (aq) ][OH - (aq) ] = K [H 2 O (l) ] [H + (aq) ][OH - (aq) ] = K[H 2 O (l) ] [H + (aq) ][OH - (aq) ] = K w

 note the 1:1 ratio of hydrogen ions and hydroxide ions, therefore the concentration of hydrogen and hydroxide ions in pure water must be EQUAL [H + (aq) ] = [OH - (aq) ] = 1.0 x mol/L  At SATP, K w = [H + (aq) ][OH - (aq) ] K w = (1.0 x mol/L) (1.0 x mol/L) K w = 1.0 x

 autoionization occurs in all aqueous solutions  In all aqueous solutions at SATP, [H + (aq) ][OH - (aq) ] = K w =1.0 x  What makes a solution neutral, acidic or basic? In neutral solutions [H + (aq) ] = [OH - (aq) ] In acidic solutions [H + (aq) ] > [OH - (aq) ] In basic solutions [H + (aq) ] < [OH - (aq) ] 12

 K w is valid at SATP, however at different temperatures it will change (equilibrium constant is temperature dependent)  the ion product constant for water, K w, can be manipulated to solve for either hydrogen ion or the hydroxide ion concentration SinceK w = [H + (aq) ] [OH - (aq) ] then[H + (aq) ] = K w __ [OH - (aq) ] and [OH - (aq) ] = K w __ [H + (aq) ] 13

p. 537 Practice UC # 4, 5, 6 p. 540 Practice UC # 8, 9, 10 14