Polyprotic Acids and Bases Section 6.5 pg. 258

Polyprotic Acids Some acids have only one acidic hydrogen atom in their compound formula (HA); these are called monoprotic acids i.e. HI(aq), HCl(aq), HBr(aq), HF(aq) HCN(aq) Some acids contain more than one acidic hydrogen and can react more than once with water (HxA); these are called polyprotic acids i.e. H2SO4(aq), H3PO4(aq) Example: H3PO4(aq) + H2O(l)  H3O+(aq) + H2PO4-(aq) H2PO4-(aq) + H2O(l)  H3O+(aq) + HPO42-(aq) HPO42-(aq) + H2O(l)  H3O+(aq) + PO43-(aq) <50% Each of the resulting ions can react with an additional water to produce more hydronium ions, but these reactions are even less complete <1% ~o%

Polyprotic Acids With all of these possible reactions you might think that phosphoric acid is very acidic, but only the first reaction is significant. Also, the first reaction is only 50% complete, so it is actually a weak acid and it has a pH noticeable higher than a strong acid at the same concentration. Based on this evidence: In general, polyprotic acids are weak acids whose reaction with water decreases with each successive step. Acid Concentration pH HCl(aq) 0.1 mol/L 1.0 H3PO4(aq) 1.7 NOTE: H2SO4(aq) is a notable exception to this rule. It is a strong acid because it’s first reaction with water is essentially complete. However the second reaction is much less than 50% complete.

Polyprotic Bases The same concept applies to bases. Some bases, like CH3COO-(aq), are monoprotic bases, meaning they can react with water only once to produce hydroxide ions CH3COO-(aq) + H2O(l)  OH-(aq) + CH3COOH (aq) Others, like CO32- (aq) are polyprotic bases because they can react more than once with water, but the reactions are much less than 50% CO32-(aq) + H2O(l)  OH-(aq) + HCO3-(aq) HCO3-(aq) + H2O(l)  OH-(aq) + H2CO3(aq) <50% <1%

Polyprotic Bases Bases like Ba(OH)2 are not polyprotic bases. This is because there is no possibility of successive reactions with water. Barium hydroxide dissociates to produce hydroxide ions, it does not matter that there are two moles of hydroxide ions for every one mole of Ba(OH)2 Ba(OH)2 (aq)  Ba2+(aq) + 2OH- (aq)

Polyprotic Bases In general, polyprotic bases are weak bases, whose reaction with water decreases with each successive step. Evidence shows that a carbonate ion solution is less basic than a strong base at the same concentration Base Concentration pH NaOH(aq) 0.1 mol/L 13.0 Na2CO3(aq) 11.4 Practice pg. 259 #1-8

Unit B Review We just finished Unit B  Things to work on to prepare for Unit 2 Exam 1. Complete Chapter 6 Review pg 262-264 Q 1-21 2. Complete Unit 3(B) Review pg 265-269 Q 1-32, 35, 37, 39, 44, 45, 56 All solutions will be posted on the wiki We will review tomorrow, go over our quiz and then write on Thursday