N EUTRALIZATION R EACTIONS FOR P OLYPROTIC AND P OLYBASIC S PECIES

P OLYPROTIC A CIDS The titration curves we have considered so far have a single defined equivalence point, because the acids are monoprotic. They have a single proton (H + ) that may dissociate. Polyprotic acids have more than one proton that dissociates. Oxalic acid (H 2 C 2 O 4 ) is diprotic It has two protons that can dissociate. Phosphoric acid (H 3 PO 4 ) is triprotic It has three protons that can dissociate.

P OLYPROTIC A CIDS Polyprotic acids do not donate all their H+ at the same time when they dissociate. The dissociation of a polyprotic acid occurs as a series of separate steps. For example: H 3 PO 4(aq) + H 2 O (l) H 2 PO 4 - (aq) + H 3 O + (aq) K a = 6.9 x H 2 PO 4 - (aq) + H 2 O (l) HPO 4 2- (aq) + H 3 O + (aq) K a = 6.2 x HPO 4 2- (aq) + H 2 O (l) PO 4 3- (aq) + H 3 O + (aq) K a = 4.8 x

P OLYPROTIC A CIDS Notice that each of these dissociation steps is an equilibrium with its own K a value. Notice also how these acid dissociation values change with each successive dissociation (H 3 PO 4 is a much stronger acid than H 2 PO 4 - ). In other words, for phosphoric acid: K a1 > K a2 > K a3 Remember that the mole ratio for neutralization reactions of this and other polyprotic acids depends on the number of protons donated to the base. When there are more than one proton that may dissociate, the proton will be donated one at a time.

P OLYBASIC C OMPOUNDS Basic compounds and ions that can accept two or more protons are polybasic. For example, the sulfide ion, S 2- is dibasic, because it can accept up to two protons.

W RITING E QUATIONS F OR A P OLYBASIC I ON Problem The borate ion, BO 3 3-, is tribasic. Write balanced, stepwise net ionic equations for a solution of sodium borate titrated with hydrochloric acid, and write the overall neutralization reaction for the titration.

W RITING E QUATIONS F OR A P OLYBASIC I ON Step 1 For all bases, remember that the metal ion is a spectator. Therefore, from a solution of sodium borate, only the borate ion is included in the first stepwise equation. You know that any strong acid will dissociate completely in water, so you can write it as H +. Stepwise equation 1: BO 3 3- (aq) + H + (aq) HBO 3 2- (aq) Stepwise equation 2: HBO 3 2- (aq) + H + (aq) H 2 BO 3 - (aq) Stepwise equation 3: H 2 BO 3 - (aq) + H + (aq) H 3 BO 3(aq)

W RITING E QUATIONS F OR A P OLYBASIC I ON Step 2 To write the overall equation, notice that there are three protons on the left side of the equation: these add. All borate-related conjugate bases cancel, leaving only the BO 3 3- reactant and the H 3 BO 3 product. Overall equation: BO 3 3- (aq) + 3H + (aq) H 3 BO 3(aq)

T ITRATION C URVES F OR P OLYPROTIC AND P OLYBASIC S PECIES Sulfurous acid is a weak diprotic acid (K a1 = 1.4 x ; K a2 = 6.3 x ). What you will see in a titration curve for a diprotic acid is separate equivalence points for the removal of each mole of H +. You can think of the acid as neutralizing in two steps, one for each dissociation.

T ITRATION C URVE FOR S ULFUROUS A CID