Kaplan MCAT General Chemistry Review

In the early part of the curve (when little base has been added), the acidic species predominates, so the addition of small amounts of base will not appreciably change either the [OH^-] or the pH. Similarly, in the last part of the titration curve (when an excess of base has been added), the addition of small amounts of base will not change the [OH^-] significantly, and the pH remains relatively constant. The addition of base most alters the concentrations of H⁺ and OH^- near the equivalence point, and thus the pH changes most drastically in that region. The equivalence point for strong acid/strong base titration is always at pH 7 (for monovalent species).

If you are using a pH meter so that you can chart the change in pH as a function of volume of titrant added, you can make a good approximation of the equivalence point by locating the midpoint of the region of the curve with the steepest slope.

WEAK ACID AND STRONG BASE

Titration of a weak acid, HA (e.g., CH₃COOH), with a strong base, such as NaOH, produces the titration curve shown in Figure 10.2.

Figure 10.2

Key Concept

Note that any monoprotic weak acid titrated with a strong base will give a similar curve. The exact pH of the equivalence point depends on the identity of the weak acid, but the “tug-of-war” rule tells us it will be above 7.

Let’s compare Figure 10.2 with Figure 10.1. The first difference we should notice is that the initial pH of the weak acid solution is greater than the initial pH of the strong acid solution. This makes sense because we know that weak acids don’t dissociate to the same degree that strong acids do; therefore, the concentration of H₃O⁺ will generally be lower (and pH will be higher) in a solution of weak acid. The second difference we should catch is the shapes of the curves. The pH curve for the weak acid/strong base titration shows a faster rise in pH for given additions of base. The pH changes most significantly early on in the titration, and the equivalence point is in the basic range. The third difference we should notice is the position of the equivalence point. While the equivalence point for strong acid/strong base titration is pH 7, the equivalence point for weak acid/strong base is above 7 (in the basic range). This is because the reaction between the weak acid (HA) and strong base (OH^-) produces a stronger conjugate base (A^-) and a weaker conjugate acid (H₂O). This produces a greater concentration of hydroxide ions than that of hydrogen ions at equilibrium (due to the common ion effect on the auto-ionization of water). The equivalence point for weak acid/strong base titration is always in the basic range of the pH scale.

WEAK BASE AND STRONG ACID

The appearance of the titration curve for a weak base titrand and strong acid titrant will look like the “inversion” of the curve for a weak acid titrand and strong base titrant. The initial pH will be in the basic range (typical range: pH 10-12) and will demonstrate a fairly quick drop in pH for additions of the strong acid. The equivalence point will be in the acidic pH range, because the reaction between the weak base and strong acid will produce a stronger conjugate acid and weaker conjugate base. The stronger conjugate acid will result in an equilibrium state with a concentration of hydrogen ions greater than that of the hydroxide ions. The equivalence point for weak base/strong acid titration is always in the acidic range of the pH scale.

POLYVALENT ACIDS AND BASES

The titration curve for a polyvalent acid or base looks different from that for a monovalent acid or base. Figure 10.3 shows the titration of Na₂CO₃ with HCl in which the divalent (the term diprotic is equivalent) acid H₂CO₃ is the ultimate product.

Figure 10.3

In region I, little acid has been added, and the predominant species is CO₃^2-. In region II, more acid has been added, and the predominant species are CO₃^2- and HCO₃^-, in relatively equal concentrations. The flat part of the curve is the first buffer region (see next section), corresponding to the pK_a of HCO₃^- (K_a = 5.6 × 10^-11 implies pK_a = 10.25).

Region III contains the equivalence point, at which all of the CO₃^2- is finally titrated to HCO₃^-. As the curve illustrates, a rapid change in pH occurs at the equivalence point; in the latter part of region III, the predominant species is HCO₃^-.