CHM 2046C Sample Module 12 Name: ____Answers____

Part D Acid-Base/pH Titrations Curves 10 points

How do titration curves differ for strong acid-strong base, strong acid-weak base; weak acid-strong base, and weak acid-weak base titrations? Sketch curves for each.

(a) Strong Acid-Strong Base titrations

If a titration is a strong acid-strong base, the equivalence point will occur at a pH of 7. Below is a sample strong-strong base titration curve.

Sample Strong Acid/Base Curve: Chapter 18 Figure 18.4, p. 863

(b) Strong Acid-Weak Base titrations

If in a titration the acid is strong and the base is weak, the equivalence point will be at a pH <7. There are five regions of possible points of interest see M-12 Part E.

Sample Weak Base/Strong Acid: Chapter 18 Figure 18.7, p 868

(c) Weak Acid-Strong Base titrations:

If in a titration the acid is weak and the base is strong, the equivalence point will be at a pH >7. There are five regions of possible points of interest see M-12 Part E.

Below is a sample Strong Base-Weak Acid titration curve:

(d) Weak Acid-Weak Base titrations

If both acid and base are weak, the equivalence point will depend upon the relative strengths of the weak and weak base. There are three cases: K_a > K_b; K_a=K_b; and K_b>K_a and the titration curves will each have equivalence points over the range of the pH scale.

If K_a=K_b the equivalence point will be at 7 on the pH scale.

If K_a > K_b the equivalence point will be less than 7 on the pH scale

If K_b>K_a the equivalence point will be greater than 7 on the pH scale

An indicator can not be used to determine the equivalence point in any of the three cases, because the vertical section of the curve is too short to be noticed due to the buffering on both sides of the equivalence point.

(3) Titrations against polyprotic acids can be divided into separate titrations for each hydrogen ion removed. Explain and demonstrate with a titration curve.

Usually in polyprotic acids, the successive K_a values differ by 10⁵ so that at any one time, there is only one conjugate acid-base pair present in the solution. (There are no polyprotic strong acids, except H₂SO₄ ~10^2-)

During the first portion of the titration only the K_a1 value and its equilibrium constant expression is used because the amount of ions furnished by the calculation using K_a2 is so small it is neglected in the calculation. As the conjugate base from the stage one reaction becomes larger and larger, buffering action takes place stage 1 weak acid and its conjugate base.

During the second portion of the titration, there is none of the original acid molecules left unionized, and the problem becomes a calculation of the second reaction and its K_a2.

For sulfurous acid the titration curve has two points of inflection:

For oxalic acid the first equivalence point has not vertical portion because of buffering action of the weak acid and its conjugate base, while the second stage has a more normal vertical section.

(4) What are acid-base indicators. Choose appropriate indicators for the four titrations listed in question #1.

An acid-base indicator is itself a weak acid or base. It is usually a large organic molecule that has slightly different structures in acid and base (the weak acid has one structure and its conjugate base another structure.) The two structures have different colors and observing the change in color durig the titration, you can monitor the change in pH. The idea is to choose an indicator with a Ka near that of the acid being titrated, so that the color change occurs at the correct stage during the titration.

Additional Titration Curves from Another Book: