Titrations. Acid-Base Indicators and Titration Curves. Shapes of Titration Curves. A titration curve is a graphical history of a titration

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Douglas Bridges
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1 Acid-Base Indicators and Titration Curves Titrations In a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point the point at which the reaction is complete Indicator substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL The indicator changes color (pink) A titration curve is a graphical history of a titration For acid-base titrations, the titration curve is typically a plot of ph vs. volume of titrant (base) added p H Titrations Titrant volume, ml Shapes of Titration Curves Strong Acid + Strong Base HNO 3 + NaOH H 2 O + NaNO Purely a stiochiometry problem 3 4 At point 1: only H + and NO 3- present ph dependent on H + At point 2: less H +, Na +, NO 3-, ph dependent on H +, half-way to endpoint of the neutralization reaction. At point 3: No H +, Na +, NO 3-. ph is 7.0, equivalence point, endpoint. At point 4: OH -, Na +, NO 3-. ph is dependent on OH -, over titration. Strong Acid-Strong Base Titrations NaOH (aq) + HCl (aq) H 2 O (l) + NaCl (aq) OH - (aq) + H + (aq) H 2 O (l) 1

Equivalence point the point at which the reaction is complete Indicator substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL The indicator changes

2 Weak Acid + Strong Base CH 3 CO 2 H + NaOH H 2 O + NaCH 3 CO Two problems: 1-stiochiometry 2-equilibrium of weak acid At point 1: H +, CH 3 CO 2-, CH 3 CO 2 H. Buffering effect (little ph change) At point 2: Same [H+], same [CH 3 CO 2- ], CH 3 CO 2 H. Buffering effect, halfway to endpoint. At point 3: No H +, Na +, CH 3 CO 2-, endpoint H + neutralized, however, ph is not 7.0, but dependent on [CH 3 CO 2- ] (a weak base), notice small equivalence point. At point 4: OH -, Na +, CH 3 CO 2-, over titration, ph is dependent on OH - Weak Acid + Strong Base With weaker acids, the initial ph is higher and ph changes near the equivalence point are more subtle. Weak Acid-Strong Base Titrations CH 3 COOH (aq) + NaOH (aq) CH 3 COONa (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) At equivalence point (ph > 7): CH 3 COO - (aq) + H 2 O (l) CH 3 COO - (aq) + H 2 O (l) OH - (aq) + CH 3 COOH (aq) 1. Draw a titration curve for a strong acid added to a weak base (use ammonia and hydrochloric acid). Then, describe each part of the curve. Strong Acid-Weak Base Titrations HCl (aq) + NH 3 (aq) NH 4 Cl (aq) H + (aq) + NH 3 (aq) NH 4 Cl (aq) At equivalence point (ph < 7): NH + 4 (aq) + H 2 O (l) NH 3 (aq) + H + (aq) 2

At point 3: No H +, Na +, CH 3 CO 2-, endpoint H + neutralized, however, ph is not 7.0, but dependent on [CH 3 CO 2- ] (a weak base), notice small equivalence point.

3 2. Using your data collected in lab, describe the weak acid and weak base titration curve. Weak Acid - Weak Base Titrations CH 3 COOH (aq) + NH 3 (aq) CH 3 COO - (aq) + NH + 4 (aq) At equivalence point (ph = 7): CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) NH 4 + (aq) + H 2 O (l) NH 3 (aq) + H 3 O + (l) And: H 3 O + (l) + OH - (aq) 2H 2 O (l) Polyprotic Acid Titrations In these cases there is an equivalence point for each dissociation. 3. Make a table showing the expected relative ph for each of the five titrations done in lab at equivalence point. 4 primary uses for titration curves: 1. Determining Equivalence point 2. Determining Possible Indicators 3. Determining Optimum Buffering Regions 4. Determining the number of ionization reactions Determination of Equivalence Point The equivalence point of a titration marks the point in the titration where there are equal numbers of moles of hydronium and hydroxide ions On the titration curve, the point of inflection between the two curves marks the equivalence point. 3

NH 3 (aq) + H 3 O + (l) And: H 3 O + (l) + OH - (aq) 2H 2 O (l) Polyprotic Acid Titrations In these cases there is an equivalence point for each dissociation. 3. Make a table showing the expected relative ph for each of the five titrations done in lab at equivalence point.

4 equivalence point Determination of Possible Reaction Indicators Different indicators change color at different ph values. By knowing these ph values, and using a titration curve for a given acid, indicators can be chosen to indicate endpoint for a neutralization of that acid. The weaker the acid/base, the smaller the range for an acceptable indicator. 4

By knowing these ph values, and using a titration curve for a given acid, indicators can

5 Determination of Optimum Buffer Ranges Weak acids and bases have a natural buffering region, seen by the small change in ph vs. volume base added. These regions identify the ph values that could be utilized by buffering with a salt of the conjugate. Remember, a buffer has a high concentration of acid-base molecules and their conjugates Determining the Number and Buffer Regions for Polyprotic Ionization Reactions Polyprotic acids have titration curves with end points corresponding to the neutralization of each proton lost by the species. And an equivalence point where complete neutralization has occurred. End Point 2 End Point 1 5

Remember, a buffer has a high concentration of acid-base molecules and their conjugates Determining the Number and Buffer Regions for Polyprotic Ionization

6 Notice how the buffering effect is diminished with diminishing acid strength One final thing that can sometimes be inferred from a titration curve is the relative acid strength. As acid strength increases, the greater the initial H + concentration, and the greater the change in ph during neutralization. Therefore, stronger acids have much larger distances between initial ph and ph at equivalence point. 4. For the titration curve that follows on the next slide, identify: A. Number of ionization reactions B. Equivalence point (s) C. End Point (s) D. Buffer region (s) E. Names of indicators required to analyze the acid 5. For the same curve, identify the species in solution at the first equivalence point if acid H 2 A were titrated with NaOH 7 6

Therefore, stronger acids have much larger distances between initial ph and ph at equivalence point. 4. For the titration curve that follows on the next slide, identify: A.

CHAPTERS 15 FAKE TEST QUESTIONS. 1. According to the Brønsted Lowry definition, which species can function both as an acid and as a base?

You might need to know the following K values: CHAPTERS 15 FAKE TEST QUESTIONS CH 3 COOH K a = 1.8 x 10 5 Benzoic Acid K a = 6.5 x 10 5 HNO 2 K a = 4.5 x 10 4 NH 3 K b = 1.8 x 10 5 HF K a = 7.2 x 10 4