1. Sections 17.1-2 Acid-Base Reactions Buffer Solutions Bill Vining SUNY Oneonta

2. Acid-Base Reactions Predict reaction for different combinations of strong and/or weak acids and bases. 1) Extent of Reaction 2) Acid-Base Properties of Products

3. Extent of Acid-Base Reactions

4. All acid-base reactions involving a strong acid or a strong base go to completion (100%) strong acid + strong base goes 100% strong acid + weak base goes 100% weak acid + strong base goes 100% weak acid + weak base extent depends on acid and base strengths Key: The stronger the base and the acid, the greater the extent of reaction.

5. Extent of Acid-Base Reactions weak acid + weak base reaction extent depends on relative strengths. Key: The stronger the base and the acid, the greater the extent of reaction. Acid (Ka)Base (Kb)Extent (%) HF (7.2 x 10 -4 )NH 3 (1.8 x 10 -5 )99.9 H 2 CO 3 (4.2 x 10 -7 )NH 3 (1.8 x 10 -5 )96 HCN (4.0 x 10 -10 )NH 3 (1.8 x 10 -5 )46 H 2 CO 3 (4.2 x 10 -7 )NH 3 (1.8 x 10 -5 )96 H 2 CO 3 (4.2 x 10 -7 )CH 3 CO 2 - (5.6 x 10 -10 )13 H 2 CO 3 (4.2 x 10 -7 )NO 2 - (2.2 x 10 -11 )3 HCN (4.0 x 10 -10 )NO 2 - (2.2 x 10 -11 )0.1

6. Determining K rxn for Acid-Base Reactions Strong acid + Strong base Full Ionic Equation: Net Ionic Equation:

7. Determining K rxn for Acid-Base Reactions Strong acid + Weak base Net-Ionic Equation

8. Determining K rxn for Acid-Base Reactions Weak acid + Strong base Net-Ionic Equation

9. Determining K rxn for Acid-Base Reactions Weak acid + Weak base Calculation of K

10. Sections 17.2 Acid-Base Buffer Solutions Bill Vining SUNY Oneonta

11. Acid-Base Buffer Solutions A buffer solution contains a mixture of a weak acid and a weak base, typically the conjugate base of the weak acid. The important property of a buffer solution is that it experiences a relatively small change in pH when a strong acid or a strong base is added.

12. How a Buffer Works When a strong acid is added to a buffer, the acid reacts with the conjugate base and is completely consumed. When a strong base is added to a buffer, the base reacts with the weak acid and is completely consumed.

13. How a Buffer Works CH 3 CO 2 H + NaCH 3 CO 2 Buffer If HCl is added, H 3 O + reacts with CH 3 CO 2 - If NaOH is added, OH - reacts with CH 3 CO 2 H

14. Identifying Buffer Solutions Key: Solution must contain a weak acid AND a weak base. Usually the acid and base are a conjugate pair. 0.16 M HNO 2 + 0.11 M NaNO 2 0.11 M Ba(OH) 2 + 0.25 M BaCl 2 0.22 HI + 0.24 M NaI 0.31 M NH 4 Br + 0.36 M NH 3

15. Predicting Buffer pH: Part 1 ICE Tables What is the pH of a buffer solution that is 0.100 M NH 4 + and 0.300 M NH 3 ? K a = 5.6 x 10 -10 NH 4 + + H 2 O(l)  H 3 O + (aq) + NH 3 (aq) Initial Change Equilib.

16. Predicting Buffer pH: Part 1 ICE Tables Solve for x and determing pH: NH 4 + + H 2 O(l)  H 3 O + (aq) + NH 3 (aq) 0.100 –x x 0.300 + x

17. Predicting Buffer pH: Part 1 ICE Tables General relationship: Caveat: This is true only for a buffer solution. Not for a solution of just an acid or a base.

18. Predicting Buffer pH: Part 2 Henderson-Hasselbalch Equation Take –log of both sides: Henderson-Hasselbalch Equation

19. Predicting Buffer pH: Part 2 Henderson-Hasselbalch Equation What is the pH of a solution of 0.31 M NH 4 Br + 0.36 M NH 3 ? K a (NH 4 + ) = 5.6 x 10 -10

20. Predicting pH change of a Buffer Solution Following Addition of a Strong Base Steps: 1. Use Henderson-Hasselbalch equation to determine initial pH 2. Assume strong base reacts completely with weak acid 3. Determine “final” concentrations after reaction 4. Use Henderson-Hasselbalch equation to determine new pH

21. 1. Use Henderson-Hasselbalch equation to determine initial pH 2. Assume strong base reacts completely with weak acid 3. Determine “final” concentrations after reaction 4. Use Henderson-Hasselbalch equation to determine new pH What is the pH change when 16 mL of 2.5 M NaOH is added to 500. mL of a solution containing 0.32 M NH 4 Br + 0.46 M NH 3 ?

22. 1. Use Henderson-Hasselbalch equation to determine initial pH 2. Assume strong base reacts completely with weak acid 3. Determine “final” concentrations after reaction 4. Use Henderson-Hasselbalch equation to determine new pH What is the pH change when 16 mL of 2.5 M NaOH is added to 500. mL of a solution containing 0.32 M NH 4 Br + 0.46 M NH 3 ?

23. Preparing Buffer Solutions: Two Methods Direct Addition or Acid-Base Reaction Steps: 1. Choose an acid-base pair with pK a near (within 1 unit) desired pH 2. Determine ratio: [weak acid]/[weak base] 3a. Preparation by Direct Addition Determine masses of acid and base to use. 3b. Preparation by Acid-Base Reaction Determine volume of solutions to use.

24. Preparing a Buffer by Direct Addition Prepare 250 mL of a buffer of pH = 4.32 using one of the acid-base pairs here. The concentration of the weak acid should be 0.500 M.

25. Preparing a Buffer by Acid-Base Reaction What volume of 2.00 M NaOH solution should be added to 250. mL of 0.500 acetic acid solution to create a buffer of pH 4.32?

26. Preparing a Buffer by Acid-Base Reaction What volume of 2.00 M NaOH solution should be added to 250. mL of 0.500 acetic acid solution to create a buffer of pH 4.32?