1. General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 10.6 Buffers Chapter 10 Acids and Bases © 2013 Pearson Education, Inc. Lectures

2. © 2013 Pearson Education, Inc. Chapter 10, Section 6 2 A buffer solution  maintains pH by neutralizing added acid or base.  in the body, absorbs H 3 O + or OH − from foods and cellular processes to maintain pH. Blood contains buffers that maintain a pH close to 7.4. A change in blood pH can disrupt oxygen absorption and metabolic processes. Buffers

3. © 2013 Pearson Education, Inc. Chapter 10, Section 6 3 Buffers (continued) When an acid or base is added  to water, the pH changes drastically.  to a buffer solution, the pH does not change very much; pH is maintained.

4. © 2013 Pearson Education, Inc. Chapter 10, Section 6 4 The components of a buffer solution  are acid–base conjugate pairs that have nearly equal concentrations.  can be a weak acid and a salt of its conjugate base.  can also be a weak base and a salt of its conjugate acid. An acid must be present to react with any OH −. A base must also be available to react with any added H 3 O +. Components of a Buffer

5. © 2013 Pearson Education, Inc. Chapter 10, Section 6 5 Learning Check Will each of the following combinations produce a buffer solution or not? Explain. A. HCl and KCl B. H 2 CO 3 and NaHCO 3 C. H 3 PO 4 and NaCl D. HC 2 H 3 O 2 and KC 2 H 3 O 2

6. © 2013 Pearson Education, Inc. Chapter 10, Section 6 6 Solution Will each of the following combinations produce a buffer solution or not? Explain. A. HCl + KClNo; HCl is a strong acid. B. H 2 CO 3 + NaHCO 3 Yes; this is a weak acid and its salt. C. H 3 PO 4 + NaCl No; NaCl is not a conjugate base of H 3 PO 4. D. HC 2 H 3 O 2 and KC 2 H 3 O 2 Yes; this is a weak acid and its salt.

7. © 2013 Pearson Education, Inc. Chapter 10, Section 6 7 An acetic acid/acetate buffer contains acetic acid (HC 2 H 3 O 2 ) and the salt of its conjugate base, sodium acetate (NaC 2 H 3 O 2 ).  Acid dissociation occurs.  The salt provides a higher concentration of the conjugate base C 2 H 3 O 2 − than provided by the dissociation of the weak acid by itself. Buffer Action

8. © 2013 Pearson Education, Inc. Chapter 10, Section 6 8 Weak Acids React with Bases  The function of the weak acid in a buffer is to neutralize added base.  The acetate ion produced by the neutralization becomes part of the available acetate. acetic acid base acetate ion water

9. © 2013 Pearson Education, Inc. Chapter 10, Section 6 9 Conjugate Bases React with Acids  The function of the acetate ion C 2 H 3 O 2 − is to neutralize added H 3 O +.  The acetic acid produced by the neutralization contributes to the available weak acid. acetate ion acid acetic acid water

10. © 2013 Pearson Education, Inc. Chapter 10, Section 6 10 Summary of Buffer Action Buffer action occurs because  the weak acid in a buffer neutralizes the base.  the conjugate base in the buffer neutralizes the acid.  the pH of the solution is maintained.

11. © 2013 Pearson Education, Inc. Chapter 10, Section 6 11 Calculating the pH of a Buffer The [H 3 O + ] in the K a expression is used to determine the pH of a buffer.

12. © 2013 Pearson Education, Inc. Chapter 10, Section 6 12 Guide to Calculating pH of a Buffer

13. © 2013 Pearson Education, Inc. Chapter 10, Section 6 13 Calculating Buffer pH The weak acid H 2 PO 4 − in a blood buffer, H 2 PO 4 − /HPO 4 2− has a. What is the pH of the buffer if Analyze the Problem.

14. © 2013 Pearson Education, Inc. Chapter 10, Section 6 14 Calculating Buffer pH The weak acid H 2 PO 4 − in a blood buffer, H 2 PO 4 − /HPO 4 2− has a. What is the pH of the buffer if Step 1 Write the K a expression. Step 2 Rearrange K a for [H 3 O + ].

15. © 2013 Pearson Education, Inc. Chapter 10, Section 6 15 Calculating Buffer pH The weak acid H 2 PO 4 − in a blood buffer, H 2 PO 4 − /HPO 4 2− has a. What is the pH of the buffer if Step 3 Substitute K a, [HA] and [A  ]. Step 4 Use [H 3 O + ] to calculate pH.

16. © 2013 Pearson Education, Inc. Chapter 10, Section 6 16 Learning Check What is the pH of a H 2 CO 3 buffer that is 0.20 M H 2 CO 3 and 0.10 M HCO 3 − ? A. 6.66 B. 6.36 C. 6.07

17. © 2013 Pearson Education, Inc. Chapter 10, Section 6 17 Solution What is the pH of a H 2 CO 3 buffer that is 0.20 M H 2 CO 3 and 0.10 M HCO 3 − ? Analyze the Problem.

18. © 2013 Pearson Education, Inc. Chapter 10, Section 6 18 Solution What is the pH of a H 2 CO 3 buffer that is 0.20 M H 2 CO 3 and 0.10 M HCO 3 − ? Step 1 Write the K a expression. Step 2 Rearrange K a for [H 3 O + ].

19. © 2013 Pearson Education, Inc. Chapter 10, Section 6 19 Solution What is the pH of a H 2 CO 3 buffer that is 0.20 M H 2 CO 3 and 0.10 M HCO 3 − ? Step 3 Substitute K a, [HA] and [A  ]. Step 4 Use [H 3 O + ] to calculate pH.

20. © 2013 Pearson Education, Inc. Chapter 10, Section 6 20 Buffer Preparation A buffer solution is needed to maintain a pH of 3.5 to 3.8 in a urine sample. Which of the following buffers would you use if 0.1 M solutions of the weak acid and conjugate base are available? Formic acid/formate Carbonic acid/bicarbonate Ammonium/ammonia

21. © 2013 Pearson Education, Inc. Chapter 10, Section 6 21 Buffer Preparation To estimate the pH of a buffer system, we can use the exponent of the K a. pH estimate Formic acid/formate 4 Carbonic acid/bicarbonate7 Ammonium/ammonia 10 The formic acid/formate buffer will have a pH close to 4, and, therefore, will be the best choice.