Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Chapter 8 Acids and Bases 8.6 Buffers 1

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Buffers When an acid or base is added to water, the pH changes drastically. In a buffer solution, the pH is maintained; pH does not change when acid or base is added. 2

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. How Buffers Work Buffers work because they resist changes in pH from the addition of acid or base in the body, absorb H 3 O + or OH  from foods and cellular processes to maintain pH are important in the proper functioning of cells and blood maintain a pH close to 7.4 in blood. A change in the pH of the blood affects the uptake of oxygen and cellular processes 3

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Components of a Buffer A buffer solution contains a combination of acid–base conjugate pairs For example: HA and NaA may contain a weak acid and a salt of its conjugate base typically has equal concentrations of a weak acid and its salt may also contain a weak base and a salt of the conjugate acid 4

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. How Buffers Work In the acetic acid/acetate buffer with acetic acid (HC 2 H 3 O 2 ) and sodium acetate (NaC 2 H 3 O 2 ), the salt produces acetate ions and sodium ions NaC 2 H 3 O 2 (aq)  C 2 H 3 O 2  (aq) + Na + (aq) the salt is added to provide a higher concentration of the conjugate base C 2 H 3 O 2  than the weak acid alone HC 2 H 3 O 2 (aq) + H 2 O(l) C 2 H 3 O 2  (aq) + H 3 O + (aq) Large amount Large amount 5

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Function of a Weak Acid in a Buffer The function of the weak acid in a buffer is to neutralize a base. The acetate ion produced in the neutralization reaction adds to the concentration of acetate already in solution from the salt. HC 2 H 3 O 2 + OH   C 2 H 3 O 2  + H 2 O acetic acid base acetate ion water 6

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Function of Conjugate Base in a Buffer The function of the acetate ion, C 2 H 3 O 2 , is to neutralize H 3 O + from acids. The acetic acid produced contributes to the available weak acid. C 2 H 3 O 2  + H 3 O +  HC 2 H 3 O 2 + H 2 O acetate ion acid acetic acid water 7

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Working Buffers Buffers work because the weak acid in a buffer neutralizes bases and the conjugate base in the buffer neutralizes acids the pH of the solution is maintained 8 The buffer described here consists of about equal concentrations of acetic acid (HC 2 H 3 O 2 ) and its conjugate base, acetate ion (C 2 H 3 O 2  ). Adding H 3 O + to the buffer reacts with C 2 H 3 O 2  whereas adding OH  neutralizes HC 2 H 3 O 2. The pH of the solution is maintained as long as the added amounts of acid or base are small compared to the concentrations of the buffer components.

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check Which of the following combinations make a buffer solution? 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 9

Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution Which of the following combinations make a buffer solution? A buffer consists of a weak acid and salt of its conjugate base. A. HCl and KClNot a buffer. B. H 2 CO 3 and NaHCO 3 Buffer, a weak acid and its salt. C. H 3 PO 4 and NaClNot a buffer. D. HC 2 H 3 O 2 and KC 2 H 3 O 2 Buffer, a weak acid and its salt. 10