Weak acids and buffers

1. Weak acids and buffers Numerical estimates.

2. Weak acids • Weak acids are characterized by less than 100% dissociation. A “weak” acid is not necessarily dilute. • Weak acids participate in an equilibrium

3. Weak acid dissociation • The concentration of water is a constant and its value is incorporated into the overall equilibrium constant.

4. This expression is the basis for studying acid dissociation, and buffer behavior, in aqueous solution. Consider acetic acid Weak acid equilibrium

5. Acetic acid dissociate • At 25oC, Ka = 1.8 x 10-5 • Calculate the % dissociation of a 0.05 M acetic acid solution.

7. Acetic acid dissociation • What is pH of this solution?

8. Acetic acid dissociation • What is concentration of hydroxide ion?

9. Buffer behavior A buffer consists of a weak acid and its conjugate base – or – of a weak base and its conjugate acid. e.g. buffer pairs – acetic acid and acetate ion (from, say, sodium acetate) -- ammonia and ammonium ion (from, say, ammonium chloride)

11. Buffer behavior • Purpose of a buffer is to maintain the pH of a solution even if hydronium or hydroxide ions are evolved or used in the course of a reaction. • Buffers are essential in biological processes. Bodily fluids are buffers. Clinical treatments often deal with excursions from the desired pH of these fluids.

12. Buffer behavior • Henderson—Hasselbalch equation • Begin with equation for weak acid (most common)

13. Buffer behavior • Use simplified version • Take log of both sides • Multiply by -1

14. Henderson-Hasselbalch Equation • Use the p function and rearrange. We obtain the usual form

15. Examples • What is the pH of an acetic acid solution that was prepared to be 0.20 M in potassium acetate and 0.10 M in acetic acid?

17. Buffer after adding acid • To this buffer, add 0.01 mole acid per liter of solution keeping the volume constant. What is its new pH?

19. Buffer with base • To the original buffer, add 0.01 mole NaOH per liter keeping the volume constant. What is its new pH?

21. Hydrolysis • Solutions of salts of strong acids and weak bases are acidic (e.g., ammonium chloride) • Solutions of salts of weak acids and strong bases are alkaline (basic) (e.g., sodium acetate) • Why?

22. Hydrolysis reaction • The hydrolysis reaction is a Bronsted-Lowry reaction. • It involves the conjugate base of the weak acid or the conjugate acid of the weak base. • Acetic acid --

23. hydrolysis • Ammonium chloride