1. Chemical Buffers Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

2. 6/10/2015 Chemical Buffers  Reading Assignment: Zumdahl: Chapter 8.1-8.2, 8.4  This lecture introduces the topic of buffered solutions, their preparation, behavior and applications.

3. 6/10/2015 Chemical Buffer H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ strong acid addition No significant pH decrease 7.006.996.98 H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ 7.004.502.00 Strong acid addition Significant pH decrease A solution which maintains a relatively constant pH, even when a small amount of strong acid or base is added to it.

4. 6/10/2015 Chemical Buffer OH – strong base addition No significant pH increase 7.007.017.02 OH – 7.009.5012.00 Strong base addition Significant pH increase A solution which maintains a relatively constant pH, even when a small amount of strong acid or base is added to it.

5. 6/10/2015 Features of a Buffer  Generally contains two reactive compounds, or one compound which can react in two ways. Alkaline character to neutralize the addition of a small amount of strong acid. Acidic character to neutralize the addition of a small amount of strong base.

6. 6/10/2015 Buffer Reactions buffer + H + (aq)slight increase in acidity buffer + OH – (aq)slight increase in basicity A buffer has to be able to perform both functions Explain with chemical equationschemical equations why bicarbonate ion is effective when used on either acid or base spills.

7. 6/10/2015 Buffer Reactions HCO 3 – (aq) + H + (aq)H 2 CO 3 (aq) Less acidic than H + HCO 3 – (aq) + OH – (aq)CO 3 2 – (aq) + H 2 O(l) Less basic than OH –

8. 6/10/2015 Features of a Buffer  If the buffered solution contains two reactive compounds, they should not cancel each others reactive ability.

9. 6/10/2015 Potentiometric Titration & Buffer Region 20.0 mL of 0.400 M HCl with Aqueous Ammonia 0481216202428 pH Volume NH 3 (mL) 0 2 10 8 6 4 12 14 Buffer Region Typical buffers maintain a pH between 4 and 10,

10. 6/10/2015 Potentiometric Titration & Buffer Region 0481216202428 pH Volume NH 3 (mL) 0 2 10 8 6 4 12 14 20.0 mL of 0.400 M HCl with Aqueous Ammonia Buffer Region Typical buffers maintain a pH between 4 and 10, with a variation of less than 1 pH unit when a strong acid or strong base is added. Buffer

11. 6/10/2015 Buffer Reactions  When hydrochloric acid is reacted with excess ammonia, a buffer is produced, consisting of: ammonium ion from the neutralization of hydrogen ion with ammonia. excess ammonia, a weak base. Explain with chemical equations whychemical equations the ammonia/ammonium ion system can behave as a buffer.

12. 6/10/2015 Buffer Reactions NH 3 (aq) + H + (aq)NH 4 + (aq) Less acidic than H + NH 4 + (aq) + OH – (aq)NH 3 (aq) + H 2 O(l) Less basic than OH –

13. 6/10/2015 Buffer Capacity  The amount of acid or base a buffer can neutralize.  Although a buffer can neutralize a small amount of strong acid or base, it cannot neutralize an infinite amount, since the neutralizing compound will eventually be used up.

14. 6/10/2015 Applications of Buffers  Chemical reactions Some chemical reactions only occur in a certain pH range. ·Outside the range the reaction might be too slow (or too fast). ·Different products might be produced.

15. 6/10/2015 Applications of Buffers  Biochemical reactions. Specific pH ranges are crucial for proper functioning of enzymes in the body Blood is buffered by bicarbonate ion and carbonic acid at a pH of 7.35-7.45 Other buffering systems include the ·phosphate buffering system ·protein buffering system

16. 6/10/2015 Preparation  Weak acid and its salt  Weak base and its salt  Weak acid partially neutralized by base  Weak base partially neutralized by acid  Conjugate base of a weak acid (salt) acidified with some strong acid  Conjugate acid of a weak base (salt) reacted with some strong base

17. 6/10/2015 Buffer Preparation  How many different ways can you make a buffer from these materials? 1 M HCl 1 M CH 3 COOH 1 M NH 3 solid potassium acetate solid sodium hydroxide

18. Buffer Preparation HCl CH 3 COOH NH 3 CH 3 COOK NaOH CH 3 COOK NH 3 Which five different combinations can be used to make a buffer?