buffer solutions report presentation.pptx

buffer solutions

1. USE OF BUFFER SOLUTIONS BY: JERT ALLONES NESSY ALIDO JOMMEL REOLA CLARENZE CARL SANOY RAJA JULIANA TOLEDO

2. USE OF BUFFER SOLUTIONS BY: JERT ALLONES NESSY ALIDO JOMMEL REOLA CLARENZE CARL SANOY RAJA JULIANA TOLEDO

3. OBJECTIVES: Describe how buffer solution maintains its pH Calculate the pH of a buffer solution using the Henderson Hasselbalch equation

4. Buffer solutions A solution of weak acid or weak base and its salt. A buffered solution is one that resists a change in its pH when either hydroxide ions or protons (H3O+ ) are added. Very little change in pH is witnessed even when a strong acid or base is added. The components of a buffer are a conjugate acid-base pair. Buffer solutions function effectively since they contain comparable amounts of acid and its conjugate base over a limited Ph range called buffer capacity.

5. Buffer Solution EXAMPLES: HF and NaF CH₃COOH and CH₃COONa HCN and NaCN NH₃ and NH₄Cl

6. The effect of adding acid or base to an unbuffered solution. A 100-mL sample of dilute HCl is adjusted to pH 5.00. The addition of 1 mL of strong acid ( left ) or strong base ( right ) changes the pH by several units.

7. The effect of adding acid or base to a buffered solution. A 100-mL sample of an acetate buffer is adjusted to pH 5.00. The addition of 1 mL of strong acid ( left ) or strong base ( right ) changes the pH very little. The acetate buffer is made by mixing 1 M CH 3 COOH ( a weak acid) with 1 M CH 3 COONa (which provides the conjugate base, CH 3 COO - ).

8. Acid-Base Buffer Systems Buffers function by reducing changes in [H 3 O + ] that result from additions of acid or base to the solution. Buffers are composed of the conjugate acid-base pair of a weak acid. Buffers function via the common ion effect . CH 3 COOH( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) The common ion effect occurs when a reactant containing a given ion is added to an equilibrium mixture that already contains that ion and the position of the equilibrium shifts away from forming more of it.

9. Buffers and the Common-ion Effect A buffer works through the common-ion effect . CH 3 COOH ( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) acetic acid acetate ion Acetic acid in water dissociates slightly to produce some acetate ion: If NaCH 3 COO is added, it provides a source of CH 3 COO - ion, and the equilibrium shifts to the left. CH 3 COO - is common to both solutions. The addition of CH 3 COO - reduces the % dissociation of the acid.

10. The Effect of Added Acetate Ion on the Dissociation of Acetic Acid [CH 3 COOH] init [CH 3 COO - ] added % Dissociation * [H 3 O + ] pH 0.10 0.00 1.3 1.3x10 -3 2.89 0.10 0.050 0.036 3.6x10 -5 4.44 0.10 0.10 0.018 1.8x10 -5 4.74 0.10 0.15 0.012 1.2x10 15 4.92 * % Dissociation = [CH 3 COOH] dissoc [CH 3 COOH] init x 100

11. How a buffer works. Buffer has equal concentrations of A - and HA. H 3 O + Buffer has more HA after addition of H 3 O + . H 2 O + CH 3 COOH ← H 3 O + + CH 3 COO - OH - Buffer has more A - after addition of OH - . CH 3 COOH + OH - → CH 3 COO - + H 2 O

12. How a Buffer Works The buffer components (HA and A - ) are able to consume small amounts of added OH - or H 3 O + by a shift in equilibrium position . CH 3 COOH ( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) Added H 3 O + reacts with CH 3 COO - , causing a shift to the left. Added OH - reacts with CH 3 COOH, causing a shift to the right. The shift in equilibrium position absorbs the change in [H 3 O + ] or [OH - ], and the pH changes only slightly.

13. Buffer Capacity The buffer capacity is a measure of the “ strength ” of the buffer, its ability to maintain the pH following addition of strong acid or base. The greater the concentrations of the buffer components, the greater its capacity to resist pH changes. The closer the component concentrations are to each other, the greater the buffer capacity.

14. The relationship between buffer capacity and pH change Buffer capacity refers to the ability of a buffer to resist pH change; buffer capacity increases as the concentrations of its components ( i.e ., the weak acid and its conjugate base) increase . Buffer pH and buffer capacity are different concepts. For an acetic acid/ acetate buffer

15. Buffer Systems at Different pH Values

16. The Henderson-Hasselbalch Equation An equation that relates pH, p K a and the ratio of weak acid to its conjugate base for a buffer solution. HA + H 2 O H 3 O + + A - K a = [H 3 O + ][A - ]/[HA] [H 3 O + ] = K a x [HA]/[A - ] -log [H 3 O + ] = -log K a - log ([HA]/[A - ]) pH = p K a + log ([A - ]/[HA]) pH = p K a + log ([base]/[acid]) Special case : when [base] = [acid], the pH of the buffer solution equals the p K a of the weak acid. or

17. SAMPLE PROBLEMS: Calculate the pH of a 0.20 M of CH₃COOH. What would be the pH of the solution if it is also 0.30 M with CH3COONa (Ka= 1.8 x )  

18. SOLUTION: Let a = [ ] at equilibrium CH 3 COOH( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) Ka =   I 0.20 C - a +a +a E 0.20 - a a a