Buffers
Objective
lesson 5
chapter 17
You will be able to explain the properties of
buffers in acid solutions.
Buffer:
-a solution that can resist drastic changes in pH when small
amounts of a strong acid or base are added
-a mixture of a weak acid and its conjugate base
(in approximately equal amounts)
Example: hydrofluoric acid and fluoride ion
HF(aq) and F-(aq)
-the conjugate base is usually in the form of an ionic
compound (ie. NaF ➔ Na+ + F-)
-a buffer can resist changes in pH because it has an acidic
species to neutralize bases and a basic species to neutralize
acids
There are two ways to make a buffer solution
1) mix equal amounts of a weak acid and its conjugate base
2)adding a strong base (OH-) to a weak acid solution.
Example:
A 5.0 mol/L solution of sodium hydroxide is added to a 10
mol/L acetic acid
Net Ionic Equation
Initial
Change
Final
Example
What would you mix with sodium oxalate to produce a buffer
solution?
·
Which of the following could be used to produce a buffer
solution?
· ammonium chloride and ammonia
· citric acid and dihydrogen citrate
· acetic acid and sodium fluoride
· nitric acid and sodium nitrate
· sodium hydrogen carbonate and potassium carbonate·
Buffer region on a titration curve:
reaction taking place: HA + OH- ---> A- + HOH
(during the beginning of the titration you are
actually making a buffer)
A buffer is a system at equilibrium.
EQ Reaction: the same as a weak acid in water
Example: A Sodium Acetate/Acetic Acid Buffer
Equilibrium taking place:
CH3COOH(aq) + H2O(aq) ⇌ H3O+(aq) + CH3COO-(aq)
The ability of a buffer to resist drastic changes in pH can be explained
using
·
Le Chatelier's Principle
·
Bronstead-Lowry acid base reactions
Example: Sodium Acetate/Acetic Acid Buffer
When a strong acid is added to the buffer:
The Bronstead-Lowry Acid Base Reaction:
CH3COO-(aq) + H3O+(aq)⇌ CH3COOH(aq) + H2O(l)
(the strong acid gets consumed)
According to Le Chatelier's Principle
CH3COOH(aq) + H2O(aq) ⇌ H3O+(aq) + CH3COO-(aq)
(EQ shifts left so [H3O+] decreases and pH increases slightly)
Biological Buffers
Blood
Carbonic acid/hydrogen carbonate
Assignment
do the buffer worksheet