Transcript Chem. Concepts: Buffers

Chem. Concepts: Buffers
A General Overview
Eric Liu
Mr. Michael Gangluff
Chemistry AP
What is a “Buffer”?
A buffer is a solution that resists changes in pH
when small amounts of acid or base are added to it.
What is it made of?
Buffers can be placed into two categories: acidic buffers and basic buffers

Acidic buffers

Consists of a weak acid and a salt of its
conjugate base

Basic buffers

 e.g. CH3COOH and CH3COONa

pH less than 7
Consists of a weak base and a salt of its
conjugate acid
 e.g. NH3 and NH4Cl

pH greater than 7
Our blood is a buffer, as cells can only survive within a narrow pH range
The pH of a buffer can be changed by altering the ratio of acid and its conjugate
base or base and its conjugate acid. The amount of acid/conjugate base or
base/conjugate acid determine its buffer capacity, or amount of acid or base that
can be added for it to still maintain constant pH
How does it work?
Buffers follow one of the following equilibriums:
HA H 2O  A  H 3O
B   H 2 O  HB  OH 
There are both products and reactants present in both cases
Buffers use Le Chatelier’s Principle, which states that a system will relieve stress put on a
system by “shifting” to the left or right side of the equation accordingly.
In other words, if you add reactant to the equilibrium, the system would respond by removing
reactant (and thus creating product), and vise versa
How does it work?
If acid is added to the system…
HA H 2O  A  H 3O +
++

+ +
The H3O+ from the acid will cause the system to shift left
B   H 2 O  HB  OH 

Or react with OH- to form H2O
How does it work?
If base is added to the system…
B   H 2 O  HB  OH  +
++

+ +
The OH- from the base will cause the system to shift to the left
HA H 2O  A  H 3O

Or react with H3O+ to form H2O
Summary
Buffers are solutions that resist changes in pH with the addition of
small amounts of acid or base
They consist of a weak base and its conjugate acid salt or a weak acid
and its conjugate base salt
They work through Le Chatelier’s Principle, shifting to the left or
right to counterbalance the addition of acid or base