Catalyst: Friday February 7, 2014

Today’s Objective: -SWBAT explain how a buffer works. SWBAT design an appropriate buffer system

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A Problem I have two beakers Beaker 1 – 1 L of pure

water Beaker 2 – 1 L of

human blood I pour 5 mL of NaOH in

the pure water and the pH goes from 7 up to 13.2

I pour 5 mL of NaOH into the blood and it goes from a pH of 7.2 to 7.3

A Problem I have the same two

beakers Beaker 1 – 1 L of pure

water Beaker 2 – 1 L of human

blood I pour 5 mL of HCl in the

pure water and the pH goes from 7 up to 2.2

I pour 5 mL of NaOH into the blood and it goes from a pH of 7.2 to 7.1

Buffers A buffer is any solution that resists

changes in pH Two components of a buffer:1. A component that neutralizes an acid2. A component that neutralizes a base

A weak acid or a weak base are capable of creating a buffer because they have both of these components, but a strong acid or base cannot create a buffer.

Weak Acids/Bases Make Excellent Buffers

A weak acid or base make an excellent buffer because they have a component that can react with an acid and a component that can react with a base.

CH3COOH + H2O CH3COO- + H+

⇌

Can react with a base!

Can react with an acid!

Therefore, reacting each component produces a part of the equilibrium and little change in pH is observed!

How to Make a Buffer The best buffers have close to equal

concentrations of the conjugate acid/base pair. We can think about the Ka expression for a weak

acid

Therefore, pH is determined by the ratio of conjugate acid/base pair and the value of Ka.

As long as the change in ratio of [HA]/[A-] is small, the change in pH will be small.

Adding Strong Acid/Base to Buffer

Adding base to a buffer shows minimal change

Adding base to a neutral solution causes huge changes

CFU

Could each of the following solutions can be classified as buffer systems? Why/why not?1. KH2PO4/H3PO42. NaClO4/HClO43. C5H5N/C5H5NHCl(C5H5N is a base)

Animation

Students explore using http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf

Class Example Calculate the pH of a buffer that is

0.12 M lactic acid and 0.10 M sodium lactate. The Ka for lactic acid is 1.4 x 10-4

Table Talk Calculate the pH of a buffer

composed of 0.12 M benzoic acid and 0.20 M sodium benzoate. The Ka for the solution is 6.4 x 10-5

When Buffers Stop Working The pH at which any buffer works

most effectively is when pH = pKa

This is known at the ½ equivalence point

Buffers usually have a useable range within ±1 pH unit of the pKa