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Catalyst: Friday February 7, 2014
Today’s Objective: -SWBAT explain how a buffer works. SWBAT design an appropriate buffer system
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Saturday School: 10-11ish Monday: Cold Call Quiz
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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