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

Catalyst: Friday February 7 , 2014. Welcome. Today’s Objective : - SWBAT explain how a buffer works. SWBAT design an appropriate buffer system. Announcements. Saturday School: 10-11ish Monday: Cold Call Quiz. Photo of the day. A Problem. I have two beakers Beaker 1 – 1 L of pure water

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

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  1. Catalyst: Friday February 7, 2014 Welcome Today’s Objective: -SWBAT explain how a buffer works. SWBAT design an appropriate buffer system

  2. Announcements • Saturday School: 10-11ish • Monday: Cold Call Quiz

  3. Photo of the day

  4. 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

  5. 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

  6. Buffers • A buffer is any solution that resists changes in pH • Two components of a buffer: • A component that neutralizes an acid • 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.

  7. Weak Acids/Bases Make Excellent Buffers ⇌ 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! 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+

  8. 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.

  9. Adding Strong Acid/Base to Buffer Adding base to a buffer shows minimal change Adding base to a neutral solution causes huge changes

  10. CFU • Could each of the following solutions can be classified as buffer systems? Why/why not? 1. KH2PO4/H3PO4 2. NaClO4/HClO4 3. C5H5N/C5H5NHCl (C5H5N is a base)

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

  12. 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

  13. 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

  14. 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

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