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Ch 12 - Chapter Twelve Acids and Bases CHEM 1212K Spring...

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Chapter Twelve Acids and Bases CHEM 1212K Spring 2012 1
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Objectives To learn to manage the pH of solutions of acids, bases, and their salts. The common theme of the chapter is that all of these equilibria can be treated in a similar way: Identify the solute species present in the solution. Identify the stoichiometric relations between the concentrations of solute species (often by setting up an equilibrium table). Use those relations together with equilibrium constants to calculate the equilibrium concentrations of the solute species. 2
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Key Ideas The concentrations of dissolved ions satisfy the equilibrium constants for the reactions in which they participate. 3
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Why Do We Need to Know It? The techniques described in this chapter provide tools that we need to analyze and control the concentrations of ions in solution. Many reactions are carried out in solution, and so this material is fundamental to understanding chemistry. The ionic compounds released into waterways by individuals, industry, and agriculture can impair the quality of our water supplies. However, these hazardous ions can be identified and removed if we add the right reagents. Aqueous equilibria govern the stabilization of the pH in blood, seawater, and other solutions encountered in biology, medicine, and the environment. 4
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Buffer Action (12.1) A ______ is a solution in which the pH resists change when small amounts of strong acids or bases are added. 5
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When a drop of strong acid is added to pure water, the pH changes significantly. When the same amount is added to a buffer, the pH hardly changes at all. To understand why, we have to consider the dynamic equilibrium between a weak acid and its conjugate base in water. CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO - (aq) 6 Buffer Action (12.1)
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CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO - (aq) Both the acid and the base must be present in similar concentrations. The acid portion of the pair reacts with any strong bases that might be added to the solution. The base portion of the pair reacts with any strong acids that might be added to the solution. 7 Buffer Action (12.1)
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Buffer Action (12.1) Summary A buffer is a mixture of a weak conjugate acid —base pair that stabilizes the pH of a solution by providing both a source and a since of protons (see Figure 12.2) 8
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Designing a Buffer (12.2) An ___________ is a mixture of a weak acid and the salt of its conjugate base and acts as a buffer at pH < 7. A ___________ is a mixture of a weak base and the salt of its conjugate acid and acts as a buffer at pH > 7. 9
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Example: What is the pH of a buffer solution that is 0.040 M NaCH 3 COO(aq) and 0.080 M CH 3 COOH(aq)? The K a for CH 3 COOH is 1.8 x 10 -5 10 Designing a Buffer (12.2)
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11 We can use the K a expression to derive an equation that allows us to more easily calculate the pH of a buffer.
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