CH302 Chapter 7 notes part 4

CH302 Chapter 7 notes part 4 - CH302 Chapter 7 notes part 4...

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CH302 Chapter 7 notes part 4 ACIDS and BASES ACID-BASE PROPERTIES OF SALTS: Dilute aqueous solutions of salts that contain no free acid or base come in five types: Salts of Strong Bases and Strong Acids Salts of Strong Bases and Weak Acids Salts of Weak Bases and Strong Acids Salts of Weak Bases and Weak Acids and Salts of Strong Acid with Highly Charged Metal Ion. Salts of Strong Bases and Strong Acids These form neutral aqueous solutions . The conjugate acid of the strong base is VERY weak so does not react with OH - available from water. The conjugate base of the strong acid is VERY weak so does not react with H 3 O + available from water. The only source of OH - and H 3 O + is from the autoionization of water so pH = 7 as for pure water!! Example: Potassium nitrate, KNO 3 : made from nitric acid and potassium hydroxide. K + does NOT recombine with OH - NO 3 - does NOT recombine with H 3 O + Therefore pH = 7 due to autoionization of water. Salts of Strong Bases and Weak Acids These form basic aqueous solutions The conjugate acid of the strong base is VERY weak so does not react with OH - available from water. The conjugate base of the weak acid is a (stronger) weak base so does react with H 3 O + available from water. As H 3 O + is used up, autoionization of water is disturbed. K w still holds, so OH - increases and pH = >7!! Example: Sodium hypochlorite, NaClO, made from sodium hydroxide and hypochlorous acid. Na + does NOT react with OH - ClO - + H 3 O + HClO + H 2 O (lowers [H 3 O + ]) OR , ClO - + H 2 O HClO + OH - (raises [OH - ]) Either way, pH > 7 due to [OH - ] > [H 3 O + ]. [] K= HClO OH ClO b - - The equilibrium constant for ClO - + H 2 O HClO + OH - is written as: How do you find K b when you are only given K a for a weak acid? Easy! A VERY useful Equation: The equilibrium constants for conjugate acid-base pairs are related to that for the autoionization of water via: K w = K a x K b We can now find K b for the hypochlorite ion given K a for hypochlorous acid = 3.5 x 10 -8
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Conjugate Acid Base Strengths The expression linking K a and K b shows how the conjugate acid of a very weak base is less weak (i.e., stronger). Try for yourself and see that a large K a (i.e. a strong acid) means that K b for the conjugate base is extremely small. Calculate the Kb's for the following anions of weak acids (i.e., find K b for these acid’s conjugate bases) 1. The fluoride ion, F-, the anion of hydrofluoric acid, HF. For HF, K a =7.2 x 10 -4 .
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CH302 Chapter 7 notes part 4 - CH302 Chapter 7 notes part 4...

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