Chem 1120 1st Edition Lecture 14 Outline of Last Lecture I Strong Weak Acids and Bases II Solving Quadratic Equilibrium Equations III Polyprotic Acids Outline of Current Lecture I Weak Bases Relation to Weak Acids II Ka and Kb Equilibrium Calculations III Acid Base Properties of Salt Solutions IV Lewis Acids and Bases Current Lecture I Strong Bases strong electrolytes NaOH aq H2O Na aq OH aq Weak Bases weak electrolytes NH3 aq H2O NH4 aq OH aq Base Ionization Constant Kb For previous equation Kb NH4 OH NH3 1 8 X 10 5 Note that liquid water solvent does not go into the K equations Weak Bases Most bases are weak bases Make sure you know the strong bases Kb is a measure of base strength the smaller the K b the weaker the base and vice versa and for all strong bases Kb 1 These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 2 major categories of weak bases 1 Neutral molecules with lone pairs of electrons ex NH 3 CH3NH2 2 Conjugate bases of weak acids ex HCO 3 FII Kb equilibrium calculations same steps as K a calculations Example Calculate the pH of 0 10 M CH3NH2 Kb 4 4 X 10 4 Solution Relationship between Ka and Kb Kw at 25oC is equal to 1 X 10 14 pKa log Ka pKb log Kb pKa pKb 14 00 Example The pKa of HF is 3 17 What is the value of Kb for the F ion Solution 14 3 17 pKb 10 83 Kb 10 10 83 Kb 1 5 X 10 11 Example Tell how many moles of either H3O or OH you must add to 87 5 mL of HA solution to adjust its pH from 8 92 to 6 33 Assume a negligible volume change III Salts are not always neutral they can be acids or bases as well Examples acidic salts ammonium nitrate ammonium chloride basic salts sodium cyanide potassium fluoride neutral salts sodium bromide potassium chloride We can predict whether a salt is acidic or basic by seeing if and how its ions react with water Hydrolysis the reaction of ions with water to produce H or OH ions The conjugate base of a strong acid will not hydrolyze the conjugate base of a weak acid will hydrolyze the cation of a strong base will not hydrolyze the conjugate acid of a weak base will hydrolyze The bigger the Ka Kb value the stronger the acid base Acidity increases with the number of oxygens IV Acid electron pair acceptor Base electron pair donor A lewis acid does not need to contain H ions a lewis base does not need to contain OH ion Atoms with an empty valence orbital can be Lewis acids Anything that could be a Bronsted Lowry base is a Lewis base Lewis bases can interact with things other than protons however Example Identify the Lewis acid and Lewis base for each reaction
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