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Previously in Chem104: K a , K b K w p -functions (pH, pK a , pK w )

Previously in Chem104: K a , K b K w p -functions (pH, pK a , pK w ) using the K w / pK w circle structural aspects of acids & bases Neutralization reactions Calc’g pH and pOH Note: Lewis Acids/Bases next week with solubility. Today in Chem104: more acid /base reactions:

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Previously in Chem104: K a , K b K w p -functions (pH, pK a , pK w )

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  1. Previously in Chem104: • Ka, KbKw • p-functions (pH, pKa, pKw) • using the Kw/ pKw circle • structural aspects of acids & bases • Neutralization reactions • Calc’g pH and pOH • Note: Lewis Acids/Bases next week with solubility • Today in Chem104: • more acid/base reactions: • weak / weak • strong / strong • strong / weak • calculations • Polyproticacids

  2. Weak Acid /Weak Base Reactions First type: you start on one side, with one acid and one base. What does the reaction HAVE TO do?  methylamine citric acid methylammonium citrate The reaction HAS TO go forward to make some product until it reaches equilibrium. How much product it makes depends on the Ka and Kb. X X  

  3. Weak Acid /Weak Base Reactions First type: you start on one side, with one acid and one base. What does the reaction HAVE TO do?  methylamine citric acid methylammonium citrate (pKb 3.35) Kb = 4.47 x 10-4 (pKa 3) Ka = 10-3 Calculation of amounts is very difficult unless you have information on most of the species. You won’t have problems like that. You will be asked about which direction the equilibrium favors in a weak acid / weak base problem.  

  4. Weak Acid /Weak Base Reactions Second type: you start with all four species, both acid/base pairs. Which side does the equilibrium favor?  Methylamine pKb 3.35 citric acid pKa 3.0 methylammonium citrate The equilibrium favors the side with weakest acid and base pair. Or, the stronger acid and base push the reaction to the side with weaker acid /base.  

  5. Example weak-weak problems Equimolar solutions of acetic acid, acetate, cyanide and hydrocyanic acid are mixed. How will the equilibrium shift?

  6. Polyprotic acids …are acids that can donate more than one proton! Phosphoric acid, H3PO4 … does having 3 H’s automatically mean it acts as a polyprotic acid? Consider H4C2O2 Step 1. H3PO4 + H2O H2PO4 -+ H3O+ Ka1 = 7.6 x 10-3 Step 2. H2PO4-+ H2O HPO4 2-+ H3O+ Ka2 = 6.2 x 10-8 Step 3. HPO42-+ H2O PO4 3-+ H3O+ Ka3 = 2.12 x 10-13 Net. H3PO4 + 3H2O PO4 3-+ 3H3O+ Keq = Ka1Ka2Ka3

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