Previously in Chem104: structural characteristics of acids & bases Today in Chem104: acid/base reactions: the easy, the difficult, the impossible calculations Polyprotic acids

Let’s do some chemistry: Acid / Base Reactions There’s 3 types of acid /base reaction problems: Easy: strong acid + strong base Difficult: strong acid + weak base or (strong base) + (weak acid) Impossible: weak acid + weak base These reactions go to completion: 1 step These reactions are neutral’n & equil’n: 2 steps These reactions depend on magnitudes of K a and K b.

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

methylaminecitric acid methylammoniumcitrate  First type: you start on one side, with one acid and one base. What does the reaction HAVE TO do? 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.   Weak Acid /Weak Base Reactions

Methylamine pK b 3.35 citric acid pK a 3.0 methylammoniumcitrate  Second type: you start with all four species, both acid/base pairs. Which side does the equilibrium favor?   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. Weak Acid /Weak Base Reactions

Example weak-weak problems Sodium carbonate (0.1 mol) is dissolved in a solution of HF (0.01 M). Describe the chemistry that occurs.

Example weak-weak problems A solution of HF (0.01 M) is mixed with a solution of hypochlorous acid (0.01 M). Does anything happen?

Example weak-weak problems A 1M solution of acetic acid is mixed with a 1 M solution of propanoic acid acid. Does anything happen?

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

Polyprotic acids …are acids that can donate more than one proton! Phosphoric acid, H 3 PO 4 … does having 3 H’s automatically mean it acts as a polyprotic acid? Step 1. H 3 PO 4 + H 2 OH 2 PO H 3 O+ K a1 = 7.6 x Step 2. H 2 PO H 2 OHPO H 3 O+ K a2 = 6.2 x Step 3. HPO H 2 OPO H 3 O+ K a3 = 2.12 x Net. H 3 PO 4 + 3H 2 OPO H 3 O+ K eq = K a1 K a2 K a3