Problem 16.28

Predict the products of the following acid-base reactions, and also predict whether the equilibrium lies to the left or to the right of the equation:

Here's how to think about this: in every acid-base reaction, the position of the equilibrium favors transfer of the proton from the stronger acid to the stronger base to form the weaker acid and the weaker base. In other words, the equilibrium mixture contains more of the weaker acid and base and less of the stronger acid and base. So, if the stronger acid and base are on the right, the equilibrium lies left, and if the stronger species are on the left, the equilibrium lies right. So, we first predict the products of the reactions, and then identify the acids and bases on each side, and compare acid and base strengths to find the position of equilibrium. Another good little hint here is that water is almost negligibly weak in terms of acid and base strength when we are doing these comparisons.

(a)NH₄⁺(aq) + OH^-(aq) = H₂O(aq) + NH₃(aq)

On the left, hydroxide ion is the base; ammonia is the base on the right. Hydroxide ion is the strongest base which can exist in aqueous solution, so hyrodxide ion is a stronger base than ammonia. Ammonium ion is the acid on the left, and water acts as the Bronsted acid on the right. Water is a weaker acid than ammonium ion, so here, the stronger acid and base are both on the left, and the equilibrium therefore lies to the right.

(b) CH₃COO^-(aq) + H₃O⁺(aq) = CH₃COOH(aq) + H₂O(aq)

Hydronium ion (the acid on the left) is the strongest acid which can exist in aq. solution and is therefore stronger than the acid on the right (acetic acid.) Acetate ion (the base on the left) is a stronger base than water, so the stronger acid/base exist on the left, and the equilibrium therefore lies to the right.

(c) HCO₃^-(aq) + F^-(aq) <===> HF(aq) + CO₃^2-(aq)

Here, HF is a stronger acid than HCO₃^-, and carbonate ion is a stronger base than fluoride ion, so the equilibrium lies to the left. How did we know the strengths of these species? Look at 16.4 - HF is a stronger acid than hydrogen carbonate ion, and carbonate ion is a stronger base than fluoride ion. So do you have to memorize this figure?? NO - very soon we'll have a quantitative way to keep track of acid/base strength.

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