A closed reaction vessel initially contains only pure liquid water
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A closed reaction vessel initially contains only pure liquid water. Into the vessel a chemist injects equimolar amounts of boric acid (B(OH)3), the polyprotic base carbonate ion (CO32−), and the amphiprotic amino acid glycine in its zwitterionic form (H3N+CH2COO−). No other solutes are present, and temperature is held constant.
The species are allowed to interact until equilibrium is reached.
(a) Considering pairwise interactions among the three solutes, list every unique proton-transfer equation that could occur prior to any significant reaction with water.
(b) For each equation in part (a), identify the acid and base on the reactant side and specify the conjugate acid–base pair produced.
(c) Boric acid is a weaker acid than glycine’s carboxyl group, and glycine’s ammonium group is a weaker acid than the bicarbonate species that would form upon protonation of CO32−. Using these qualitative relationships, determine which of the proton transfers from part (a) will be favored at equilibrium. Provide a concise justification referencing acid strength comparisons.
(d) Construct a logically organized argument predicting whether the final equilibrium solution will be acidic, basic, or approximately neutral overall. Your argument should reference (i) the dominant reaction identified in part (c), (ii) the amphoterism of water, and (iii) any secondary reactions you expect to contribute measurably to the pH.