A chemist is comparing the acid–base properties of four hypothetical substances at 25 °C

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A chemist is comparing the acid–base properties of four hypothetical substances at 25 °C. The available equilibrium data are:
SubstanceRelevant equilibrium in waterKK value
Substance DHD⇌H++D−\text{HD} \rightleftharpoons \text{H}^+ + \text{D}^- Ka=2.8×10−9K_a = 2.8 \times 10^{-9}
Substance EHE⇌H++E−\text{HE} \rightleftharpoons \text{H}^+ + \text{E}^- Ka=6.5×10−7K_a = 6.5 \times 10^{-7}
Substance FF−+H2O⇌HF+OH−\text{F}^- + \text{H}_2\text{O} \rightleftharpoons \text{HF} + \text{OH}^- Kb=1.6×10−11K_b = 1.6 \times 10^{-11}
Substance GG−+H2O⇌HG+OH−\text{G}^- + \text{H}_2\text{O} \rightleftharpoons \text{HG} + \text{OH}^- Kb=9.8×10−5K_b = 9.8 \times 10^{-5}
All equilibrium constants are measured at 25 °C. (a) Arrange the four substances in order of decreasing tendency to act as Brønsted–Lowry bases in aqueous solution (strongest base first). (b) Two 0.10 M solutions are prepared, one of Substance E and one of Substance G. The chemist mixes equal volumes of these two solutions. Predict whether the resulting mixture is acidic, basic, or neutral at equilibrium and justify your prediction quantitatively. (c) A hypothetical proton-transfer reaction is proposed: HD+G−→D−+HG\text{HD} + \text{G}^- \rightarrow \text{D}^- + \text{HG} Predict the direction in which the equilibrium lies (toward products or reactants) and defend your answer with a brief but complete argument based on the data provided.

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