GRE Subject Test: Chemistry - Acid-Base Reaction

$\begin{matrix} \text{Acid} &\text{pK}_{\text{a}} \ HC_2H_3O_2 & 4.76\ HO_2CCF_3 & 0\ HCl & -2\ H_2O& 16\ C_6H_5OH&10 \ MeOH& 16\ H_2 & 35\ NH_3 & 33\ \end{matrix}$

For which of the following acid-base reactions will the equilibrium lie on the left side?

$HC_{2}H_{3}O_{2} + NaO_{2}CCF_{3} \rightleftharpoons NaC_{2}H_{3}O_{2} +HO_{2}CCF_{3}$

$HCl + NaOH \rightleftharpoons H_{2}O + NaCl$

$C_{6}H_{5}OH (phenol) + MeONa \rightleftharpoons C_{6}H_{5}ONa + MeOH$

$MeOH + LiNH_{2} \rightleftharpoons MeOLi + NH_{3}$

$H_{2}O + NaH \rightleftharpoons H_{2} + NaOH$

Explanation

The pKa value indicates how strong an acid is, and acid strength increases as pKa decreases. The side of a reaction with a lower pKa is going to dissociate more, pushing the equilibrium over to the other side. The equilibrium will thus lie on the side with the HIGHER pKa.

$HC_{2}H_{3}O_{2} + NaO_{2}CCF_{3} \rightleftharpoons NaC_{2}H_{3}O_{2} +HO_{2}CCF_{3}$

Since the pKa of acetic acid (4.76) is higher than the pKa of trifluoroacetic acid (0), the reaction will shift to the left to reach equilibrium.

Acid-Base Reaction

Help Questions

Explanation