## Q. 18.5

Predicting the Net Direction of an Acid-Base Reaction

Problem Predict the net direction and indicate whether $K_c$ is greater or less than 1 for each of the following reactions (assume equal initial concentrations of all species):

(a) $H_2PO_4^−(aq) + NH_3(aq) \xrightleftharpoons[]{} NH_4^+(aq) + HPO_4^{2−}(aq)$

(b) $H_2O(l) + HS^−(aq) \xrightleftharpoons[]{} OH^−(aq) + H_2S(aq)$

## Verified Solution

Plan We identify the conjugate acid-base pairs and consult Figure 18.8 to see which acid and base are stronger. The reaction proceeds in the direction in which the stronger acid and base form the weaker acid and base. If the reaction aswritten proceeds to the right, then [products] is higher than [reactants], so $K_c > 1$.

Solution (a) The conjugate pairs are $H_2PO_4^−/HPO_4^{2−}$ and $NH_4^+/NH_3$. Since $H_2PO_4^−$ is higher on the list of acids, it is stronger than $NH_4^+$; since $NH_3$ is lower on the list of bases, it is stronger than $HPO_4^{2−}$. Therefore,

The net direction is to the right, so $K_c > 1$.
(b) The conjugate pairs are $H_2O/OH^−$ and $H_2S/HS^−$. Since $H_2S$ is higher on the list of acids, and $OH^−$ is lower on the list of bases, we have

The net direction is to the left, so $K_c < 1$.