Question

For the following reaction, label each of the below species as an acid or a base. Use lower case letters only (e.g. acid)

Answer 1

Explanation:

According to Arrhenius, bases are the species which dissociate to give hydroxide ions, that is, $OH^{-}$.  

For example, $NaOH + H_{2}O \rightarrow Na^{+} + OH^{-}$

Arrhenius acids are the species which dissociate to give hydrogen ions, that is, $H^{+}$.

For example, $CH_{3}COOH + H_{2}O \rightleftharpoons H_{3}O^{+} + CH_{3}COO^{-}$

In the given example, HCN is donating the hydrogen ion therefore, it is an acid whereas $HPO^{2-}_{4}$ is accepting the hydrogen ions so it acts as a base.

Now, standard $\Delta G^{o}$ values of the given species are as follows.

 HCN = 124.7 kJ/mol,      $HPO^{2-}_{4}$ = -1089.3 kJ/mol

 $H_{2}PO^{-}_{4}$ = -1130.4 kJ/mol,       $CN^{-}$ = 172.4 kJ/mol

Therefore, $\Delta G^{o}$ of the given reaction will be calculated as follows.

      $\Delta G^{o} = \Delta G_{products} - \Delta G_{reactants}$

                   = [-1130.4 + 172.4] - [124.7 + (-1089.3)]

                   = (-958 + 964.6) kJ/mol

                   = 6.6 kJ/mol

Since, $\Delta G^{o}$ is greater than zero. Hence, the reaction will be non-spontaneous.