Question

Predict the sign of the enthalpy change for the following processes. Give a reason for your prediction.

(a) $\mathbf{N}\mathbf{a}\mathbf{N}{\mathbf{O}}_{\mathbf{3}}\mathbf{(}\mathbf{s}\mathbf{)}\to \mathbf{N}{\mathbf{a}}^{\mathbf{+}}\mathbf{(}\mathbf{a}\mathbf{q}\mathbf{)}\mathbf{+}\mathbf{N}{{\mathbf{O}}_{\mathbf{3}}}^{\mathbf{-}}\mathbf{(}\mathbf{a}\mathbf{q}\mathbf{)}$

(b) the freezing of liquid water

(c) $\mathbf{C}{\mathbf{O}}_{\mathbf{2}}\mathbf{(}\mathbf{s}\mathbf{)}\to \mathbf{C}{\mathbf{O}}_{\mathbf{2}}\mathbf{(}\mathbf{g}\mathbf{)}$

(d) $\mathbf{C}\mathbf{a}\mathbf{C}\mathbf{O}\mathbf{(}\mathbf{s}\mathbf{)}\to \mathbf{C}\mathbf{a}\mathbf{O}\mathbf{(}\mathbf{s}\mathbf{)}\mathbf{+}\mathbf{C}{\mathbf{O}}_{\mathbf{2}}\mathbf{(}\mathbf{g}\mathbf{)}$

Short answer

1. Positive; The solid dissolves to give an increase of mobile ions in solution.
2. Negative; The liquid becomes a more ordered solid.
3. Positive; The relatively ordered solid becomes a gas.
4. Positive; There is a net production of one mole of gas.

Step-by-step solution

Define phase transitions

• For the phase transitions or changes involving the breaking of bonds or intermolecular attractions like hydrogen bonding, heat is added or absorbed and the enthalpy of the system increases and the$\mathrm{\Delta }\mathrm{H}$is +ve and the reaction is endothermic.
• The phase changes or transitions involving the formation of bonds or formation of intermolecular interactions, the heat is released and the $\mathrm{\Delta }\mathrm{H}$ is -ve and the reaction is exothermic.

Determine the decomposition reaction.

(a) The reaction of converting $\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_3(\mathrm{s})$ to its aqueous ions involves the breaking of bond and heat is added up. The reaction is endothermic in nature and the $\mathrm{\Delta }\mathrm{H}$ is +ve.

(b). The freezing of the liquid water is an exothermic reaction as the heat is evolved from the system. The $\mathrm{\Delta }\mathrm{H}$ is -ve.

(c). solid $\mathrm{C}{\mathrm{O}}_2$ (s) sublimes to $\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$. The heat is absorbed from the surroundings for the change. The process is endothermic and $\mathrm{\Delta }\mathrm{H}$ is +ve.

(d). solid $\mathrm{C}{\mathrm{O}}_2$ (s) sublimes to $\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$. The heat is absorbed from the surroundings for the change. The process is endothermic and $\mathrm{\Delta }\mathrm{H}$ is +ve.