Question

Commercial cold packs often contain solid NH4NO3 and a pouch of water. The temperature of the pack drops as the NH4NO3 dissolves in water. Therefore, for the dissolving of NH4NO3 in water, a. ΔHsoln is positive and ΔSsoln may be negative or positive. b. ΔHsoln is positive and ΔSsoln is positive. c. ΔHsoln is negative and ΔSsoln is positive. d. ΔHsoln is negative and ΔSsoln may be negative or positive.

Answer 1

Answer: b. $\Delta H_{soln}$ is positive and $\Delta S_{soln}$ is positive.

Explanation:-

As the temperature of the pack drops, the energy has been absorbed from the pack for dissolution of $NH_4NO_3$ in water. Thus as the energy has been absorbed in the reaction, the reaction is endothermic and the change in enthalpy i.e. $\Delta H_{soln}$ is positive.

The entropy is the measure of degree of randomness. The entropy increases when the randomness increases and the entropy decreases when the randomness decreases. When a substance dissolves in water, it dissociate into ions and hence the randomness increases thus the change in entropy i.e. $\Delta S_{soln}$  is positive.

$NH_4NO_3\rightarrow NH_4^++NO_3^-$