In the case of dissolving solid NH4NO3 in water, the process is endothermic, thereby causing a decrease in the temperature of the solution. To understand why the enthalpy change is positive, let's break it down.
Enthalpy (ΔH) represents the heat energy transferred during a process. When a substance dissolves in water, the bonds between the particles of the solute (NH4NO3) are broken, as well as the bonds between the particles of the solvent (water).
In this case, solid NH4NO3 dissociates into NH4+ and NO3- ions in solution. The formation of these ions requires energy to break the bonds between the particles of NH4NO3. Since energy is being absorbed from the surroundings, the process is endothermic.
To simplify this concept, think about dissolving salt in water. When you pour salt into water and stir it, the salt crystals break apart, and the sodium (Na+) and chloride (Cl-) ions separate from each other. This separation requires energy. Even though the process is endothermic and absorbs heat from the surroundings, it doesn't necessarily mean the water gets colder. The temperature decrease might occur due to the overall heat absorbed in the process.
So, in the case of dissolving NH4NO3 in water, both the entropy (disorder) and enthalpy (heat energy transfer) are positive. The positive entropy indicates an increase in the randomness (disorder) of the system, while the positive enthalpy implies an endothermic process that absorbs heat energy.