When solid NH4NO3 is dissolved in water at 25celcius, the temp of the solution decreases. What is true about the signs of enthalpy and entropy for this process?
I understand entropy is positive, however, i do no understand why enthalpy is also positive. Can someone please explain this to me, possibly a simple example with a chemical in everyday life?
Yes, but first, note the correct spelling of celsius.
By definition, delta H (enthalpy) is - if heat is emitted from a reaction and + if heat is absorbed by a reaction. Since the temperature of the water decreases that means the solvation of NH4NO3 absorbed energy and it told you that by taking the heat (enthalpy) from the water.
NH4NO3 + H2O ==> NH4NO3(solution)- heat which can be re-written as
NH4NO3 + H2O + heat = NH4NO3(aq)
and delta H is + for an endothermic reaction (again, by definition).
I'm a retired chemistry professor from a university in Louisiana (but not living in Louisiana now). Taught chemistry for 40 years.
What is your profession Dr Bob
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.