system and surroundings? solid ammonium chloride?

Question

Solid ammonium chloride is added to water in a beaker and dissolves. The beaker becomes cold to the touch.
a. Make an appropriate choice of system and surroundings and describe it unambiguously.
b. Explain why you chose the system and surroundings you did.
c. identify transfers of energy and material into and out of the system that would be important for you to monitor in your study.
d. Is the process of dissolving NH4Cl solid in water exothermic or endothermic?

Answer 1

a) You could go a couple of ways on this, but I will take the compound, and water as the system.

The surroundings then would be the beaker, any thermometer, stirrer, room, universe.

b) I chose this system because it includes only the dissolving salt and the liquid that's doing the dissolving. Everything else is outside of the process of dissolving, and is surroundings.

c) Transfer of energy out of the system to the thermometer, and sides of beaker (touch). Monitor this by reading the thermometer.
Transfer of energy in from the surroundings to allow the dissolving to take place. This could be monitored if the beaker holding the dissolving salt were placed in a closed insulated container with another thermometer.

d). The process absorbs heat from the surroundings. It is endothermic.

Answer 2

The system is the NH4Cl and (1) the breaking of the ionic bond between the NH4+ ion and the Cl- ion plus (2) the process of water molecules hydrating the ions, forming their respective solvation spheres.

The surroundings are all the water molecules into which the ions are entering.

I chose this system & surroundings because that's the region of chemical interest: the items chemically reacting and the area (the surroundings) from which energy is provided.

I also chose to limit my surroundings to the water because of the shortness of the actual dissolving. It is true that the atmosphere extends beyond the water, making for more surrounding, but the energy transfer between atmo and water is very slow compared to the dissolving process.

Transfers of energy (1) from surroundings into the ionic bond, allowing it to be broken. This is an endothermic process. (2) the formation of solvation spheres, an exothermic process.

The entire process is endothermic, as seen by the water getting colder. There has been a net transfer of energy from the surroundings into the system.

HTH

Answer 3

You are trying to work out whether a process involving ammonium chloride is exothermic or endothermic, and you are finding this out by seeing whether it takes in heat from, or give seek out to, the water.

So the ammonium chloride is your system, and your water is the surrounds.

That should get you started.