# When ammonium chloride dissolved in water the temperature decreases. The energy change for the dissolution of ammonium chloride is +284Jg^-1 ?

Aug 3, 2017

$\left(i\right)$

Well, $\left(i\right)$ is just a unit conversion; the grams are of the solute, so use the molar mass of ammonium chloride, and the conversion $\text{kthings}$ $=$ $\text{1000 things}$:

DeltaH_"diss" = (284 cancel"J")/cancel"g" xx ((14.007 + 4 xx 1.0079 + 35.453) cancel"g")/"mol" xx ("1 kJ")/(1000 cancel"J")

$=$ $\underline{\text{15.2 kJ/mol}}$

$\left(i i\right)$

It's easier to start with the $\text{J/g}$ value here... The energy absorbed FROM the solution (i.e. released out of the solvent into the solute) is

DeltaH_("diss") = "284 J"/cancel"g" xx 3 cancel("g NH"_4"Cl") = ul"852 J"

That is, the system with respect to which the process is endothermic is the $\text{NH"_4"Cl}$ solute itself, and the surroundings are the solvent.

Hence, the solute gains the heat from the solvent, and the temperature of the solution decreases.