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

1 Answer
Truong-Son N.
Aug 3, 2017

#(i)#

Well, #(i)# is just a unit conversion; the grams are of the solute, so use the molar mass of ammonium chloride, and the conversion #"kthings"# #=# #"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")#

#=# #ul"15.2 kJ/mol"#

#(ii)#

It's easier to start with the #"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 #"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.

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