When 1.0 g of potassium chloride, KCl, is dissolved in 25 mL (=25 g) of water, it causes the temperature to drop from 24.33^o C to 22.12^o C. What is the molar enthalpy of dissolution for KCl?

1 Answer
Apr 10, 2016

The energy absorbed from the water by this endothermic reaction is approximately Delta H=17.3 kJmol^-1.

Explanation:

The reaction in question is:

KCl_"(s)" to K^"+"(aq) + Cl^"-" (aq)

The reaction is endothermic, so the value of Delta H we calculate at the end will be positive. First, though, we need to calculate the amount of energy absorbed from the water, which will be negative:

DeltaH=mCDeltaT=mC(T_2-T_1)
=25xx4.186xx(22.12-24.33)=-232.05 J

This is the amount of energy absorbed from the water by the process of dissolving the KCl. This is the same as the energy gained by the 'system' of dissolving KCl. That means energy gained of 232.05 J for the 1.0 g sample. What we want to know, though, is the total energy absorbed when 1 mole of KCl dissolves.

One gram of KCl is: n=m/M=1/74.55=0.0134 mol

That means the total energy gain if we had dissolved an entire mole, 74.55 g, of KCl, is 74.55 times as great, which gives:

Delta H=74.55 mol^-1xx232=17295.6 Jmol^-1 ~~ 17.3 kJmol^-1