Purpose: To calculate the Molar enthalpy of dissolving for two ionic solids.
Materials:




Reagent grade ammonium chloride (NH
4
Cl)
Reagent grade calcium chloride. (CaCl
2
)
100-200 ml coffee cup, and lid graduated cylinder
 water (distilled is preferred by not absolutely necessary)
 thermometer
 stirring rod.
Procedure:
1.
Measure out 100 ml of distilled water in a graduated cylinder and record it's temperature.
2.
Add the water to the coffee cup.
3.
Measure out 0.1 moles of ammonium chloride.
4.
Add this amount to the water in the coffee cup and place the lid on top. Stir the solid until it dissolves.
5.
Record the lowest temperature reached .
6.
Empty out the coffee cup , and rinse and then repeat the procedure (step #1-5) with a second sample.
7.
Now repeat steps #1- #6 using 0.1 moles of calcium chloride.
Data:

Mass of water in calorimeter
(1ml = 1g)
Mass of solid used
Moles of solid used
Temperature change
Ammonium Chloride
Trial #1 Trial #2
Calcium Chloride
Trial #1 Trial #2
Calculations:
1.
Calculate the molar heat ( H) for the dissolving of the two solids. The formula is given below m * c * t
H substance
= n m = mass of the substance in surroundings (water) in kilograms c= specific heat capacity of water (4.19 J/g* o C) t = temperature change in water in calorimeter. n = moles of substance reacted
Calculation of molar enthalpy
Example: Calculate the molar enthalpy of the reaction if when 0.25 moles of a substance is reacted the heat released produces a 4.5 o
C temperature increase in 325 ml of water in a coffee cup calorimeter.
1) Write down the information given. a) moles of substance = 0.25 moles b) mass of substance in surroundings = 325 ml = 325 g

c) temperature change = 4.5 C d) specific heat capacity of water is 4.19 J/g
o
C
2) Determine the information required.
Molar enthalpy
3) Analysis the problem and make a plan to solve for molar enthalpy.
m * c * t
H substance
= n
4) Apply formula and solve problem.
325g * 4.19 J/g o
C * 4.5 C
H substance
=
0.25 mol.
H substance
= 24,511.5 J/mol divide by 1000 to get kJ
5) State answer.
The molar enthalpy of the substance is 24.5 kJ/ mol