LAB 9: Hess’s Law & The Heat of Formation
Objective:
To determine the heat of formation using Hess’s Law for solid ammonium chloride and ammonium sulfate.
Procedure:
1. Set up the Styrofoam cup system as a calorimeter.
2. Neutralization: Place 50mL of 1.50 M NH3 in the bottom of calorimeter and 50 mL of 1.50 M HCl
in a beaker. Record the temperature of each solution. Add all the acid, replace the top of
thermocup , stir gently with the thermometer and record the highest temperature obtained within
the next minute. Clean up.
3. Dissolving: Calculate the mass of NH4Cl (aq) in the final solution in step 2. Obtain the exact same
amount of NH4Cl(s). Put 100mL of distilled water in the cup and record temperature. Add the
correct amount of NH4Cl(s) to the water, stir and determine the final temperature of the solution
when all the solid is dissolved (this will take a few minutes of stirring, be sure all the solid has
dissolved).
4. Repeat step 2 with 50 mL of 1.50 M NH3 and 25mL of H2SO4.
5. Calculated amount of (NH4)2SO4(aq) in solution of step 4. Obtain the exact same amount of
(NH4)2SO4(s). Put 100mL of distilled water in the cup and record temperature. Add (NH 4)2SO4(s) to
the water, stir and determine the final temperature of the solution when all the solid is dissolved
(this will take a few minutes of stirring, be sure all the solid has dissolved).
Data:
Neutralization HCl + NH3:
Volume of 1.5 M HCl
Volume of 1.5 M NH3
Mass of NH4Cl
Intial Temp of HCl
Intial Temp of NH3
Final Temp HCl + NH3
Dissolving of NH4Cl:
Initial Temp of 100mL water
Final Temp of 1.5 M NH4Cl
Neutralization H2SO4 + NH3:
Volume of 1.5 M H2SO4
Volume of 1.5 M NH3
Mass of (NH4)2SO4
Intial Temp H2SO4
Intial Temp of NH3
Final Temp H2SO4 + NH3
Dissolving of (NH4)2SO4:
Initial Temp of 100mL water
Final Temp 0.75 M (NH4)2SO4
Calculations/Questions:
1. Show your calculations used to determine the mass NH4Cl and (NH4)2SO4 required for the Dissolving
experiments.
2. Determine the Hneut in kilojoules per mole for the reaction of NH3(aq) with
a) the HCl(aq)
b) the H2SO4(aq)
3. Determine the Hdiss in kilojoules per mole for dissolving:
a) NH4Cl(s)  NH4Cl(aq)
b) (NH4)2SO4(s)  (NH4)2SO4(aq)
4. Write the chemical equation for the Standard Heat of Formation of solid NH4Cl.
5. Write the chemical equation for the Standard Heat of Formation of solid (NH4)2SO4.
6. Using Hess’s Law, and knowing that the Hf of 1.5 M NH3(aq) is –81.2 kJ/mol and the Hf of 1.5 M
HCl(aq) is –165.1 kJ/mol; calculate the heat of formation (Hf) for NH4Cl(s).
7. . Using answers to previous questions, Hess’s Law, and knowing that the Hf of 1.5 M NH3(aq) is –81.2
kJ/mol and the Hf of 1.5 M H2SO4(aq) is –884.2 kJ/mol; calculate the heat of formation (Hf) for
(NH4)2SO4(s).