Hess's Law
In this lab activity, you will calculate the enthalpy change for the thermal decomposition
of sodium bicarbonate:
Problem:
What is the molar enthalpy of reaction for the thermal decomposition of sodium
hydrogen carbonate at standard conditions?
Materials
weighing boats
graduated cylinder
250 mL beaker
styrofoam cups
2.00 g of solid sodium carbonate
2.00 g of sodium hydrogen carbonate (baking soda)
100 mL of 1.0 M hydrochloric acid
temperature probe
Pre lab:
1. Determine
for each of the equations above.
2. Then determine the
for the final reaction using Hess’s Law
Part 1:
1. Measure out 2.00 g of NaHCO3 (baking soda)
2. Measure out 50 mL of HCl.
3. Add the HCl to the Styrofoam cup
4. Record initial temperature of the HCl
5. After 5-10 seconds, add the baking soda, stir
6. Click stop when the temperature has a reached a minimum and starts to increase
again
7. Dump the liquid in the sink, rinse and dry the Styrofoam cup.
8. Repeat steps 1-7 using Na2CO3 instead of the baking soda
Analysis:
1.
2.
3.
4.
Calculate the change in temperature for the HCl with the baking soda
Calculate the mass of the Styrofoam cup water (mass of solid and mass of HCl)
Calculate q
Repeat 1-3 with the Na2CO3
5. Determine the
using Hess’s law using your actual lab values
6. Determine percent error of Hess’s law values from the pre-lab and your actual lab
values.
CP Hess Law lab Worksheet
Pre-lab work:
Data table
Actual mass of baking soda (NaHCO3)
Moles of baking soda (NaHCO3)
Volume of HCl
Initial temperature of HCl
Final temperature of HCl and NaHCO3
Change in temperature
Value of q (Cp=4.184J/gC)
-q = value of ΔH
ΔH / mol NaHCO3
Actual mass of Na2CO3
Moles of Na2CO3
Volume of HCl
Initial temperature of HCl
Final temperature of HCl
Change in temperature
Value of q (Cp=4.184J/gC)
-q = value of ΔH
ΔH / mole Na2CO3
Heat of formation using Hess’s law with
lab values
Percent Error
Show all work for analysis calculations on back of this paper!
Names