SCH4U
The Experimental Application of Hess’ Law
There are many reactions for which the heat of reaction cannot be easily measured
experimentally. Some are dangerous to perform in the lab. Others simply cannot be done using a
calorimeter. In such cases it is practical to examine a series of reactions whose NET effect is the
desired reaction, but whose heats of reaction are more easily measured. The required heat of
reaction can then be calculated using Hess’s Law. This will be a FORMAL LAB WRITE-UP.
Pre-lab [
/10]:
1. Make a safety chart for all the chemicals that will be used in this lab. Refer to the Esters lab for
an example. [
/4]
2. Draw a sketch of the set-up of your experiment. [
3. Design your observation chart. [
/2]
/2]
4. Make a check list for the procedure you are going to take. Make sure each step is ONLY ONE
TASK. [
/2]
Materials
1.5 M sulphuric acid solution
Magnesium oxide
Magnesium metal
Styrofoam cup
Thermometer
100 mL gradulated cylinder
Watchglass
Procedure
1. Measure the change in temperature that occurs when about 2 g (weight accurately) of MgO is
added to 50 mL of 1.5 M H2SO4 solution. Record the temperature every 10 seconds.
2. Measure the change in temperature that occurs when about 0.5 g (weigh accurately) of
magnesium is added to 50 mL of 1.5 M H2SO4 solution. Record the temperature every 10
seconds.
SCH4U
Required Calculations
1. Calculate ΔH for the reaction of magnesium oxide with sulphuric acid solution using your
experimental results.
2. Calculate ΔH for the reaction of magnesium with sulfuric acid solution using your
experimental results
3. Look up ΔH for the combustion of hydrogen gas
4. Write the balanced chemical equation for the combustion of magnesium
5. Use Hess’ Law to calculate ΔH for the combustion of magnesium
Results [
/20]
Your results section must contain ALL of your calculations. Your calculations must lead up to
your finding the HEAT OF COMBUSTION OF MAGNESIUM. Due to this, your results section will be
weighed more heavily than your discussion section for this lab.
You will also need to include two graphs outlining the temperature change during the two
reactions. Join each set of points with a SMOOTH curve.
Discussion [
/10]
Your discussion section must include the following:
-
What is the theoretical value for the heat of combustion of magnesium? Show all your
calculations.
-
How does your experimental value compare to the theoretical value?
-
What are some possible sources of error? Specifically how would those sources of error
affect your results?