Experimental Applications of Hess’s Law
There are many reactions for which the heat of reaction cannot be easily measured
experimentally. Some of these are dangerous to perform in the lab. Others generate so
much heat that simple calorimeters cannot be used. 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. Once that has been done, the required heat of reaction
can be calculated using Hess’s Law.
In this lab, you will conduct two such investigations. In the first one we will examine the
combustion of magnesium. However, since we don’t have a bomb calorimeter to do this
reaction in, you will have to perform the elementary steps in the mechanism to determine
the heat for the reaction. The equation for the combustion of magnesium can be
obtained by manipulating these three equations:
MgO + 2HCl  MgCl2 + H2O
Mg + 2HCl  MgCl2 + H2
2H2 + O2  2H2O
In the second investigation, we will examine the heat of formation of solid calcium
hydroxide. Once again, this cannot be measured in a single step. You will have to
measure the heat by examining the following reactions:
Ca + 2H2O Ca(OH)2 + H2
2H2 + O2  2H2O
Apparatus and Material:
Thermometer
Mg ribbon
2 Styrofoam cups
water
100 mL Grad. Cylinder
Ca metal
Magnesium oxide
Stir Rod
balance
1M HCl
Safety:
Hydrochloric acid is corrosive. The calcium hydroxide solutions produced is very
caustic. Avoid spilling either of these on yourself, your clothes, or your friends. Wash
immediately if the solutions contact your skin. Safety goggles are required. Calcium
metal reacts vigorously with water. Be extremely careful that pieces of calcium metal do
not become wet. It is advisable to wear a lab coat or an apron throughout this
experiment.
Procedure~
Part One: The Combustion of Magnesium
 Use 100.0 mL/ 1.0 g or less of any substance to determine the heat of reaction for
the mixture of magnesium oxide in hydrochloric acid and for magnesium in
hydrochloric acid.
 The heat of reaction for the formation of water is -285 kJ. You will not be able to
perform that step reaction.
Part Two: The heat of formation of solid calcium hydroxide
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Use 100.0 mL/ 1.0 g or less of any substance to determine the heat of reaction for
the mixture of calcium and water.
Caution: do not allow any pieces of calcium metal to become wet while on the
balance or the lab table. Use only the amount needed, and place the rest of the
calcium back in its container.
The heat of reaction for the formation of water is -285 kJ. You will not be able to
perform that step reaction.
Calculations:
1. From your data in Part 1 (Magnesium), calculate the following:
a. The T for each experiment.
b. The mass of each solution used.
c. The heat (q) absorbed by the solution (use Cp= 4.184 J/g*K)
d. The number of moles of (1) magnesium oxide and (2) magnesium used.
e. The heat of the reaction when hydrochloric acid reacted with (1)
magnesium oxide and (2) magnesium.
2. From your data in Part 2 (Calcium), calculate the following for the reaction.
a. The T for each experiment.
b. The mass of each solution used.
c. The heat (q) absorbed by the solution (use Cp= 4.184 J/g*K)
d. The number of moles of calcium metal used.
e. The heat of the reaction when water reacted with calcium metal
3. Show how the equations for the magnesium reactions can be added together to get
the overall reaction using Hess’s Law. Show also your calculations of the heat
of the overall reaction using the heats found in calculation # 1.
4. Show how the equations for the actions found in the introduction can be added
together using Hess’s Law to form the overall reaction for the calcium hydroxide.
Show also your calculations of the heat of the overall reaction using the heats
found in calculation # 2.