Heat of Reaction: MgO(s) + HCl(aq)
VCL 5-11: Heat of Reaction: MgO(s) + HCl(aq)
Energy is either absorbed or released for all chemical reactions, and we call this energy the enthalpy of
reaction (∆Hrxn). If the enthalpy of reaction is positive, then we say that the energy was absorbed or that
the reaction was endothermic. If the enthalpy of reaction is negative, then we say that energy was released
or that the reaction was exothermic. Most chemical reactions are exothermic. In this problem, you will
measure the amount of heat released when solid MgO is reacted with aqueous HCl to form water and then
you will calculate the heat of reaction.
MgO(s) + 2HCl(aq) = H2O(l) + MgCl2(aq)
1. Start Virtual ChemLab and select Heat of Reaction: MgO + HCl from the list of assignments. The lab
will open in the Calorimetry laboratory.
2. There will be a bottle of MgO near the balance. A weigh paper will be on the balance with
approximately 2.81 g MgO on the paper. Record the mass of MgO in the data table.
3. The calorimeter will be on the lab bench and filled with 100 mL 1.000 M HCl. Make certain the
stirrer is On (you should be able to see the shaft rotating). Click the Lab Book to open it. In the
thermometer window click Save to begin recording data. Allow 20-30 seconds to obtain a baseline
temperature of the HCl solution. Drag the weigh paper containing the MgO sample over to the
calorimeter and drop the sample in. Observe the change in temperature until it reaches a maximum
and then record data for an additional 20-30 seconds. Click Stop in the temperature window. (You
can click on the clock on the wall labeled Accelerate to accelerate the time in the laboratory.) A blue
data link will appear in the lab book. Click the blue data link and record the temperature before
adding the MgO and the highest temperature after adding the MgO in the data table. (Remember that
the water will begin to cool down after reaching the equilibrium temperature.)
Data Table
NaOH/HCl
Mass MgO
initial temperature (°C)
final temperature (°C)
4. Is the observed reaction endothermic or exothermic? What will be the sign of ∆Hrxn?
5. Calculate the change in temperature, ∆T. Record your results in the results table on the following
page.
6. Calculate the mass of the reaction mixture in the calorimeter. (To do this, assume that the density of
the HCl solution originally in the calorimeter can be approximated with the density of water (1.0
g/mL). Record your results in the results table.
7. Calculate the total heat released in the reaction, in J, assuming that the specific heat capacity of the
solution is the same as that of pure water, 4.184 J/K⋅g. Record the result in the results table.
Remember: heat of reaction = m × C × ∆T
73
Chapter 5
8. Calculate the number of moles of MgO used in the reaction. The molar mass of MgO is 56.305
g/mole. Record the results in the results table.
9. Calculate ∆Hrxn, in kJ/mol, of MgO for the reaction and record the results in the results table. Make
sure the sign of ∆Hrxn is correct.
Results Table
Mass of Rxn
Mixture
∆T
Total Heat Released
74
mol MgO
∆Hrxn/mol