# Capital Community College Hesss Law Worksheet

Name _____________________________ Date ________________Lab Partner Name(s) _______________________________________
Hess’s Law and the heat of combustion of Mg
In this experiment you will use Hess’s law to measure the H of combustion of Mg. To measure the heat
of combustion directly would be cumbersome. You will measure the H of two reactions that can be
carried out easily in the lab. These values and the known H of formation of water will be used to
calculate the heat of combustion of Mg:
(Reaction 1) Mg(s) + ½ O2(g)
MgO(s)
HRxn1
This equation can be obtained by combining equations :
(Reaction 2) MgO(s) + 2 HCl(aq)
MgCl2(aq) + H2O(l)
HRxn2
(Reaction 3) Mg (s) + 2 HCl(aq)
MgCl2(aq) + H2 (g)
HRxn3
(Reaction 4) H2(g) + ½ O2(g)
H2O(l)
HRxn4
The pre-lab portion of this experiment requires you to combine equations from Reactions 2, 3, and 4 to
obtain the equation for Reaction 1 before you do the experiment. Heats of reaction for equations (2) and
(3) will be determined in this experiment. As you may already know, H for Reaction 4 is -285.8 kJ.
Prelab:
1) Write the definition of Hess’s law.
2) Illustrate that by combining Equations 2, 3 and 4 you are left with Equation 1 above.
3 ) How does Hess’s Law allow you to determine the ∆H for Equation 1 using ∆H’s for Equations 2, 3 and 4?
4) Why is H called a “state function”? Explain.
Procedure
Part 1. Enthalpy change for MgO plus HCl
Set up the “calorimeter” as shown in the following figure:
1) Measure the mass of 100 mL of HCl 1.0 M and place in the Styrofoam
cup (place Styrofoam cup on balance and zero it. Add the HCl in the cup
outside the balance and measure the mass of solution).
2) Place the Styrofoam cup in the beaker for added stability.
3) Measure about 1 g of MgO exactly. Record this mass in data table.
4) Open Experiment 19 in the folder from Chemistry with Computers.
5) Put the temperature probe into the HCl solution and record the initial temperature of the solution.
6) Click COLLECT.
7) Add the MgO to the HCl solution and stir constantly.
8) Record the maximum temperature reached.
Mass of HCl solution
Mass of MgO
Tinitial
Tmax
Part 2. Enthalpy change for Mg plus HCl
1) Measure the mass of 100 mL of HCl 1.0 M and place in the Styrofoam cup,
2) Measure about 0.5 g of Mg exactly. Record the mass in data table below.
3) Put the temperature probe into the HCl solution and record the initial temperature of the solution.
4) Click COLLECT.
5) Add the Mg to the HCl solution and stir constantly.
6) Record the maximum temperature reached (until it begins to drop)
Mass of HCl solution
Mass of Mg
Tinitial
Tmax
Calculations
1) Use the following information to find the H for Reaction 2 (Part 1 of procedure).
qsoln = heat absorbed by solution
qsoln = msoln·Csoln·Tsoln
msoln= mass of solution = mHCl + mMgO
Csoln ≈ CH2O = 4.18 J/g∙°C (an approximation)
Tsoln = Tfinal – Tinitial
qRxn2 = heat given off by reaction
qRxn2 = – qsoln
nMgO = mMgO / MMMgO
H Rxn2 = qRxn2 / nMgO
a) Calculate the Tsoln for Reaction 2:
b) Calculate the msoln for Reaction 2:
c) Calculate the qsoln for Reaction 2:
d) Determine the qRxn2 :
e) Calculate the nMgO:
f) Determine the HRxn2: Express this in kJ/mol.
2) Now you will repeat these calculations to find the H for Reaction 3 (Part 2 of procedure).
**Make sure you use Mg in place of MgO in the calculations.
a) Calculate the Tsoln for Reaction 3:
b) Calculate the msoln for Reaction 3:
c) Calculate the qsoln for Reaction 3:
d) Determine the qRxn3 :
e) Calculate the nMg:
f) Determine the HRxn3: Express this in kJ/mol.
3) Calculate Hrxn for Reaction 1. Start by fill in the your experimental values for H’s to the right of
each thermochemical equation below:
(Reaction 2) MgO (s) + 2 HCl (aq)
MgCl2 (aq) + H2O(l) HRxn2 = ___________
(Reaction 3) Mg (s) + 2 HCl(aq)
MgCl2 (aq) + H2 (g)
HRxn3 = ___________
(Reaction 4) H2 (g) + ½ O2 (g)
H2O (l)
HRxn4 = ___________
Manipulate these equations above to Calculate the H for Reaction 1 (Use Hess’s Law!)
(Reaction 1) Mg (s) + ½ O2(g)
MgO (s)
HRxn1 = ?
Show all work and manipulations:
4) Look up the accepted value for the Hf° for Reaction 1. Calculate a percent error.
accepted value – experimental value
% error =
x 100
accepted value
5) How would your reported H for Reaction #1 have changed (explain) if…
a) Some of the MgO remained in the weigh boat and never made it into Reaction #2.
My reported H value for Reaction #1 would have been ___________________, b/c:
b) Some of the Mg remained in the weigh boat and never made it into Reaction #3.
My reported H value for Reaction #1 would have been ___________________, b/c:
c) You missed the Tmax for the Reaction #2 and recorded a lower temperature instead.
My reported H value for Reaction #1 would have been ___________________, b/c:
d) You missed the Tmax for Reaction #3 and recorded a lower temperature instead.
My reported H value for Reaction #1 would have been ___________________, b/c:

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