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Experiment 7: Enthalpy of Formation of Magnesium Oxide
Objective: In this experiment, a simple calorimeter will be constructed and calibrated,
and Hess' law of constant heat summation will be used to determine the enthalpy of
formation of magnesium oxide, MgO
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However, the heat of reaction can be obtained indirectly by measuring the
enthalpies of other reactions and using Hess' law
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Virtually all chemical processes involve energy transfer of one type or another, and the
branch of chemistry that explores this transfer is called thermochemistry
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In accordance with Hess' Law, enthalpy changes for chemical reactions are additive
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When applying Hess' law, it is important to establish a convention for the signs, positive
or negative, of the enthalpy changes
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An endothermic reaction is one which absorbs

energy and is assigned a positive value
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For the combustion of magnesium, reaction (1) [previous page], one possible set of
reactions is:
(2)

Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g)

∆H°rxn(2) = negative number

(3)

MgO(s) + 2 H+(aq) → Mg2+(aq) + H2O (l)

∆H°rxn(3) = negative number

(4)

1/2 O2 (g) + H2(g) → H2O (l)

∆H°rxn(4) = negative number

Reaction (1) is equal to reaction (2) minus reaction (3) plus reaction (4):
∆H°rxn(1) = ∆H°rxn(2) – ∆H°rxn(3) + ∆H°rxn(4)
The enthalpy of formation of liquid H2O has been measured and is given by:
∆H°rxn(4) = ∆H°f (H2O) = –285840 Joules/mole = –285
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Heat is released by the
addition of excess strong acid to Mg, thus the quantity ∆H°rxn(2) will be a negative
number
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Measuring the Heat Capacity of a Calorimeter
A calorimeter is a reaction vessel that is well insulated in order to prevent heat transfer
between the reaction and its surroundings
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Thus, all heat effects that
are observed in the calorimeter should be the result only of the ∆H° value
of the reaction or process occurring in the calorimeter
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The reactions you are observing today are both exothermic, so the system is
losing heat and the water and calorimeter are gaining heat
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Therefore the
∆Hsystem = - (swater x mwater x ∆Twater + Ccal x ∆Tcal)

This is just like what you did last week
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The same Ccal from last week will be used this week
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0mL of HCl
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The ΔHorxn is then calculated by ∆Ho = - (swater x mwater x ∆Twater + Ccal x ∆Tcal)
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The ΔHo needs to be expressed as a per mole
quantity however
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ΔHorxn/mole = = - (swater x mwater x ∆Twater + Ccal x ∆Tcal)/moles of reactant (Mg or MgO
solid)
Measuring ΔT
Instead of a thermometer, a temperature probe interfaced to a computer will be used to
acquire the temperature readings in this experiment
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Figure 1 below shows a temperature vs
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After the initial sharp rise in temperature when the
reaction is begun (at time of mixing, t = 0 seconds), the temperature begins to decrease
due to heat loss from the calorimeter
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Extrapolation of this linear portion of the curve back to t = 0 seconds gives an accurate
value for the maximum temperature attained, Tf, if equilibrium were to take place
instantaneously with no thermometer lag
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Open the program on
the Mac and, if the Temperature probe is properly connected, you will see a screen
labeled Temperature vs
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The program is now ready for data acquisition
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When you are
finished collecting a data set, click stop (red square)
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0 mL of 1 M HCl and 0
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0 mL of 1 M HCl and
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Run each experiment 3 times
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You are going to perform
the heat of reaction with Mg three times and the heat of reaction with MgO three times
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Print a copy of your graphs of temperature vs time for each trial to include in your lab
writeup
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ΔHorxn/mole = - (swater x mwater x ∆Twater + Ccal x ∆Tcal)/moles of reactant (Mg or MgO
solid)
Don’t average the change in temperature (ΔTcal
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Do 6 separate calculations
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Show all calculations and clearly indicate your answers
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Determine an average value of ΔHorxn/mole for Mg and MgO from your three trials for
each
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Using the answer from question 2 along with the enthalpy of formation for water
(given earlier in the lab) use Hess’s law to calculate the enthalpy of formation for
magnesium oxide (see equations 1-4 on pages 1 and 2)
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Compare your value of the enthalpy of formation of magnesium oxide to the value in
your text
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Show all your calculations and clearly indicate your answer
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Discussion
The discussion is a qualitative and quantitative description of the experiment
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For each
value where you determined a percent error indicate a possible source of error (indicate if
it was random or systematic) and describe exactly how it affected your data
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Post Lab Question
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