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Hess Lab

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SCH 4U – U3 Hess’s Law Lab

(refer to Investigation 5.4.1 “Hess’s Law”, Nelson Chemistry 12, P. 351-352)


You will use calorimetry and Hess’s Law to determine the molar enthalpy of combustion, ΔHc, of magnesium metal (Mg):

Mg(s) + ½ O2 ( MgO(s)

In open air, this combustion occurs too rapidly so that a calorimetric determination in the student lab would be ineffective (and impossible, considering flames!). However, a different pathway can be constructed using 3 different equations, and the ΔHc (Mg) can be calculated using Hess’s Law. Two of these reactions can be conducted in a student calorimeter to obtain first their q values, and then converted to ΔH using the relationship n ΔH = -q. This last relationship can also be written as ΔH = -q/n, and can be used only if adiabatic conditions are assumed.

Lab Preparation:

1. Calculate the expected (accepted) values for ΔH1 and ΔH2 using the ΔHfo tables (source: Brady and Holum, 1988), plus the ones below:

Hfo (MgCl2 (aq)) = -801.15 kJ/mol Hfo (HCl (aq)) = -162.2 kJ/mol Hfo (MgCl2 (aq)) = -601.6 kJ/mol

2. Calculate the expected ΔHc (Mg) value using Hess’s Law and the expected values for ΔH1, ΔH2, and ΔH3.

3. Show all of your work in tidy and thorough solutions.


1. New Question to start the lab:

“What is the ΔHc (Mg), determined by using a student calorimeter? How effective is the student calorimeter (in producing results close to the accepted values of ΔHc (Mg), ΔH1, and ΔH2)?”

2. Hypothesis:

The ΔHc (Mg) can be determined with a good degree of certainty, using both calorimetry and Hess’s Law if the following assumptions can be made:

< list 6 here>

3. Prediction:

With a brief introduction, organize all of the expected values in a table.…...

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