AP Chemistry-Experiment 8 Heat of Reaction
For the Combustion of Magnesium (Hess’s Law)
When a reaction can be expressed as the algebraic sum of a sequence of two or more simpler
reactions, the heat of reaction is the algebraic sum of the heats of these simpler reactions. This
generalization has been found to be true for every reaction tested. This is called Hess's Law.
In this experiment, you will use this generalization to determine the change in enthalpy for a
reaction that is difficult to measure directly. Magnesium burns very rapidly, as you have
observed, in photo flashbulbs, releasing much heat and light. The reaction is represented by the
equation:
(1) Mg (s) + 1/2 O2 (g)  MgO (s)
The equation can be obtained by combining the following equations:
(2) MgO (s) + 2 HCl (aq)  MgCl2 (aq) + HOH (l)
(3) Mg (s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)
(4) H2 (g) + 1/2 O2 (g)  HOH (l)
Before doing the experiment, combine these three equations to obtain the first equation. Heats
of reactions for the second and third equations can easily be experimentally determined. The
change in enthalpy for the last reaction can be obtained from an enthalpy table.
Procedure:
For both these reactions, pour the measured hydrochloric acid solution into a Styrofoam
calorimeter (cup). Record the temperature of the solution and add either MgO or Mg to the
solution. Stir the mixture and record the highest temperature reached during each reaction.
Reaction 2
Measure the change in temperature that occurs as 1.0 gram of magnesium oxide, MgO is
added to 50.0 ml of hydrochloric acid, HCl, solution.
Reaction 3
In millimeters, measure and record the length of the magnesium ribbon. The magnesium
ribbon has a mass of 0.______ g / 1000 mm.
Measure the change in temperature that occurs as this magnesium ribbon is reacted with 50.0 ml
of 1.0 M hydrochloric acid, HCl, solution.
AP Chemistry-Experiment 8 Heat of Reaction
For the Combustion of Magnesium (Hess’s Law)
Processing the Data
1. Combine the equations (2), (3), and (4) to obtain equation (1). Use an enthalpy table to
calculate the theoretical heat of reaction for equation (1).
The enthalpies for MgO(s), HCl (aq), MgCl2(aq), and HOH(l) are respectively
– 601.2 kJ/mole, - 167.5 kJ/mole, - 795.0 kJ/mole, and – 285.8 kJ/mole.
2. Calculate the change in temperature for each reaction, (2) and (3).
3. Calculate the kilojoules, kJ, of energy released for each reaction. Assume that 4.18 joules of
energy is required to change the temperature of 1.0 g (1.0 ml) of solution 1.0 Celsius degree.
4. Calculate the kilojoules/mole, (kJ/mole), of MgO and kJ/mole of Mg.
5. Determine the enthalpy for reaction (1). Report the change in enthalpy in kJ/mole of MgO.
6. Determine the percentage error by using:
Percent Error = (Experimental value - Theoretical value) X 100 %
(Theoretical value)