Hess's Law - Heat of Reaction for the Combustion of Magnesium
INTRODUCTION
The heat of reaction, when expressed in ΔHof notation, has the unit kJ/mole of substance
formed. Example:
H2(g) + ½ O2(g) → H2O(l) ΔHof = -285.8 kJ/mole. If the number
of moles of reactants is doubled then: 2H2(g) + O2(g) → 2H2O(l) ΔHo = -571.6 kJ
If we reverse the reaction and start with water as the reactant, the equation becomes:
H2O(l) → H2(g) + ½ O2(g) ΔHo = ?
The decomposition of water is an endothermic process and the energy absorbed in the
decomposition must equal the energy released in the formation of the compound. ΔHo for
the decomposition of water is ΔHo = +285.8 kJ/mole of water.
When a reaction can be expressed as the algebraic sum of a sequence of two or more
other reactions, the heat of reaction ΔHo is the algebraic sum of the heats of these other
reaction.
(ΔHorxn = ΔHo1 + ΔHo2 + ΔHo3 + …) Using the above information and Hess's Law of
additivity, you will be finding the heat of combustion of Mg (s).
Mg(s) + ½ O2(g) → MgO(s) ΔHo = ?
The heat of combustion of Mg will be calculated by the means of an indirect method,
using the following equations:
Mg(s) + 2HCl(aq) → MgC12(aq) + H2(g)
ΔHo1 = ?
MgO(s) + 2HCl(aq) → MgC12(aq) + H2O(l) →
ΔHo2 = ?
H2(g) + ½ O2(g) → H2O(l)
ΔHo3 = ?
Combine these three equations to obtain the equation for the combustion of magnesium.
For the first two reactions measure the HCl solution into a Styrofoam calorimeter. Record
the temperature of the solution and add either MgO or Mg to the solution. Stir the
mixture and record the final temperature.
Procedure
Reaction 1
Obtain 0.50 grams of the solid Magnesium metal. Measure and pour 100.0 mL of 1.0M
HCl into the calorimeter. Record the initial temperature of the solution. Add the
magnesium metal to the solution in the calorimeter, stir and record the final temperature.
CAUTION: DO NOT breathe in the fumes.
Reaction 2
Obtain 1.00 grams of the MgO(s). Add 100.0 mL of 1.00M HC1 to the calorimeter and
record the initial temperature. Pour the solid magnesium oxide into the acid solution, stir
and record the final temperature.
Reaction 3 Information is given above.
MAKING SENSE OF THE DATA
Calculate ΔHo1 per mole of Mg(s).
Calculate ΔHo2 per mole of MgO(s).
Rearrange any of the equations (1,2 or 3) so that their sum yields:
Mg(s) + ½ O2(g) → MgO(s) ΔHco = ____
Include the ΔH so that the heat of combustion of Mg may be determined.