Heat of Reaction and Hess’s Law
Purpose: Determine the heat of reaction for the combustion of magnesium using the individual heats of reaction for the
equations involved in this reaction.
Equations:
Combustion of Magnesium: Mg(s) + ½O2(g) → MgO(s) + heat
The algebraic sum of the heats of reaction for the following three equations can be used to calculate the heat
of reaction for the combustion of magnesium:
Equation A:
Equation B:
Equation C:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
H2(g) + ½O2(g) → H2O(l)
The heats of reaction will be determined experimentally for Equations A and B using the equation:
Δ Hreaction = mass x specific heat x Δtemperature
The heat of reaction will be calculated using the standard heats of formation for Equation C. The reactions will then be
rearranged to calculate the overall heat of reaction for the combustion of magnesium.
Pre-Lab:
1. Rearrange Equations A, B and C to equal the combustion of magnesium reaction.
2. Using the ∆Hf° for each of species included in these reactions, calculate the ∆H for each of the reactions.
3. Using the ∆H values calculated in problem 2, calculate the overall ∆H for the combustion of Mg.
Materials: 1 M HCl, Mg ribbon, MgO, Balance, calorimeter, thermometer, forceps, graduated cylinder, ruler, scissors,
spatula, stirring rod, deionized water and weighing dish
Safety: HCl is toxic by ingestion/inhalation and is corrosive to skin and eyes. Mg metal is a flammable solid. Keep away
from flames. Wear goggles.
Procedure:
Equation A
1. Using a forceps, obtain a 3-4 cm strip of magnesium ribbon.
2. Measure the exact length of each piece of magnesium ribbon to the nearest 0.1 cm.
3. Multiply the length of each piece of Mg ribbon by the conversion factor (1 cm = .01 g) to obtain the mass of each piece
of Mg. Record the mass in the data table.
4. Mass a clean, dry calorimeter to the nearest 0.01 g. Record in the data table.
5. Using a graduated cylinder, add 15 ml of 1M HCl to the calorimeter and measure the combined mass of the
calorimeter and acid. Record in the data table.
6. Measure the initial temperature of the HCl solution to the nearest 0.1 ˚C. Record in the data table.
7. Add the piece of magnesium ribbon to the acid and stir the solution until the magnesium has dissolved and the
temperature of the solution remains constant.
8. Determine the final temperature of the solution to the nearest 0.1 ˚C. Record in the data table.
9. Rinse the contents of the calorimeter down the drain with excess water.
Equation B
1. Mass a clean, dry calorimeter to the nearest 0.01g. Record in the data table.
2. Using the graduated cylinder, add 15 ml of 1M HCl to the calorimeter and measure the combined mass of the
calorimeter and HCl. Record in the data table.
3. Tare a small weighing dish and add about 0.2 g of MgO. Measure the exact mass of MgO to the nearest 0.01 g.
Record in the data table.
4. Using the thermometer, measure the initial temperature of the HCl solution to the nearest 0.1 C. Record in the data
table.
5. Using a spatula, add the MgO to the acid. Stir the reaction mixture until the temperature remains constant for
several five second intervals. Determine the final temperature of the solution to the nearest 0.1 C. Record in the data
table.
6. Pour the reaction mixture down the drain with excess water. Rinse and dry the calorimeter.
Data and Analysis:
Reaction A(Mg + HCl)
Reaction B (MgO + HCl)
Mass of Calorimeter(g)
Mass of Calorimeter +
HCl Solution (g)
Mass of Mg(Reaction A) or
MgO(Reaction B)
Initial Temperature (°C)
Final Temperature (°C)
Conclusion:
1. Calculate the mass of the HCl for both Reactions A and B.
2. Calculate the total mass of the reactants (HCl + Mg or MgO) for Reactions A and B.
3. Calculate the change in temperature for Reactions A and B.
4. Calculate the Heat of Reaction for Reactions A and B. (The specific heat of water is 4.18 J/g C)
5. Convert the mass of Mg and MgO to moles.
6. Calculate the enthalpy change (Heat of Reaction/moles) for Reactions A and B. Because Reactions A and B are both
exothermic, express the enthalpy change as a negative value.
7. Using enthalpies of formation, calculate the Heat of Reaction for Reaction C.
8. When the three equations used to calculate the standard heat of formation have been rearranged, Hess’ Law can be
used to calculate the heat of formation of solid MgO. Calculate this value.
9. Use the actual value for the overall ∆H (calculated in the pre-lab) to determine your percent error.