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)
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.
Pre-Lab: The heats of reaction will be determined experimentally for Equations A and B using the equation:
Δ Hreaction = mass x specific heat x Δtemperature
(Use the specific heat of water 4.180 J/g˚C)
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.
1. Using the standard heats of formation, calculate the heat of reaction for Equation C.
2. Rearrange the Equations A, B and C to give you the reaction for the combustion of magnesium:
Mg(s) + ½O2(g) → MgO(s)
Procedure:
Equation A
1. Using a forceps, obtain a piece 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 the piece of Mg ribbon by the conversion factor (1 cm = .01 g) to obtain the mass of the Mg.
4. Mass a clean, dry calorimeter to the nearest 0.01 g.
5. Using a graduated cylinder, add 15 ml of 1M HCl to the calorimeter and measure the combined mass of the
calorimeter and acid.
6. Measure the initial temperature of the HCl solution to the nearest 0.1 ˚C.
7. Add the piece of magnesium ribbon to the acid and stir the solution until the magnesium has completely reacted and
the temperature of the solution remains constant.
8. Determine the final temperature of the solution to the nearest 0.1 ˚C.
9. Pour the reaction mixture down the drain with excess water. Rinse and dry the calorimeter.
Equation B
1. Mass a clean, dry calorimeter to the nearest 0.01g.
2. Using the graduated cylinder, add 15 ml of 1M HCl to the calorimeter and measure the combined mass of the
calorimeter and HCl.
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.
4. Using the thermometer, measure the initial temperature of the HCl solution to the nearest 0.1 C.
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.
6. Pour the reaction mixture down the drain with excess water. Rinse and dry the calorimeter.
Heat of Reaction and Hess’s Law
Data and Analysis:
-Copy the following data table into your lab and then transfer your data from your lab to the data table.
Data Table
Reaction A (Mg + HCl)
Reaction B (MgO + HCl)
Mass of Calorimeter
Mass of Calorimeter + HCl solution
Mass of Mg(A) or MgO(B)
Initial Temperature
Final Temperature
Conclusion:
1. Solve for the following for Reaction A:
a. Calculate the mass of HCl used in this lab for Reaction A.
b. Calculate the total mass of reactants (HCl + Mg).
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of Mg used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of Mg).
2. Solve for the following for Reaction B:
a. Calculate the mass of HCl used in this lab for Reaction B.
b. Calculate the total mass of reactants (HCl + Mg) .
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of MgO used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of MgO).
3. Calculate the heat of formation of solid MgO. (Hint: Look at Question #2 in your Pre-lab)
4. The actual value for the heat of formation of solid MgO is -601.6 kJ/mol. Use this value to calculate your percent error.
Heat of Reaction and Hess’s Law
Data and Analysis:
-Copy the following data table into your lab and then transfer your data from your lab to the data table.
Data Table
Reaction A (Mg + HCl)
Reaction B (MgO + HCl)
Mass of Calorimeter
Mass of Calorimeter + HCl solution
Mass of Mg(A) or MgO(B)
Initial Temperature
Final Temperature
Conclusion:
1. Solve for the following for Reaction A:
a. Calculate the mass of HCl used in this lab for Reaction A.
b. Calculate the total mass of reactants (HCl + Mg).
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of Mg used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of Mg) (This is an exothermic reaction so the change is negative.)
2. Solve for the following for Reaction B:
a. Calculate the mass of HCl used in this lab for Reaction B.
b. Calculate the total mass of reactants (HCl + MgO).
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of MgO used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of MgO) (This is an exothermic reaction so the change is negative.)
3. Calculate the heat of formation of solid MgO. (Hint: Look at Question #2 in your Pre-lab)
4. The actual value for the heat of formation of solid MgO is -601.6 kJ/mol. Use this value to calculate your percent error.
Heat of Reaction and Hess’s Law
Purpose, Materials, Safety, Table of Contents, Date
Pre-Lab:
1. Using the standard heats of formation, calculate the heat of reaction for Equation C.
2. Rearrange the Equations A, B and C to give you the reaction for the combustion of magnesium:
Mg(s) + ½O2(g) → MgO(s)
Procedure:
Equation A
1.
2.
3.
4.
5.
6.
7.
8.
9.
Length
Mass
Mass
Mass
Temperature
Temperature
Equation B
1.
2.
3.
4.
5.
6.
Mass
Mass
Mass
Temperature
Temperature
Heat of Reaction and Hess’s Law
Data and Analysis:
-Copy the following data table into your lab and then transfer your data from your lab to the data table.
Data Table
Reaction A (Mg + HCl)
Reaction B (MgO + HCl)
Mass of Calorimeter
Mass of Calorimeter + HCl solution
Mass of Mg(A) or MgO(B)
Initial Temperature
Final Temperature
Conclusion:
1. Solve for the following for Reaction A:
a. Calculate the mass of HCl used in this lab for Reaction A.
b. Calculate the total mass of reactants (HCl + Mg).
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of Mg used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of Mg).
2. Solve for the following for Reaction B:
a. Calculate the mass of HCl used in this lab for Reaction B.
b. Calculate the total mass of reactants (HCl + Mg).
c. Calculate the change in temperature.
d. Calculate the heat of reaction, in Joules, for this reaction. (See Pre-Lab)
e. Convert the mass of MgO used in this lab to moles.
f. Calculate the enthalpy change (Heat of Reaction/Moles of MgO).
3. Calculate the heat of formation of solid MgO. (Hint: Look at Question #2 in your Pre-lab)
4. The actual value for the heat of formation of solid MgO is -601.6 kJ/mol. Use this value to calculate your percent error.