As in the previous experiment, this experiment will investigate Hess’s Law of the transfer of heat, however, this time the calculated value will be the standard heat of formation, Hof. Remember that Hess’s Law states that “if a reaction can be expressed as the sum of two or more reactions, then the heat flow for the overall reaction is equal to the sum of the heat flows.” The standard heat of formation is defined as the heat involved in the formation of one mole of a compound from the corresponding elements as they are found usually at 25oC and 1 atmosphere pressure. To better understand the distinction “elements as they are found at standard conditions”, note the examples in Eq. 1 through Eq. 4.
CH4 (g) + 2 O2 (g) 2 H2O (g) + CO2 (g) Eq. 1
C diamond (s) + O2 (g) CO2 (g) Eq. 2
C graphite (s) + O2 (g) CO2 (g) Eq. 3
H2 (g) + ½ O2 (g) H2O (l) Eq. 4
The first three reactions all form carbon dioxide and can occur at 25oC and 1 atmosphere pressure. However, in equation 1, the starting materials are not all elements since methane is a compound, and two products are formed. Hence equation 1 is excluded from the definition of standard heat of formation. The natural state of carbon is accepted as graphite, not diamond, and thus equation 2 does not fit the definition of standard heat of formation. Finally equation 3 represents “the formation of the compound CO2 from the elements as they are found at standard conditions.” Equation 4 does the same for liquid water.
When magnesium metal is burned in the presence of oxygen, the heat evolved per mole of magnesium is the Hf o for MgO(s), (Eq. 5). This process will be demonstrated for you in the lab. The o indicates standard conditions.
Mg(s) + ½O2(g) MgO(s) Eq. 5
You will observe that it would be difficult to measure the heat evolved without an elaborate apparatus. Therefore we will use Hess’s Law to devise an alternate, practical method to determine the Hf o for MgO(s).
First you will dissolve magnesium metal with zero oxidation state, (Mg0) in a dilute HCl solution. This reaction will form magnesium chloride in solution along with hydrogen gas. The reaction is exothermic and the resulting temperature change can be easily measured in a coffee cup calorimeter. Next we will dissolve magnesium oxide in dilute HCl to form dissolved magnesium ions and water. This reaction also is exothermic and its temperature change easily measured. Finally we will use this information along with a standard heat of formation of liquid water (Eq. 6) to calculate the Hf o for MgO(s) using Hess’s Law.
H2(g) + ½ O2(g) H2O(l) ΔHf o Eq. 6
Procedure: Part 1
Obtain a clean coffee cup calorimeter and add 75-100 mL of 3.0 M HCl solution and record the volume to the nearest 0.1 mL. Place the thermometer in the cup for 2 minutes to record the temperature of the solution. In a weigh boat, measure approximately 0.6g of solid magnesium metal. Record the exact mass. Carefully add the magnesium metal to the calorimeter and place the cover back on the system. Gently swirl the cup to ensure mixing for 3-4 minutes and record the “highest” temperature that is obtained. Discard the liquid into the proper waste container and repeat the process using fresh 3.0 M HCL solution. For trial 2 use approximately 0.8 g of magnesium metal.
Procedure: Part 2
Obtain a clean coffee cup calorimeter and add 75-100 mL of 3.0 M HCl solution and record the volume again to 0.1 mL. Place the thermometer in the cup for 2 minutes to record the temperature of the solution. In a weigh boat, measure approximately 1.5g of solid magnesium oxide and record the exact mass. Carefully add the magnesium oxide to the calorimeter and place the cover back on the system. Gently swirl the cup to ensure mixing for 3-4 minutes and record the “highest” temperature that is obtained, making sure that all the magnesium oxide dissolves. Discard the liquid into the proper waste container and repeat the process using fresh 3.0 M HCL solution. For trial 2 use approximately 2 g of magnesium oxide. Return washed top and bottom of coffee cup calorimeter to the back counter for use by other classes.
Calculations
The equation that will be used to determine the heats of the reactions is:
Use the density of 1.01 g/ml when calculating the mass of the solutions for which you know only the volume. The heat capacity of the solution can be approximated by the value 4.06 J/goC. The standard heat of formation for H2O (l) can be found in Appendix C. When determining the limiting reagent, pay attention to the stoichiometry of the equations.
Pre-Lab
1. What is calorimetry?
2. Write the balanced equation for the reaction of magnesium metal with hydrochloric acid.
3. Write the balanced equation for the reaction of magnesium oxide with hydrochloric acid.
4. Combine the above two equations with that for the formation of liquid water (Eq. 6 in handout) and use addition and/or subtraction to arrive at the equation for the formation of magnesium oxide (Eq. 5 in handout). Show work.
5. What will be some possible sources of error (internal and external) that could lead to a decrease in accuracy?
Trial 1 Trial 2
Volume of 3.0 M HCl______
Mass of Mgo______
Mass of 3.0 M HCl______(calculated)______(calculated)
Total Mass______(calculated)______(calculated)
Mole of HCl______(calculated)______(calculated)
Mole of Mgo______(calculated)______(calculated)
Limiting reagent______
Initial Temperature ______
Final Temperature______
H rxn 1______
Average H rxn 1______
Trial 1 Trial 2
Volume of 3.0 M HCl______
Mass of MgO______
Mass of 3.0 M HCl______(calculated)______(calculated)
Total Mass______(calculated)______(calculated)
Mole of HCl______(calculated)______(calculated)
Mole of MgO______(calculated)______(calculated)
Limiting reagent______
Initial Temperature ______
Final Temperature______
H rxn 2______
Average H rxn 2______
Write the balanced equation for the reaction of magnesium metal with hydrochloric acid.
Write the balanced equation for the reaction of magnesium oxide with hydrochloric acid.
Combine the above two equations with that for the formation of liquid water (Eq. 6) and use addition and/or subtraction to arrive at the equation for the formation of magnesium oxide (Eq. 5). Show work.
Use Hess’s Law and average values for ΔH rxn 1, ΔH rxn 2 and ΔHf o to determine Hf o MgO (show calculation).
Calculated Hf o MgO______
Accepted Hf o MgO______
Percent Error ______
Questions