Chemlab: Chemistry 3/5


Calorimetry 1: The Enthalpy of Formation of MgO

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Before Lab
Complete the usual prelab procedure and sample calculations or analysis flowchart or outline, along with the prelab problems. Your analysis flowchart should include each of the chemical equations you must sum together to calculate for MgO and the calculation of the heat capacity Ctotal.

The reaction examined first will be

HCl(aq, 0.5 M) + NaOH(aq, 0.5 M)

H2O(l) + NaCl(aq, 0.25 M)(1)

Using the enthalpy of formation data given in Table 1, below, calculate the molar H° for this neutralization reaction. Convert this value to an enthalpy change, H, for the experiment you will perform, remembering that you must consider the moles of each reactant used in the experiment. Note that the sign of H is very important! Using the calculated value of H and the value of (T2 - T1) measured for reaction (1), you will calculate the heat capacity, Ctotal.

The second reaction examined was

Mg(s) + 2 HCl(0.5 M)

MgCl2(~0.025 M in ~0.45 M HCl) + H2(g) (2)

Using the heat capacity calculated above and the value of (T2 - T1) measured for this reaction, you will calculate its enthalpy change (again, take care with the sign of H). Finally you will convert your H to the molar H in units of kJ mol-1. You will also compare your experimental value with that calculated using the enthalpy of formation data given in Table 1. (The table does not have data for MgCl2 in 0.45 M HCl; use the MgCl2(aq) value.)

The final reaction examined was

MgO(s) + 2 HCl(0.500 M)

MgCl2(~0.08 M in ~0.35 M HCl) + H2O(l) (3)

Again use the heat capacity calculated above and the measurements for reaction (3) to calculate the enthalpy change for this reaction. Finally, you will convert your H to the molar H, and compare your experimental value with that calculated using data given in Table 1.

Now you can use the molar H values calculated for reactions (2) and (3) and the data in Table 1, to calculate the enthalpy of formation of magnesium oxide, i.e. the molar H for the reaction

Mg (s) + 1/2 O2 MgO (s).

You will need to use one molar H value for a reaction you did not measure, which appears in Table 1. Be sure that your sample calculations or analysis flowchart includes a summation of the three reactions needed to find for MgO.

In Lab
During the experiment, you will plot temperature vs. time for each reaction observed. These plots will be included in your formal report, along with calculations and results. Before leaving the lab, calculate the following and complete your Results section:

• Ctotal for the calorimeter and products from the HCl + NaOH reaction.

• Observed H and molar H for the Mg + HCl reaction and for the MgO + HCl reaction.

• Molar for MgO.

Uncertainty Analysis
While in the lab, estimate the uncertainty in your determination of the temperature change for each reaction. For the HCl + NaOH reaction, use the uncertainty in T and error propagation to calculate an uncertainty in Ctotal. For the subsequent reactions, use this uncertainty in Ctotal and error propagation to calculate the uncertainty in your H values. Follow the error propagation rules as outlined in the section at the start of this manual called Computing Uncertainty in Laboratory Data and Results. Ask your TA or go to office hours for help if you are not sure how to use error propagation. You could use the whole lab period to start the uncertainty analysis with your partner and get help from your TA.

After Lab
You will prepare a formal laboratory report for this experiment. Include sections 1, 2, 5 and 6 from page 9 of your manual in your report, which should be word-processed. Also see page 15 in your manual for further instructions on writing a formal report. Don't forget a title page and theory section. Your theory section should include background information on enthalpy, enthalpy of formation, Hess' Law of summation, and calorimetry. Results can be shown in table form. Calculations or flow charts do not have to be typed, but can be handwritten, clearly and legibly.

Your discussion should summarize your results, discuss their significance, and relate them to the objectives of the experiment. To evaluate your results, address the following points:

• Compare your results to the literature values given in Table 1. Do the literature values fall within the uncertainty range of your calculated results?

• Discuss contributions to the accuracy and precision of your final results.

• How could the heat of formation of MgO be determined more accurately? Be specific and quantitative in your answer.


Table 1: Enthalpies of Formation for Various Reagents

Reagent (kJ mol-1)
HCl (0.50 M)
(0.25 M)
(aq)
-166.19
-166.52
-167.44
NaOH (0.50 M)
(aq)
-469.07
-469.61
NaCl (0.25 M)
(aq)
-406.77
-407.10
H2O (l) -285.85
MgCl2 (aq) -796.88
MgO (s) -601.83
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