EXPERIMENT 1 : CALORIMETER HESS’S LAW INTRODUCTION The purpose of the experiment was to prove Hess’ Law through the use of calorimeter and to determine the standard enthalpy of formation of magnesium oxide through the use of the reactants magnesium, magnesium oxide and hydrochloric acid. Calorimeter is a deviced used to measure the enthalpy change of a chemical reaction. In the experiment, the enthalpy was measured from the specific heat or heat capacity of a substance. Hess’ Law states that the value of enthalpy change in any reaction can be written in steps which equal the sum of the values for each of these individual steps. This essentially means that the enthalpy change of any reaction can be determined using a series of other reaction containing the same products and reactants To prove Hess’ Law, the enthalpy change was determined for reactions of hydrochloric acid with magnesium and magnesium oxide respectively. In order to do this, several steps were followed. The basis of the experiment’s procedure was the combination of hydrochloric acid and magnesium in a calorimeter. The enthalpy change was then recorded. This process was repeated for the reactants hydrochloric acid and magnesium oxide. These reactions are shown by the following equations: MgO + 2HCl → MgCl2 + H2O Mg + 2HCl → H2 + ½ O2
MgCl2+ H2
→ H2O
The changes in temperature were recorded. The enthalpy change was then determined using the formula: ΔH = mcΔT n OBJECTIVE 1. To determine the heat capacity of a calorimeter. 2. To determine the standard enthalpy of formation of magnesium oxide, ∆H°f. CHEMICALS 1. 2.0 M Hydrochloric Acid (HCL) 2. Magnesium Oxide powder 3. Magnesium powder
APPARATUS 1. 2. 3. 4.
Thermometer (2) Polystyrene cup with cover 100 mL beaker 100 mL graduated cylinder
RESULTS A) Heat Capacity of Calorimeter Temperature of hot water: 65°C Temperature of cold water (before mixed with hot water) Time (Minute)
Temperature (°C)
1.00 2.00
24 24
3.00
24
4.00
24
Temperature of Cold + Hot water (after mixed together) Time Minute
Seconds
Temperature (°C)
1st Minute
15.00
42
30.00
42
45.00
42
60.00
42
15.00
42
30.00
42
45.00
42
60.00
41
15.00
41
30.00
41
45.00
41
60.00
41
2nd Minute
3rd Minute
B) Reaction 2: Magnesium with Hydrochloric Acid Mass of Magnesium powder: 1.08 g Temperature of Hydrochloric Acid Magnesium powder, Mg)
(HCL)
(before
Time (Minute)
Temperature (°C)
1.00 2.00
25 25
3.00 4.00
25 25
Temperature of HCL + Mg (after mixed together) Time
Temperature (°C)
Minute
Seconds
1st Minute
15.00
74
30.00
74
45.00
74
60.00
73
15.00
73
30.00
73
45.00
72
60.00
71
15.00
71
30.00
70
45.00
70
60.00
69
2nd Minute
3rd Minute
mixed
with
C) Reaction 3: Magnesium Oxide with Hydrochloric Acid Mass of Magnesium Oxide: 1.70 g Temperature of Hydrochloric Acid Magnesium Oxide powder, MgO)
HCL
(before
Time (Minute)
Temperature (°C)
1.00
26
2.00
26
3.00
26
4.00
26
Temperature of HCL + MgO (after mixed together) Time
Temperature (°C)
Minute
Seconds
1st Minute
15.00
50
30.00
51
45.00
52
60.00
52
15.00
52
30.00
53
45.00
53
60.00
52
15.00
52
30.00
51
45.00
51
60.00
51
2nd Minute
3rd Minute
CALCULATIONS
mixed
with
a) Tabulate all your results Have stated in the results above b) Make necessary temperature time plots to obtain the corrected temperature change. Extrapolate the straight line portions of the graph to the fifth minute. The difference between the temperatures at the fifth minutes corresponds to the corrected temperature change Refer to graph of reaction 1, 2 and 3. c) Calculate the heat capacity of the calorimeter
d) Calculate the heat reactions 2 and 3 in kJ/mol. You will need to find out the moles of the reactants involved in the reactions.
e) Show how reaction 2, 3 and 4 can be added to produce reaction 1
f) Show how the heat of reaction (kJ/mol) value for the reactions 2,3 and 4 can be added to yield the heat of reaction for reaction 1
g) Consult the literate to obtain the accepted value for the heat of formation of magnesium oxide. Calculate the percent error and discuss the factors that may have affected your results. The accepted value based on literature for the heat of reaction of magnesium oxide is-601.8 kJ/mol
% error = Theoretical – Actual Theoretical
x 100
DISCUSSION Through the experiments, we have calculated the value of heat capacity of the calorimeter by measuring the temperature change when equal amount of hot water and cold water was added to the calorimeter which results 58.0556 J/0C. The heat of reaction 2 between Magnesium with Hydrochloric
Acid
was
also
calculated
(91.5499
kJ/mol)
and
the
temperature in this reaction was decreases showing that the heat was absorbed which is endothermic reaction had occur. Besides, reaction 3 between Magnesium Oxide and Hydrochloric Acid show temperature rises
indicating the heat was released and exothermic reaction had occur. The heat reaction of reaction 3 calculated is -49.9375 kJ/mol. Hess’s Law have proved that the value of enthalpy change in any reaction can be written in steps which equal the sum of the values for each of these individual steps where total of heat of reaction in reaction 2, 3 and 4 can be added to obtain heat of reaction 1 (-244.1876 kJ/mol). By referring to the literature studied, the heat of reaction of magnesium oxide is -601.8 kJ/mol where a result shows 59.42% of percent error was calculated from our result. The higher percentage error obtained was maybe due to several error such as the reactant such as magnesium and magnesium oxide powder was not completely transferred as some of them was still stick in the bottom of the beaker and may also escaped into the air in the form of dust. Besides, some of heat may have transferred to the air in the Styrofoam cup hence not all the energy was released and results in higher percentage error in experimental results. The Styrofoam cup may also have bigger holes for thermometer reading as the heat can be escaped through the big spaces during measuring the temperature. To minimize these errors various precautions should be taken. For example, the container should be completely dried to ensure all the magnesium and magnesium oxide powder was completely transferred to the HCl solution and three consecutive of weighing samples should be carried out to eliminate the errors of the handlers. Besides, use better equipment such as digital thermometer and true calorimeter. Because heat is more likely to escape through the system in Styrofoam cup than a true calorimeter.
QUESTIONS
1. For the following problems, assume that the volume of the final solution is 200 cm3, the density of the solution is 1.00 g mL -1 and the heat capacity of the solution is the same as water (4.184 J g-1 0C). a) When 0.800 g of Ca metal is added to 200 cm 3 of 0.500 M HCL (aq) according to the method described in Procedure b, a temperature increase of 13.0 0C is observed. What is ∆ H at room temperature rxn
+
for the reaction of Ca(s) + 2H (aq)?
b) When 1.12 g of CaO is added to 200 cm3 of 0.500 M HCl (aq) according to the method described in Procedure b, a temperature
increase of 4.62 oC is observed. for the reaction of CaO (s) + 2H+ ?
What is
∆ H at room temperature rxn
2. A students carelessly inserts the thermometer while assembling the calorimeter and a large hole is torn in the plastic lid. How will this affect his experimental results? His experimental results will be inaccurate as the heat was escaped through the large hole in the plastic lid hence increasing the percent of error during conducting the experiment. The heat produce from the reaction will not being contained in the Styrofoam cup and measured by the thermometer as the heat was loss to the surrounding through the large hole.
CONCLUSION The heat capacity of a calorimeter is 58.0556 J/ 0C. Meanwhile, the standard enthalpy of formation of magnesium oxide, ∆ H0f is – 244.1876 kJ/mol.
REFERENCE (2013), Table of Heats of Formation, Retrieved from http://chemistry.about.com/od/thermodynamics/a/Heats-Of-Formation.htm (2004), Calorimetry 1: The Enthalpy of Formation of MgO, Chemistry Lab : Darkmouth College, Retrieved from http://www.dartmouth.edu/~chemlab/chem3-5/calor1/full_text/writeup.html Churala , (2013) , CALORIMETRY AND HESS’S LAW, Retrieved from http://www.scribd.com/doc/236674646/Calorimetry-and-Hess-Law L. Weinert (2013), Hess’ Law, Retrieved 2013, 16 November from http://www.scribd.com/doc/193515604/Hess-Law-Lab P. Sharifi, (n.d), Determining the Validity of Hess’ Law through Calorimetry, Retrieved from http://www.scribd.com/doc/109774297/Hess-s-Law-Lab R. Lepage, (n.d), Calorimeter, http://www.answers.com/Q/A
Retrieved
2013,
2
August
from
DISCUSSION Through the experiments, we have calculated the value of heat capacity of the calorimeter by measuring the temperature change when equal amount of hot water and cold water was added to the calorimeter which results 58.0556 J/0C. The heat of reaction 2 between Magnesium with Hydrochloric
Acid
was
also
calculated
(91.5499
kJ/mol)
and
the
temperature in this reaction was decreases showing that the heat was absorbed which is endothermic reaction had occur. Besides, reaction 3 between Magnesium Oxide and Hydrochloric Acid show temperature rises indicating the heat was released and exothermic reaction had occur. The heat reaction of reaction 3 calculated is -49.9375 kJ/mol. Hess’s Law have proved that the value of enthalpy change in any reaction can be written in steps which equal the sum of the values for each of these individual steps where total of heat of reaction in reaction 2, 3 and 4 can be added to obtain heat of reaction 1 (-244.1876 kJ/mol). By referring to the literature studied, the heat of reaction of magnesium oxide is -601.8 kJ/mol where a result shows 59.42% of percent error was calculated from our result. The higher percentage error obtained was maybe due to several error such as the reactant such as magnesium and magnesium oxide powder was not completely transferred as some of them was still stick in the bottom of the beaker and may also escaped into the air in the form of dust. Besides, some of heat may have transferred to the air in the Styrofoam cup hence not all the energy was released and results in higher percentage error in experimental results. The Styrofoam cup may also have bigger holes for thermometer reading as the heat can be escaped through the big spaces during measuring the temperature.
To minimize these errors various precautions should be taken. For example, the container should be completely dried to ensure all the magnesium and magnesium oxide powder was completely transferred to the HCl solution and three consecutive of weighing samples should be carried out to eliminate the errors of the handlers. Besides, use better equipment such as digital thermometer and true calorimeter. Because heat is more likely to escape through the system in Styrofoam cup than a true calorimeter.
