CHM431
PHYSICAL CHEMISTRY
EXP 1: CALORIMETRY: HESS’S LAW Name Student’s ID Lab partner
: Nurul Iman Amani Bt Che Awang : 201417996 : Siti Farah Wahida Ismail (2014773321)
Lecturer
: Mr Mohd Sufri Bin Mastuli
Date of Experiment: 2th October 2017 Date of submission: 23rd October 2017
PHYSICAL CHEMISTRY 2017 Calorimetry: Hess’s Law
Objective: A) To compare the heat capacities of a coffee cup and a copper calorimeter. B) To determine the standard enthalpy of formation of magnesium oxide, ∆𝐻° f
Introduction: According to Hess’s Law, the heat of reaction for a given reaction (in KJ mol -1) can be determined by using the values of heat of reaction for other reactions.One of the reactions can be expressed as the combination of the other two reactions. Therefore, the heat of reaction of the one reaction should be equal to the sum of the heats of reaction for the other two.
In the present experiment, the enthalpy of formation of Magnesium Oxide will be calculated from the algebraic sums of the following reactions. The algebraic sum of the following reaction. Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g)
(2)
MgO(s) + 2HCl(aq)
MgCl2 (aq) + H2O (l)
(3)
H2(g) + ½ O2(g)
H2O(l)
(4)
_________________________________________________________________________ Mg(s) + ½ O2(g)
MgO(s)
(1)
PHYSICAL CHEMISTRY 2017 The heat capacity of calorimeter can be obtained by measuring the temperature change.
qhot = qcold + qcal qhot = mhc (Th – Tf) = heat lost by hot water qcold = mcc (Tf – Tc) = heat gained by cold water qcal = Ccal (Tf – Tc) = heat gained by calorimeter
Where c = specific heat for water = 4.18 Jg-1 °C Ccal = heat capacity of the calorimeter Th = temperature of hot water Tc = temperature of cold water Tf = final temperature after mixing m = mass of water (cold or hot)
Apparatus: 1. Thermometer 2. Styrofoam cup with cover 3. 100cm3 beaker 4. 100cm3 graduated cylinder
Chemicals: 1. 2.0M hydrochloric acid 2. Magnesium oxide powder 3. Magnesium powder
PHYSICAL CHEMISTRY 2017 Method: A. Reaction 1: Heat Capacity of Calorimeter 1. 50cm3 of tap water is transferred into a calorimeter by using a burette. The thermometer and cover is replaced on the calorimeter. The water temperature is recorded for four minutes at one minute intervals. 2. 50cm3 of hot water is measured by using a graduated cylinder and poured into a beaker. The water temperature is recorded and poured completely into a calorimeter at the fifth minute. The lid is replaced and the thermometer is used to stir the water. The temperature is recorded every 15 seconds for the next three minutes. 3. Step 1 and 2 is repeated using the copper calorimeter. (cannot be done due to insufficient equipment)
B. Reaction 2: Magnesium and Hydrochloric Acid 1. 1.1g of magnesium powder is weighed and recorded. 2. 50cm3 of 2M HCl is drained from a burette into the calorimeter. The thermometer and cover is replaced. Temperature of HCl is recorded every minute for four minutes. Magnesium powder is poured into the HCl at the fifth minutes. The lid is replaced and thermometer is used to stir the water. The temperature is recorded for the next three minutes at 15 seconds intervals.
C. Reaction 3: Magnesium Oxide and Hydrochloric Acid 1. 1.8g of magnesium oxide is weighed and recorded. 2. 50cm3 of 2M HCl is drained from a burette into the calorimeter. The thermometer and cover is replaced. The temperature of HCl is recorded every minute for four minutes. Magnesium oxider is poured into the HCl at the fifth minutes. The lid is replaced and thermometer is used to stir the water. The temperature is recorded for the next three minutes at 15 seconds intervals.
PHYSICAL CHEMISTRY 2017 DATASHEET EXPERIMENT 1 CALORIMETRY: HESS’S LAW Temperature of hot water (°C)
58
Mass of Mg (g)
1.0756
Mass of MgO (g)
1.7273
Calorimeter (Styrofoam) Time (s)
T (°C)
0 min
22
1 min
22
2 min
22
3 min
22
4 min
22
15
39
30
39
45
39
60
39
75
39
90
39
105
39
120
39
135
38
150
38
165
38
180
38
PHYSICAL CHEMISTRY 2017 Mg & HCl
MgO & HCl
Time (s)
T (°C)
Time (s)
T (°C)
0 min
22
0 min
22
1 min
22
1 min
22
2 min
22
2 min
22
3 min
22
3 min
22
4 min
22
4 min
22
15
37
15
23
30
41
30
23
45
48
45
23
60
75
60
75
76
75
23.2 23.5
90
78
90
24
105
78
105
24
120
81
120
24.2
135
81
135
24.5
150
81.8
150
24.5
165
81
165
180
81
180
24.5 24.9
PHYSICAL CHEMISTRY 2017
Discussion: 1. Result has been recorded in previous page. 2. The graph of the reaction is plotted as below:
Extrapolation of temperature vs time for an exothermic process 90
80
70
Temperature (°C)
60
50 Cofee cup calorimeter Mg and HCl
40
MgO and HCl 30
20
10
0 1
2
3
4
5 Time (min)
6
7
8
PHYSICAL CHEMISTRY 2017 3. Calculation of heat capacity of both calorimeter. Heat capacity for coffee cup calorimeter qhot = qcold + qcal qhot = 50g X 4.18 J/g●°C X (58.0-39.0) °C = 3,971 J qcold = 50g X 4.18 J/g●°C X (39.0-22.0) °C = 4,089 J 3,971 = 4,089 + qcal qcal = -118 J qcal = C∆T -118 J = C (39.0-22.0) °C C = -6.941 J/°C Based on the experiment, it is assumed that the coffee cup calorimeter is a perfect insulator. In other words, it is assumed that there is no heat is lost to the surrounding neither to the cup itself, and the water gains all the heat lost by the reaction. From the calculation, the heat capacity of coffee cup calorimeter is -6.941 J/°C.
PHYSICAL CHEMISTRY 2017 4. Heat of reaction of reaction 2 Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Mol of Mg = mass / molar mass = 1.0756 / 24.31 = 0.0442 mol
1 mol Mg → 1 mol of MgCl2 0.0442 mol → 0.0442 mol
0.0442 mol is the limiting reactant. q = m C∆T q = (1.0995) (4.18) (37-22) = 68.94 J ∆ Hrxn = q / n ∆ Hrxn = 68.94 J / 0.0442 mol = -0.156 kJ / mol
Mol of HCL = mass / molar mass = 50 / 36.5 = 1.3699 mol
2 mol of HCl → 1 mol of MgCl2 1.3699 mol → 0.6850 mol
PHYSICAL CHEMISTRY 2017 Heat of reaction of reaction 3 MgO(s) + 2 HCl(aq) → MgCl2 + H2O Mol of MgO = mass / molar mass = 1.7273 / 40.31 = 0.0429 mol
1 mol Mg → 1 mol of MgCl2 0.0429 mol → 0.0429 mol
Mol of HCL = mass / molar mass = 50 / 36.5 = 1.3699 mol
2 mol of HCl → 1 mol of MgCl2 1.3699 mol → 0.6849 mol
0.0429 mol is the limiting reactant. Q = m C∆T q = (1.7273) (4.18) (23 –22) = 7.220 J ∆ Hrxn = q / n ∆ Hrxn = 7.220 J / 0.0429 mol = -0.168 kJ / mol The amount of heat reaction for reaction 2 is -0.156 kJ / mol while the amount of heat reaction of reaction 3 is -0.168 kJ / mol
PHYSICAL CHEMISTRY 2017 5. Mg(s) + ½ O2(g) → MgO(s)
(1)
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(2)
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
(3)
H2(g) + ½ O2(g)→ H2O(l)
(4)
Mg(s) + ½ O2(g) → MgO(s) 6. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
∆H = -0.156 kJ/mol
MgCl2 (aq) + H2O(l) → MgO(s) + 2HCl(aq)
∆H = +0.168 kJ/mol
H2(g) + ½ O2(g)→ H2O(l)
∆H = -285 kJ/mol
∆H = (-0.156) + (+0.168) + (-285) ∆H = -284.98 kJ/mol According to Hess’s Law, the enthalpy in any given reaction, is the sum of the enthalpies of the sub reactions. Thus, we can produce chemical equation on the formation of Magnesium Oxide by referring to the chemical equations for reaction 2,3 and 4 of the experiment.
7. Percentage Yield
Percent Yield =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑
×100%
= -284.98 kJ/mol / - 601.8 kJ/mol X 100% = 47.36 % Percent error = 100% – 47.36% = 52.64%
PHYSICAL CHEMISTRY 2017 Errors In conducting the experiment, there are some possible error may occur and affect the result; The instrument used such as Styrofoam cup, beaker and graduated cylinder are not cleaned enough and dried properly which can affect the temperature of the reaction. The stirring process of the mixture is been done by using thermometer instead of a glass rod. Measurements that are taken may also not truly accurate due to analytical balance round off and thermometer round off. Moreover, the mixture may not perfectly mix and there might be fingerprint left on the wall of the beakers that will contribute to inaccuracy in data reading.
Besides, during experiment, there are some chemical changes are not being observed. a) Whether the gas evolved are presented or not b) Whether there are exothermic reaction and endothermic reaction occur.
Safety Precaution that need to be highlighted throughout the experiment. Chemical such as Hydrochloric Acid (HCl) is very dangerous chemical and corrosive. While Magnesium Oxide is dangerous chemical when we inhale it. Thus, it is important to protect our body by wearing proper attire, goggles and gloves when handling the chemicals. Other than that, chemical waste must be disposed in a disposable container and make sure it is been kept in the fume chamber.
Suggestions to improve the accuracy of result:
When conducting the experiment, there is some factor that can be considered to improve the accuracy of results and reduce the percentage error. Each procedure must be repeated at least twice for a better result. Multiple trials can be used to detect the incorrect data by using standard deviation and graphing analysis to find an agreeing result. Instead of using the glass rod to stir, we can also use the magnetic stirrer to ensure that the chemicals are mixed thoroughly. The vernier temperature probe also can be used to ensure the temperature is recorded more accurately.
PHYSICAL CHEMISTRY 2017 Conclusion According to Hess’s Law, The enthalpy in any given reaction, is the sum of the enthalpies of the sub reactions. The calorimeter was used to determine the enthalpy changes in different reactions, relating their transferred heat with the change in temperature in solutions. The enthalpies of the formation of Magnesium Oxide is been calculated by using thermochemical equations and Hess’s Law. In conclusion, the heat capacity of coffee cup calorimeter is -6.941 J/°C and the enthalpy change value of formation MgO is -284.98 kJ/mol. There was an occurrence of percentage error of about 52.64% due to improper techniques during conducting the experiments.
PHYSICAL CHEMISTRY 2017 Question 1. b. Calorimeter is a device that have chemical or physical reaction take place. Heat will enter into the calorimeter from the surrounding. Besides, it is also leave the calorimeter into the surrounding. This is because any heat liberated by the reaction must be pick up by the calorimeter. The thermometer is used to measure the changes of the temperature of the chemical reaction. 2. a. qreaction = -(qcal + qsol) = - (C∆T + m C∆T ) = - [(4.18J/g●°C)(13.0°C) + 0.800g X 4.18J/g●°C X 13.0°C)] = - 97.81 J b. qreaction = -(qcal + qsol) = - (C∆T + m C∆T ) = - [(4.18J/g●°C)(4.62°C) + 1.120g X 4.18J/g●°C X 4.62°C)] = - 40.94 J 3. The temperature will decrease and not accurate. This is because the coffee cup cannot function as a heat insulator anymore and the heat loss to the surrounding will increase.
PHYSICAL CHEMISTRY 2017 References
Chang, R & Cruickshank, B. (2005). Chemistry Eighth Edition. McGraw-Hill International Edition. Helmenstine, A. M. (2017). Heat of Formation Table for Common Compounds. Retrieved from https://www.thoughtco.com/common-compound-heat-of-formation-table609253 on 17 th October 2017. Hess's Law. (n.d.). Retrieved from https://scilearn.sydney.edu.au/chemcal/S1GenChem/CalorimetrySyd/HessLaw.html on 17 th October 2017.
