Hess\'s Law - Heat of Combustion RESULTS AND CALCULATIONS 1. In the additional s
ID: 1045996 • Letter: H
Question
Hess's Law - Heat of Combustion RESULTS AND CALCULATIONS 1. In the additional spaces provided in the data table, calculate the change in temperature, for Reactions 1 and 2. 2. Calculate the heat released by each reaction, g, using the formula 4 = Cp.m.At Cp = 4.18 J/g°C, and m = 100.0 g of HCl solution. Convert joules to KJ in your final answer. 3. Determine the moles of MgO and Mg used. 4. What is the change in enthalpy (in kJ mol-' of Mgo or Mg) for reactions (1) and (2)? 5. What is the change in enthalpy (in kJ mol Mg-l for Reaction 4? (Use your results, your pre- lab work, and AH = -285.8 kJ for Reaction 3). 6. What is your percent error for the answer you obtained in Step 5? The accepted value for this reaction is -601.8 kJ mol''. Comment on your results. 7. Identify any sources of error that would account for your answer. Are they reasonable? Comment on the error and results. 8. Indicate any changes you would make to the procedure. PRE-LABORATORY ASSIGNMENT To be completed before attending lab: 1. Identify all potentially hazardous steps in your procedure. In your own words, explain what safety precautions should be taken and why? 2. Record the standard enthalpy of formation of water vapor. 3. In the space provided below, combine equations (1), (2), and (3) to obtain equation (4). Then provide an equation for calculating the heat of reaction in kJ/mol. (3)Explanation / Answer
Mass of 1 M HCl, m = 100 g
Change in temperature = Final temperature – initial temperature
?T = 27 – 21.3 K = 5.7 K
Cp = 4.18 J/g-K
Q = m Cp ?T
= 100 g * 4.18 J/g-K * 5.7 K
= 2382.6 J
= 2.38 kJ
?H = - Q (Heat balance, reaction is exothermic)
?H, kJ = - 2.38 kJ
Mass of solid = 1.007 g
Molar mass of solid = 40.3 g/mol
Moles of solid = 1.007 / 40.3 = 0.025
?H, kJ/mol = - 2.38 kJ / 0.025 mol
= - 95.4 kJ/mol
Similarly for reaction 2 we get results as shown below:
S No.
Reaction 1
(MgO)
Reaction 2
(Mg)
1
Volume of 1M HCl (mL)
100
100
2
Final temperature, K
27
27
3
Initial temperature, K
21.3
21.3
4
Change in temperature, K
5.7
5.7
5
Mass of solid, g
1.007
0.493
6
Heat q, kJ
2.38
2.38
7
?H, kJ
-2.38
-2.38
8
Molar mass, g/mol
40.3
24.3
9
Moles
0.0250
0.0203
10
?H, kJ/mol
-95.4
-117.5
(1) MgO(s) + 2 HCl(aq) ? MgCl2(aq) + H2O(l); ?H1 = -95.4 kJ/mol
(2) Mg (s) + 2 HCl(aq) ? MgCl2(aq) + H2 (g); ?H2 = -117.5 kJ/mol
(3) H2(g) + 1/2 O2(g) ? H2O(l); ?H3 = -285.8 kJ/mol
(4) Mg(s) + 1/2 O2(g) ? MgO(s); ?H4 = ?
?H4 = ?H2 - ?H1 + ?H3 (Using Hess’s Law)
= -117.5 kJ/mol – (-95.4 kJ/mol) -285.8 kJ/mol
= -307.9 kJ/mol
% error = (Theoretical ?H - Calculated ?H) / Theoretical ?H * 100
= (604.8 kJ/mol – 307.9 kJ/mol)/ 604.8 kJ/mol * 100
= 49%
Sources of error:
1) Heat losses cannot be accounted in the experiment.
2) Constant Cp of water is assumed at all temperatures.
S No.
Reaction 1
(MgO)
Reaction 2
(Mg)
1
Volume of 1M HCl (mL)
100
100
2
Final temperature, K
27
27
3
Initial temperature, K
21.3
21.3
4
Change in temperature, K
5.7
5.7
5
Mass of solid, g
1.007
0.493
6
Heat q, kJ
2.38
2.38
7
?H, kJ
-2.38
-2.38
8
Molar mass, g/mol
40.3
24.3
9
Moles
0.0250
0.0203
10
?H, kJ/mol
-95.4
-117.5