CHEM 162 Bellevue Community Heat of Combustion of Magnesium Lab Questions Here is the lab question: 1.Calculate the heat (q) released by each reaction. Re

CHEM 162 Bellevue Community Heat of Combustion of Magnesium Lab Questions Here is the lab question:

1.Calculate the heat (q) released by each reaction. Report your final answer, in units of kJ, to the appropriate number of significant figures.

·Evaluating error: Which of these equations represents the law of conservation of energy? How do we define the system and surroundings in this experiment and how does this definition introduce error in our measurement?

Calculate ?H of reaction for reactions 1 and 2. Reminder: ?H of reaction is reported in units of kJ/mol.

Reaction 3 is the formation of liquid water. You can find the enthalpy of formation of water in the NIST database here. Scroll down to “condensed phase thermochemistry data” and find the value for DHf of liquid water. There may be more than one value. Use some rationale to determine which is value to use as the DHf of H2O. The experimental enthalpy of reaction 4 can then be found using Hess’ Law and the enthalpies of reactions 1-3. The accepted value for reaction 4 (Note: DHcombustion of Mg is the same as DHf of MgO) can be found in this entry in the NIST database.

6. Calculate your percent error.

Here are the data for it:

Reaction 1 Reaction 2
T(initial) T(final) mass of calorimeter (g) mass of MgO T(initial) T(final) mass of calorimeter (g) mass of Mg H1 H2 H3 H4 Actual DH4
23.2 31.6 105.629 1.004 23.5 43.8 100.974 0.503 -147.758 -409.321 -285.83 -547.393 -601.83 ONLINE EXPERIMENT 1: HEAT OF COMBUSTION OF MAGNESIUM
Hess’s Law states that when are going from a particular set of reactants to a particular set of products, the
heat of reaction is the same whether the reaction takes place in one step or in a series of steps. In this
experiment, you will use Hess’s Law to determine a heat of reaction that would be difficult to obtain by
direct measurement—the heat of combustion of magnesium turnings. The reaction is represented by the
equation:
(4)
?? MgO(s)
Mg(s) + ½ O2(g) ?
This equation can be obtained by combining equations (1), (2), and (3):
??
(1) MgO(s) + 2 HCl(aq) ?
MgCl2(aq) + H2O(l)
??
(2) Mg(s) + 2 HCl(aq) ?
MgCl2(aq) + H2(g)
(3)
H2(g) + ½ O2(g)
?
?? H2O(l)
For this online lab, you will be using the following video https://www.youtube.com/watch?v=jehXd-sw9Eg
to answer basic procedural questions. Sample data has been assigned to you on the Collaboration space of
Canvas. Use the data set assigned to your group to perform calculations and interpret your results. Please
complete Part I, II and III of this lab exercises. The measurement of reaction enthalpy will be (was)
performed in a Styrofoam cup calorimeter as shown in Figure 1.
Figure 1: “Coffee cup” calorimeter
EXAMPLE PROCEDURE (This is what you would perform if you were on campus)
Reaction 1: MgO and HCl
1. Place a Styrofoam cup into a 250-mL beaker as shown in Figure 1. Use a balance to add 100g of
1.00M HCl into the Styrofoam cup. Be sure to record the actual mass added.
•
CAUTION: Handle the HCl solution with care. It can cause painful burns if it comes in contact with
the skin.
•
Evaluating error: You will need to determine the appropriate measuring devices for all volume and
mass measurements, since it is always the responsibility of the scientist to determine the correct
measuring devices for their procedure. At this point you should be familiar the purpose for each
piece of glassware at your disposal. Be sure to include the measuring device that you used in your
procedure.
•
Evaluating error: This calorimeter is obviously a simple design and will introduce a significant
amount of error into your data. This error will have a very specific effect on your data that you
should be able to recognize and explain.
2. Use a utility clamp and a slit stopper to suspend a temperature probe from a ring stand as shown in
Figure 1. Lower the temperature probe into the solution in the Styrofoam cup.
3. Weigh out about 1.00 g of magnesium oxide, MgO, on a piece of weighing paper.
•
CAUTION: Avoid inhaling magnesium oxide dust.
•
Evaluating error: It is almost impossible to weigh out 1.00 g, that’s why the procedure says “about
1.00g.” You will need to record the actual mass that you weigh out. Deviation of your mass from
1.00g will not introduce error, as long as you record the actual mass. Whether you use 1.05g or
0.92g you should get the same value for ?Hrxn. Why is this true?
Collect to begin data collection. Record the initial temperature of your reaction in triplicate.
4. Click
Space out each reading by about 30 seconds. What is the purpose of recording the temperature in
triplicate?
5. Carefully add the white magnesium oxide powder to the solution. If any powder spills during transfer
record this as an observation in your lab notebook.
6. Use a stirring rod to stir the cup contents until a maximum temperature has been reached and the
temperature starts to drop. Click
Stop
to end data collection.
7. Examine the initial readings in the Table window to determine the initial temperature, t1. To determine
the final temperature, t2, click the Statistics button,
. The maximum temperature is listed in the
statistics box on the graph. Record t1 and t2 in your data table.
8. Discard the solution as directed by your instructor.
Reaction 2: Mg and HCl
Repeat Steps 1-7 using about 0.50 g of magnesium turnings rather than magnesium oxide powder. Be
sure to record the measured mass of the magnesium.
CAUTION: Do not breathe the vapors produced in the reaction!!
Part I. Procedural questions https://www.youtube.com/watch?v=jehXd-sw9Eg
1. At 0:20 the analyst weighs some Mg. What mass of Mg did the analyst record?
a. Did the analyst tare the balance before weighing the Mg(s)?
b. Define what it means to “tare” a balance?
2. At 0:26 the analyst adds 50 mL of HCl to a Styrofoam cup. What measuring device did they use to
deliver the HCl? How many decimal places can we record from this measuring device?
3. At 0:35 the analyst pours 150 mL of water into the Styrofoam cup. What measuring device is used
for this measurement? How many decimal places can we record from this device?
4. Based on what you observed in Part A, approximately what percentage of liquid was spilled or lost
during all of the transfer steps combined?
5. Notice that the analyst is constantly stirring the reaction mixture. How would the analyst use their
data to determine whether or not they stirred sufficiently?
6. Notice that the analyst stirs and observes the temperature of the first reaction for 10 minutes. What
would happen to the data if they only recorded for 1 minute? What would happen to their data if
they recorded for 20 minutes?
7. In this experiment, one obvious source of error is the heat lost to the atmosphere outside of the
calorimeter. However, in using Hess’s Law you will find that this error should negate itself. Why is
that?
8. Describe all of the potential health hazards associated with this lab exercise. What
protections/procedures can you use to reduce the risk of these hazards?
9. Explain why you won’t be able to discard the products of your reactions into the sink or common
trash can?
PART II. DATA AND CALCULATIONS
Found in the Canvas Collaboration for this lab. To find data, click on “collaboration” and find your team
number. The excel spreadsheet will have your raw data and a place for you to enter your name and you
calculated DH values.
The following table is an example of an organized table that might go in your lab notebook. Note that you
should report the ?H in units of kJ/mol not just kJ. Use Hess’ Law to find ?H4.
Rxn #
?H (kJ/mol)
Reaction
1
MgO (s) + 2 HCl (aq)
MgCl 2 (aq) + H2O (l)
2
3
Mg (s) + 2 HCl(aq)
H2 (g) + 1/2 O 2 (g)
MgCl 2 (aq) + H2 (g)
H2O (l)
4
Mg (s) + 1/2 O 2 (g)
MgO (s)
Accepted value for ?H4
1. Calculate the heat (q) released by each reaction. Report your final answer, in units of kJ, to the
appropriate number of significant figures.
2. Some constants to consider
a. Density of 1 M HCl = 1.00 g/mL
b. Mass of calorimeter = mass of 1M HCl + mass of Mg (or MgO)
c. Cp of calorimeter = 4.18 J/g oC
3. Relevant equations:
q = Cp • m • ?T where ?T = Tfinal – Tinitial and qsystem = -qsurroundings
•
Evaluating error: Which of these equations represents the law of conservation of energy? How
do we define the system and surroundings in this experiment and how does this definition
introduce error in our measurement?
4. Calculate ?H of reaction for reactions 1 and 2. Reminder: ?H of reaction is reported in units of kJ/mol.
5. Reaction 3 is the formation of liquid water. You can find the enthalpy of formation of water in the NIST
database here. Scroll down to “condensed phase thermochemistry data” and find the value for ?Hf
of liquid water. There may be more than one value. Use some rationale to determine which is value
to use as the ?Hf of H2O. The experimental enthalpy of reaction 4 can then be found using Hess’ Law
and the enthalpies of reactions 1-3. The accepted value for reaction 4 (Note: ?Hcombustion of Mg is
the same as ?Hf of MgO) can be found in this entry in the NIST database.
6. Calculate your percent error.
% error =
experiment al – accepted
? 100%
accepted
PART III. RESULTS/DISCUSSION:
1. Data presentation. All raw data should be included here, even raw data collected in error.
Measurements should be presented in an organized fashion, tables are the most common format.
Basic charts and graphs can also be included. Insert excel spreadsheet OR show example
calculations for all calculations to be performed.
2. Quantitative analysis. Compare your experimental data for the formation of MgO with the the
value of ?H4 found on the NIST website. Are your experimental values above or below expected
values? This is where appropriately recording significant figures are important since 2 numbers that
differ only in an insignificant digit can be said to be the same number.
3. Error analysis. In scientific investigations we are typically trying to determine whether differences
between experimental and expected values are due to error or real phenomena.
a. Which limitations of your procedure likely caused the observed error?
b. When you perform this experiment again, how could you reduce the error that you
observed?
*Mistakes such as spills and incorrect use of measuring devices are not reportable types of error. If you
spill something, you should observe that and record it in your lab notebook. If you don’t know best practices
for using a piece of equipment you should ask your lab partner or your instructor for instructions before
making the measurement. If any portion of the protocol is unclear then you should ask your lab partner or
instructor before performing the procedural step.
Group Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Member 1
Member 2
3687.797476
Member 3
Reaction 1
T(initial) T(final)
23.2
30.4
22.7
30.1
21.8
28.7
24.6
31.3
22.1
30.4
21.3
30.0
23.3
31.8
22.5
31.6
23.3
29.1
21.3
31.4
23.2
30.4
22.3
31.2
21.7
30.2
23.5
30.8
22.4
31.1
23.6
29.5
22.2
29.2
23.1
29.7
22.4
30.7
23.2
31.6
22.0
30.6
23.2
30.2
21.6
30.7
23.3
30.7
23.6
29.9
21.0
29.2
Reaction 2
mass of calorimeter (g) mass of MgO T(initial) T(final) mass of calorimeter (g)mass of Mg H1
103.682
1.008
22.7
42.8
101.823
0.501 -123.826
101.203
0.998
22.6
44.6
99.150
0.499 -125.467
103.093
1.007
23.9
43.7
99.510
0.501 -118.129
101.885
0.994
20.3
43.2
97.481
0.500 -113.444
101.932
1.002
23.0
44.9
100.875
0.496
-140.07
105.484
0.998
23.6
43.8
101.563
0.490 -155.505
103.158
1.000
22.4
42.4
98.948
0.488 -146.506
102.014
1.001
22.5
41.9
99.006
0.503 -154.731
104.161
1.007
21.8
43.1
98.821
0.499 -100.028
106.801
1.002
22.2
45.0
99.559
0.500 -179.844
98.245
0.994
22.1
42.4
100.402
0.497 -118.635
104.103
0.992
22.5
43.0
102.107
0.500 -155.603
102.526
1.005
23.3
43.8
98.725
0.500 -145.587
101.998
1.025
22.7
44.4
102.711
0.498
-122.71
104.583
1.003
22.3
42.6
99.333
0.495 -151.006
99.699
0.999
23.0
43.7
102.299
0.502 -98.3425
101.480
1.016
21.5
43.1
102.122
0.500 -118.139
104.999
1.008
23.8
45.2
99.882
0.512 -114.606
102.540
0.998
22.2
44.2
99.654
0.506 -141.379
105.629
1.004
23.5
43.8
100.974
0.503 -147.758
102.587
0.992
22.9
43.8
98.466
0.500 -151.241
103.659
1.011
21.8
46.2
99.676
0.501 -120.209
101.001
0.998
22.9
45.0
101.875
0.498 -155.104
102.415
1.008
23.0
45.4
100.123
0.504 -125.705
102.1077295 1.012440862 20.7433 46.353
102.4793127 0.50836779
105.0381357 1.01743617 22.0093 41.764
98.60235611 0.50794886
H2
H3
H4
Actual DH4
-409.819
-285.83 -571.823
-601.83
-439.53
-285.83 -599.892
-601.83
-395.083
-285.83 -562.784
-601.83
-447.915
-285.83
-620.3
-601.83
-446.564
-285.83 -592.323
-601.83
-420.453
-285.83 -550.778
-601.83
-407.022
-285.83 -546.346
-601.83
-382.106
-285.83 -513.205
-601.83
-422.37
-285.83 -608.173
-601.83
-456.133
-285.83 -562.119
-601.83
-412.339
-285.83 -579.534
-601.83
-419.103
-285.83
-549.33
-601.83
-407.893
-285.83 -548.135
-601.83
-449.686
-285.83 -612.805
-601.83
-408.084
-285.83 -542.909
-601.83
-424.131
-285.83 -611.618
-601.83
-441.893
-285.83 -609.583
-601.83
-419.231
-285.83 -590.454
-601.83
-435.809
-285.83 -580.261
-601.83
-409.321
-285.83 -547.393
-601.83
-416.976
-285.83 -551.565
-601.83
-487.865
-285.83 -653.486
-601.83
-453.572
-285.83 -584.298
-601.83
-446.328
-285.83 -606.453
-601.83
-0.04986
-0.00322
-0.06488
0.03069
-0.0158
-0.08483
-0.09219
-0.14726
0.010539
-0.06598
-0.03705
-0.08723
-0.08922
0.018236
-0.0979
0.016264
0.012883
-0.0189
-0.03584
-0.09045
-0.08352
0.085832
-0.02913
0.007682
-552.83

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