Annotated Example of Lab Report. Some details about Cpd1, Cpd2, etc. are not included to
simplify the example. Although error is critical to understanding the strength of every claim, a
discussion about error has been omitted to simplify the example.
I. Summary:
This experiment is called the Reaction of Compound 1 (Cpd1) and Compound 2 (Cpd2)
in Acetonitrile (ACN). The procedure was provided in an InterChemNet handout (accessed on
September 6, 2014). This experiment involves the reaction of compound #1 (Cpd1) and
compound #2 (Cpd2) in the solvent, acetonitrile and looking at the products of the reaction as
summarized below in the equation:
Cpd1 + Cpd2
ACN
→
Products?
The reaction was followed using a UV-vis spectrometer at 385 and 621 nm, which are
the wavelengths of two possible products. We are exploring the possibility that the product of
this reaction (P1) may react with the solvent (ACN) to generate a second product (P2).
II. Evidence Collected from Experiment:
Poor Example:
The results of following the chemical reaction between Cpd1 (1.04 g, 0.23 mol) and
Cpd2 (3.1 g, 0.23 mol) in acetonitrile with time is presented here. Mixing A and B took about a
half a min after which the solution was placed into a spectrometer and monitoring commenced (t
= 0). Data was taken every 30 sec for 450 sec. The data was collected at (time in seconds,
Abs) at 385 nm is presented here: (30, 0.15), (60, 0.25), (90, 0.35, (120, 0.45), (150, 0.33),
(180, 0.18), (210, 0.11), (240, 0.08), (270, 0.07), (300, 0.06), (330, 0.05), (360, 0.04), (390,
0.03), (420, 0.02), (450, 0.00). The data was collected at (time in seconds, Abs) at 621 nm is
presented here: (30, 0.01), (60, 0.01), (90, 0.00, (120, 0.03), (150, 0.05), (180, 0.30), (210,
0.80), (240, 0.95), (270, 1.02), (300, 1.07), (330, 1.07), (360, 1.07), (390, 1.07), (420, 1.07),
(450, 1.07).
Comments: The data taken in lab is listed within a paragraph. Note how hard it is to understand
the relationships from the Abs and Time presented here. No patterns are therefore noticed,
which is critical evidence observed in this experiment. This example would not earn full marks.
See the next section for much better ways to present the data.
Excellent Example:
A clear solution of Cpd1 (1.04 g, 0.23 mol) in ACN was monitored over time and no
changes were observed (500 sec.) in the UV-vis spectra. Similarly, the clear solution of Cpd2
(3.1 g, 0.23 mol) in ACN was monitored and no changes over time (500 sec.) in the UV-vis
spectra were observed. This data was taken but because no changes were found, it is
described here but otherwise not presented.
The chemical reaction between Cpd1 (1.04 g, 0.23 mol) and Cpd2 (3.1 g, 0.23 mol) in
ACN was monitored by UV-vis spectroscopy. Cpd1 and Cpd2 were mixed, and it took about a
half a min to place the solution into a spectrometer when the timer was started (t = 0). Data in
Table 1 and Table 2 were taken at 385 and 621 nm, respectively. After mixing, the initial clear
solution turns yellow (from the absorbance at 385 nm) which after about a min. then turns blue
(from an absorbance at 621 nm). Figure 1 graphs both data sets from Tables 1 and 2. It is
1 clearly seen from Figure 1, that after mixing cpds 1 and 2, an absorbance at 385 nm grows in,
which then disappears over time, forming another absorbance at 621 nm.
Table 1. Absorbance at 385 nm as a function of time for the reaction of A and B. Mixing took
about 30 seconds, and the first measurement was made about 60 seconds after the chemicals
were mixed.
Sec
30
60
90
Abs
385nm
(Yellow)
0.15
0.22
0.35
Sec
120
150
180
Abs
385nm
(Yellow)
0.45
0.33
0.18
Sec
210
240
270
Abs
385nm
(Yellow)
0.11
0.08
0.07
Sec
300
330
360
Abs
385nm
(Yellow)
0.06
0.05
0.04
Abs
385nm
(Yellow)
390
0.03
420
0.02
450
0
Sec
Table 2. Absorbance at 621 nm as a function of time for the reaction of A and B. Mixing took
about 30 seconds, and the first measurement was made about 60 seconds after the chemicals
were mixed.
Sec
30
60
90
Abs
385nm
(Yellow)
0.01
0.01
0.00
Sec
120
150
180
Abs
385nm
(Yellow)
0.03
0.05
0.30
Sec
210
240
270
Abs
385nm
(Yellow)
0.80
0.95
1.02
Sec
300
330
360
Abs
385nm
(Yellow)
1.07
1.07
1.07
Abs
385nm
(Yellow)
390
1.07
420
1.07
450
1.07
Sec
Absorbance, at 385 and 621 nm Absorbance (at 385 and 621 nm) vs. Time (seconds) 1.200 1.000 0.800 385 Yellow Abs 621 Blue Abs 0.600 0.400 0.200 0.000 0 100 200 300 Time, Seconds 400 500 Figure 1. Absorbance as a function of time (seconds) for the reaction between A (1.04 g, 0.23 mol) and B
(3.1 g, 0.23 mol) in acetonitrile. Mixing A and B placing the solution into a spectrometer took about a half
a min when the timer was started (t = 0). The data points from monitoring the Absorbance at 385 nm and
621 nm are shown here. Data was taken every 30 sec. Data taken at 385 nm are shown as black
triangles, while the data at 621 nm are shown as black squares.
2 III. Analysis of Evidence:
Poor Example:
This lab was to mix Cpd1 and Cpd2 and see what happens. Our results indicate that a reaction
takes place and strongly supports the idea that a new compound is generated. We were unsure
what the role of acetonitrile was in the experiment. This obviously could lead to error that could
explain why this was unclear to us. We suggest that having better procedures would be helpful
to guide us to better results.
Excellent Example:
This lab involves mixing equal concentrations of Cpd1 and Cpd2 and monitoring any changes in
the UV-vis spectra with time. According to our handout, the formula Abs = ε (molar absorptivity)
x b (path length) x concentration.
A reference provided in the procedure (Journal of Inorganic General Chemistry Reactions,
Macky, W. and Jones, P., 2003, 621-641) indicates that acetonitrile can react with compounds
similar to Cpd1 and Cpd2. Our data indicates that acetonitrile did not react with either Cpd1 or
Cpd2, thus:
ACN
Cpd1
→
No reaction
(1)
Cpd2
ACN
→
No reaction
(2)
However, upon mixing Cpd1 and Cpd2, color changes and changes in the absorbances at 385
nm and 621 nm were observed. Consistent with this data is that Cpd1 initially reacts with Cpd2
to form a new product, which we identify as P1. The increase in absorbance indicates that the
concentration of P1 initially increases. Thus,
Cpd1 + Cpd2
ACN
→
P1 (yellow; absorbance at 385 nm) (3)
The fact that P1 then disappears fairly quickly (monitored at 385 nm), suggests that it is
reactive. Given that similar compounds can react with acetonitrile, the data is consistent with P1
reacting with acetonitrile to form a new product, P2, that is blue, with an absorbance at 621 nm.
Thus, from the changes in the UV-vis, we see that as the concentration of P1 decreases, the
concentration of P2 increases, suggesting the following reaction occurs:
P1 + ACN
P2 (blue; absorbance at 621 nm) (4)
→
3 IV. Claims:
Poor Example:
Our hypothesis was that when Cpd1 and Cpd2 were mixed together, a reaction would occur.
This is what happened because when mixed together, we found evidence that a reaction took
place. Since our hypothesis was proved, we can make the claim that a reaction occurred.
Excellent Example:
Reaction Scheme 1. Based on our analysis, the reaction scheme shown below summarizes
our claims from the experiment, “Reaction of Compound 1 (Cpd1) and Compound 2 (Cpd2) in
Acetonitrile (ACN)”:
Cpd1
Cpd2
Cpd1 + Cpd2
P1 + ACN
ACN
→
ACN
→
ACN
→
ACN
→
No reaction
(1)
No reaction
(2)
P1 (yellow; absorbance at 385 nm) (3)
P2 (blue; absorbance at 621 nm) (4)
Claim #1: Monitoring solutions by UV-vis spectroscopy we find that neither Cpd1 nor Cpd2
reacts individually in ACN (eq 1 and 2 in the Reaction Scheme above). (Data taken in lab but
not shown because there were no changes observed.)
Claim #2: When Cpd1 and Cpd2 were mixed, data from UV-vis spectroscopy experiments
suggests that a new compound with an absorbance at 385 nm forms over time, which we label
P1 (see eq 3).
Claim #3: We claim that P2 is formed from a reaction of P1 with ACN. This is supported by
changes in the UV-vis spectroscopy and literature that suggest that compounds like these can
sometimes react with ACN.
Optional items:
• If there are other experiments suggested by unanswered questions, you are encouraged to
summarize your thoughts about them.
• All measurements involve error. Although this example did not include error analysis (to
simply the example), attention to error can be important in evaluating your claims.
4