STUDENT ID (YOUR ID)
The Structure of
Acetylene
A Double Challenge
Your name
27/6/2013
Student No. Your ID
Date: Today’s date
The Structure of Acetylene – A double challenge
Aim:
The aim of this experiment is to use spectral data obtained during the
experiment to determine the carbon-carbon bond and the carbonhydrogen bond lengths in acetylene.
Safety information:
Compounds, reagents and
products
Calcium carbide
Acetylene
R and S numbers
TOXIC AND FLAMMABLE
R: 15
S: 8-43c
HIGHLY FLAMMABLE
R: 11
All experimentation is to be carried out in a fumecupboard. Solid waste
should be put in a plastic bag and placed in the non-toxic waste bin.
Aqueous waste should be poured down the sink with an excess of
water. Gas cells should be flushed out with air.
Experimental:
Calcium carbide is reacted with water, using 0.005 moles of calcium
carbide and 100% excess of water. The gas produced during the
reaction is collected in an IR gas cell and the cell analyzed using a
spectrometer. The procedure is then repeated using calcium carbide
and deuterium oxide.
(Switch to present tense as these are instructions? Or make it reported
text, so change the Experimental?)
Research strategy:



Perform the experiment and analyze the two gas cells containing
C2H2 and C2D2 using FTIR spectroscopy;
Obtain spectra and label 10 peaks in the R and P branches;
Use the method of combination differences to find B1 and B0 for
each molecule by plotting 2J’’ + 1 against Δν1, to find B1, and 2J’
+ 1 against Δν0 to find B0;
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Student No. Your ID



Date: Today’s date
Use the value obtained for B to calculate the moment of inertia
for each molecule;
From the moments of inertia of C2H2 and C2D2 the carbon-carbon
and hydrogen/deuterium-carbon bond lengths can be calculated;
Compare experimental values with literature values.
Results for C2H2:
Table 1: Tabulated R branch data for C2H2
Branch
Wavenumber/cm-1
R0
1330.5
R1
1332.7
R2
1335.1
R3
1337.4
R4
1339.8
R5
1342.2
R6
1344.7
R7
1347.0
R8
1349.5
R9
1351.9
R10
1354.3
Table 2: Tabulated P branch data for C2H2
Branch
Wavenumber/cm-1
P1
1325.5
P2
1323.2
P3
1320.9
P4
1318.6
P5
1316.2
P6
1313.9
P7
1311.6
P8
1309.3
P9
1307.0
P10
1304.7
P11
1302.4
See Figures 1 and 2 for spectra.
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Student No. Your ID
Date: Today’s date
Figure 1. FTIR spectrum of acetylene
Figure 2. FTIR spectrum of acetylene – R and P branches
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Student No. Your ID
Date: Today’s date
Data Analysis for C2H2:
In order to calculate B0 and B1 the method of combination differences
is used.
Δν1 and Δν0 are calculated for all the appropriate pairs of R and P
branch transitions and tabulated against the corresponding values for
2J’’ + 1 (for Δν1) and 2J’ + 1 (for Δν0).
Table 3: Tabulated data for R and P branch transitions and the
corresponding values for 2J’’ + 1
Branch
Δν1
2J’’ + 1
R1-P1
7.2
3
R2- P2
11.9
5
R3- P3
16.5
7
R4- P4
21.2
9
R5- P5
26
11
R6- P6
30.8
13
R7- P7
35.4
15
R8- P8
40.2
17
R9- P9
44.9
19
R10- P10
49.6
21
Table 4: Tabulated data for R and P branch transitions and the
corresponding values for 2J’ + 1
Branch
Δν0
2J’’ + 1
R0-P2
7.3
1
R1- P3
11.8
3
R2- P4
16.5
5
R3- P5
21.2
7
R4- P6
25.9
9
R5- P7
30.6
11
R6- P8
35.4
13
R7- P9
40
15
R8- P10
44.8
17
R9- P11
49.5
19
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Date: Today’s date
Student No. Your ID
Calculations of B0 and B1 for C2H2:
Δν1 is plotted against (2J’’ + 1) to find B1, and Δν0 is plotted against
(2J’ + 1) to find B0. The equations below show that B0 and B1 can be
found by calculating the slope of the graph and divide the value by 2.
Δν1 = 2B1 (2J’’+1) + c
Δν0 = 2B0 (2J’+1) + c
Figure 3: Plot of Δν0 against (2J’ + 1) for C2H2
The equation of the line in Figure 1 is y = 0.4255x – 0.0407, where y
is Δν0 and x is (2J’+1). The slope of the line is equal to 2B0 hence B0
can be found by dividing the value by 2.
y  2.3503x  0.0964
2.3503
2
BO  1.17515
BO 
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Date: Today’s date
Student No. Your ID
Figure 4: Plot of Δν1 against (2J’’ + 1) for C2H2
The equation of the line in figure 2 is y = 0.4239x – 0.0256, where y is
Δν1 and x is (2J’’+1). The slope of the line is equal to 2B1 hence B1 can
be found by dividing the value by 2.
y  2.3591x  0.0609
BO 
2.3503
2
BO  1.17955
Calculations of the carbon-carbon and carbon-hydrogen bond
lengths:
By using the experimentally determined value of B0 for C2H2 the
moments of inertia for both molecules can be calculate using the
equation below.
B
Rearranged for simplicity:
h
8 2 Ic
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Date: Today’s date
Student No. Your ID
I
h
B8 2 c
Where c = 2.998 x 108 msec-1 & h = 6.626 x 10-34 Jsec.
Applying this equation for C2H2:
First converting B0 into m-1:
1.17515cm 1 102  117.515m1
I
6.626 10 34 J sec
117.515m 1  8 2  2.998 10 8 m sec 1
I
6.626 1034
2.781728024 1012
I  2.371972623 10 46
With a known value for the moment of inertia the carbon-carbon and
hydrogen-carbon bond lengths can be found by applying the equation
below
I   mi ri
2
Rearranged for simplicity:
r
I

However as you can see we do not yet have a value for μ. This is
calculated by applying the following equation:

m1 m 2
m1  m 2
If finding the carbon-carbon bond lengths, both m1 and m2 equal the
mass of carbon in kg. However if calculating the carbon-hydrogen
bond lengths, m1 is the mass of carbon but m2 is the mass of
hydrogen in kg.
Mass of carbon = 1.9926 x 10-26kg
Mass of hydrogen = 1.673493466 x 10-27kg
Therefore for a carbon-carbon bond:
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Date: Today’s date
Student No. Your ID

(1.9926 10 26 ) 2
2  (1.9926 10 26 )
  9.963 10 27 kg
And for the carbon-hydrogen bond:
1.9926 10 26 1.673493466 10 27

(1.9926 10  26 )  (1.673493466 10  27 )

3.33460308 10 53
2.159949347 10 26
  1.543833926 10 27 kg
Now we have these values it is possible to calculate the bond lengths
with the previous equation:
I
r

The carbon-carbon bond length is therefore calculated as follows:
r
2.381972623 x 10 -46
9.963 x 10 -27
r  1.546227232 10 10 m
The carbon-hydrogen bond length is therefore calculated as follows:
r
2.381972623 x 10 -46
1.543833926 x 10 -27
r  3.92796937110 10 m
Conclusion:
Additional questions:
1.
2.
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Student No. Your ID
Date: Today’s date
3.
4.
Reference(s): Handbook of Chemistry & Physics, 85th Edition, CRC
Press, 2004-2005, David R. Lide Editor in Chief
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