Paul B. Dorain Brandeis University Waltham, MA 02154
The Bond Energy of CH3-H A physical chemistry experiment
T h e a d v e n t o n t h e m a r k e t of small commercial m a s s spectrometers h a s opened t h e possibility of making measurements of t h e hond energies of molecules in the undergraduate lahoratory. W i t h t h e addition of relatively low cost digital voltmeters, t h e measurements c a n he performed which a r e quantitative a n d a challenge t o t h e s t u d e n t . The specific experiment performed in our laboratory by juniors and seniors is an adaptation of a measurement of the CH3-H hond energy whieh was reported hy D. P. Stevenson.' If theappearance potentials ol'C2H5+whieh are derived from ethane, CsHs, and propane, CBHS, are measured, the reactions are C2H, C:,H8
-
-
+ H + e-
(1)
+ CHJ + e -
(2)
C2H:
C2H:
T h e appearance potential of C& AP, (CzH:) CIHB = AHI (CzH:)
for reaction (1) is
+ AHf (H) -AH,
(C2Hd
and for reaction (2) APn (CIH:) C:3Hs = AHl (CzH:)
+ AHl
(CH:,) - AHf (CaHs)
where we have made the usual simplifying assumptions.2The dissociation energy of the CH bond in CHI is D(CH3-H)
+ AHI (CaHs) -AH1 (CzHs) + 2AH1 (HI (3)
= APz - API - AH, (CHI)
The heats of formation of each of these species in eqn. (3) are commonly available in most physical chemistry texts. Alternatively some of them might he measured calorimetrically in a previous experiment. Most of the contribution to the dissociation energy in eqn. (:Oresults fiom the thermudynamie heatsofformation whichamounts to 117 kcallmul. The correction due to the appearance potentials is small (-10%) and the measurement of a significant difference presents a challenge to the students. In the actual operation of the laboratory, a Varian EM 600 Mass spectnmeter was used. Since the electron energy adjustment is crude on this instrument, n pair of outlets was installed so that a Hewlett I'nckard 3'12 digit voltmeter could be added. In the Varian instrument, the variation ofelectrunenergy is relative toa valtageofa fewhundred volts, so that considerable care in prnvidingstudent isulation is necessary. We did this by recessing the outlet terminals, under the plastic external cover. The students spend one lahoratory period with the covers off the instrument a,that under the direction of a laboratory assistant they can observe the physical parts of the apparatus. The simplicity of the Varian instrument makes this exercise particularly valuable. During this session the students also run spectra of heptane and CC4 a t various electron voltages. The heptane provides an easy calibration and is nicely illustrative of the effect of electron energy on an organic material. The CC14 spectrum is a convenient example of isotopic effects as well as that of a molecule which dues not have a stable parent ion, CClrt. These measurements also serve to demonstrate the care necessary to obtain good appearance potentials.
'Stevenson, D. P., Disc.Foroday Snc. 10,35 (1951). S. G., J. CHEM. EDUC., 48,545 (1971). :'Kiser, R. W., "Introduction to Mass Spectrometry and its Applicatiuns," Prentiee-Hall Inc., Englewood Cliffs, N.J., 1965. 'Salzhere. H. W., Morrow. J. I., Cahen, S. R.. Green, M. E., "Physical Chemistry ~aboratury,Principles and Experiments," MacMillan Pub. Co., Inc., New York, 1978. Since this paper was written a new text for lahoratory has been published which outlines a similar experiment to that descrihed here. It is doubtful that satislactory resultsare
