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CO and CH4 Boiling Points
Name: Amy S.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Sunday, December 08, 2002
Question:
Based on intermolecular forces, carbon monoxide should
boil at a higher temperature than methane. However, it does not. Why?
Replies:
Carbon monoxide does have a dipole moment but it is very small. C and O
are similar in electronegativity so the charge difference is quite small.
Methane has no dipole moment but has more electrons than CO does and is
more polarizable.
So even though CO molecules exhibit dipole-dipole and dispersion forces
and CH4 molecules only exhibit dispersion forces, CH4's dispersion forces
are apparently attractive enough to win the game.
A more extreme example can be seen if you compare n-octane, a nonpolar
hydrocarbon, with water, which exhibits hydrogen bonding, dipole-dipole
and dispersion forces. However, water's
boiling point is 100 deg.C while octane's is 125.7 deg.C . Dispersion
forces can be quite strong, and the bigger the molecule the stronger the
dispersion force.
R. Topper
Correlating boiling points (or melting points) with "intermolecular
forces" is only approximate. Both boiling and melting are much more
complicated phenomena. In addition, what experimental data was used to
determine the "intermolecular forces" can change both the absolute and
relative values assigned to a given molecule. The molecular volume and
shape also play a subtle but important role. Remember that the boiling
point is rather arbitrary. It is the temperature at which the vapor
pressure equals 1 atmosphere. Choose another vapor pressure and the order
of the list of boiling points may change. The value of the critical
constants is frequently a better parameter to compare. The critical
constants Tc, Pc, and Vc are the value of temperature, pressure, and molar
volume where the density of the liquid equals the density of the vapor. In
fact, the parameter Zc = PcVc/RTc
is often used to compare the aggregate effect of the various molecular
factors that measure things like volatility. However, the critical
constants are known for only a small number of molecules. The bottom line
is: Do not take correlations too seriously; they are only estimates (read
that "guesses").
Vince Calder
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Update: June 2012
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