Phase Change, Water, Hydrogen Compounds
Date: October 2006
How are heating and boiling point of water
different then expected of other hydrogen compounds?
The reason water has a high boiling point compared to other compounds with
hydrogen (methane for example) is because water can form hydrogen bonds.
Because oxygen is much more electronegative than hydrogen (whereas carbon is
about the same as hydrogen), it pulls the electrons from the hydrogens
closer to itself than to the hydrogen atoms. This creates a dipole moment
in water, with the oxygen being partially negatively charged and hydrogen
being partially positively charged.
Since opposite charges attract, a partially negatively charged oxygen from
one water molecule can form a hydrogen bond with a partially positively
charged hydrogen from another water molecule. Since the water molecules
then are strongly attracted to each other, it boils at a higher temperature.
The explanation from thermodynamics: going from liquid to gas phase requires
breaking bonds and therefore delta H of boiling will be positive. Since gas
molecules have more entropy than liquid molecules, delta S of boiling is
positive. If you remember from the relation delta G = delta H - T delta S
(where processes are spontaneous for delta G < 0), for a system where
boiling has a large positive delta H (like water) the T term (temperature)
will have to be higher for delta G to be negative, whereas if delta H is
positive and small (like for methane, where intermolecular interactions are
relatively weak) the T term will not have to be so high. This is why water
boils at 100C whereas methane boils at -161.6C.
Stanford Department of Chemistry
Imagine a bunch of balls connected to each other by springs. If you
drop an object onto these balls, the balls will vibrate. The amount
of vibration will depend (everything else being constant) on how
stiff the springs are. The stiffer the springs, the less vibration.
Since water molecules are held together by a very strong
intermolecular attractive force called "hydrogen bonding", adding
energy into water is not going to allow for much vibration (the
spring is very stiff). Since temperature is a measure of the average
molecular motion (vibration), then, adding energy (such as heat)
into water is not going to make it vibrate as much as compared to
other molecules with weaker intermolecular forces.
Boiling point can be thought of as that temperature at which the
intermolecular attractive forces between molecules are being broken
apart. Since, again, the intermolecular forces of water are quite
strong, water has a higher boiling point when compared to other
compounds that do not have such a strong intermolecular attractive force.
Greg (Roberto Gregorius)
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Update: June 2012