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Name: Michele
Status: educator
Grade: other
Location: VA
Country: N/A
Date: October 2006

Question:
How are heating and boiling point of water different then expected of other hydrogen compounds?



Replies:
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.

Ethan Greenblatt
Stanford Department of Chemistry


Michele,

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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