MadSci Network: Biochemistry
Query:

Re: H2O water molecules in a covalent bond repel each other, resulting

Date: Tue Jul 14 13:33:18 1998
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Biochemistry
ID: 891270157.Bc
Message:

891270157.Bc

MadSci Network: Biochemistry
Query:

H2O water molecules in a covalent bond repel each other, resulting in a distorted tetrahedral bond of 104.5 degrees. If the bond were to be linear, how would this arrangement affect life on earth?
As far as I can tell, what you are asking is the effect of water having a H-O-H bond angle of 180 rather than the actual value of 104.5. I can tell you that the effect would be profound.

Consider the water molecule as it actually is. The bonds between oxygen and hydrogen are quite polar. Oxygen holds electrons more strongly than hydrogen does, and so the oxygen atom accumulates a negative electrical charge, while the hydrogens have a positive charge.

Because the molecule is bent, the hydrogens are both on the same side of the oxygen atom. This results in one side of every water molecule having a permanent positive charge, and the other side having a permanent negative charge; such a situation is called a permanent dipole. Since opposite charges attract, water molecules are strongly attracted to each other and to other polar molecules (molecules having permanent dipoles).

Because the charge separation (or dipole) is so strong in water, it is able to induce temporary dipoles even in non-polar molecules. The result is that water is the "universal solvent." Almost anything will dissolve in water, even if only to a very small extent. Thus, water is able to bring together a wide variety of substances, which appears to be a prerequisite for life.

Water molecules attract each other strongly (but not too strongly), and so water is a liquid over a wide range of temperatures and pressures. This is important if you want to have liquid solutions of anything...

Even water's high surface tension (the tendency to bead up, rather than spread out the way alcohol or gasoline does) helps it be uniquely suited for supporting life -- and water would not have so much surface tension if it were not such a polar molecule.


Now, if water were linear the O-H bonds would still be polar -- they would each have a positive and a negative end -- but because they point in opposite directions the molecule as a whole would be completely non-polar.

To understand the consequences of this, consider the similar (but much more massive) molecule carbon dioxide (CO2). Unlike water, carbon dioxide is extremely reluctant to liquify. CO2 cannot be liquified at any temperature, if the external pressure is lower than 4.1 atmospheres. The temperature range in which CO2 remains liquid, even at that rather high pressure, is not large.

Furthermore, CO2 is completely non-polar and so is not able to dissolve very many other substances.

So if water were linear rather than bent,

  1. it would not liquify except at quite high pressures, and it would probably not remain liquid over more than about a 20 C. temperature range at best.
  2. it would dissolve very few other substances.
In consequence, life as we know it could not exist, not only on earth but anywhere.

Dan Berger
Bluffton College
http://cs.bluffton.edu/~berger


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