Sam Ellick 13/11/2010
Water otherwise known as H2O is a necessity for life to exist. It has many unique features compared to other molecules of its size. For example solid ice is less dense than liquid ice, which ponds and lakes freeze from the top down. This is important in insulating the aquatic life in the winter. This essay however will be looking at why water is so important in biochemical terms.
Water, as its formula H2O suggests, consists of one oxygen bonded singularly to two hydrogen molecules (see fig 1.) Common sense would dictate that each hydrogen will be located at opposite ends of the oxygen making a linear shape, however this is not the case. H2O is tetrahedral due to the oxygens two lone pairs. In a tetrahedral molecule every bond is 109˚ from each other. However due to the repulsion from the oxygens two lone pairs, the angle H-O-H is approximately 104.5˚. This feature gives H2O its dipole property. The pole where the two lone pairs are located are more electronegative than the opposite pole, of which hydrogens are located. In addition, the electrons that compose the single (sigma) bonds are pulled closer to the oxygen than the hydrogen, which adds to the difference in electronegativty at each pole of the molecule.
The dipole has a massive effect on H2O intermolecularly. In adjacent atoms the positive dipole (where the hydrogens are) will interact with another molecules negative dipole (where the lone pairs are) forming a hydrogen bond. A hydrogen bond is a bond that is weaker than a sigma or π bond, yet stronger than van der waals forces. Each H2O molecule can make up to four hydrogen bonds. This strong intermolecular interaction is the reason H2O (molecular weight 18) is a liquid at standard temperature and pressure (STP) but not a gas. Molecules such as butane (molecular weight 60) and hydrogen sulphide (molecular weight 34) are gasses at room temperature, so the trend would... [continues]
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