Chemistry

Explaining Hydrogen Bonding in Water



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Water, water and water, almost everywhere water earth appears like this when you see from space. Almost 2/3 of earth's surface is covered by water. It is most abundant substance on earth, essential for every living creature in this planet. Water is a major component of human body and many bodily fluids e.g. blood, urine, saliva. In our lifetime, 50 tons of water passes through our bodies. It is also considered as a universal solvent because many substances dissolve in it.

No matter whether it is salty water or drinking water, chemists describe this miracle liquid as H2O. It exhibits many physical properties that distinguish it from other small molecules of comparable mass, contributing to its anomalous' property. Water expands, whereas most substances contract when they solidify, making it less dense as a solid than as a liquid. Water at room temperature should be a gas due to its light molecular weight but we are drinking it everyday. These unique properties are result of interaction of individual H2O molecules with each other.

In water, two hydrogen atoms are covalently bonded with one oxygen atom. This covalent bond occurs by sharing electrons of each atom. Since this is about sharing, it could be equal or non-equal sharing. If it is equal sharing of bonding electrons then the bond is non-polar covalent bond and if it is non-equal sharing then it is polar covalent bond. In case of water, it is polar covalent bond.

As the bonding electrons are shared unequally by the hydrogen and oxygen atoms in water molecule, a partial negative charge forms at the oxygen end (as oxygen atom has a stronger affinity for electron than hydrogen atoms) and a partial positive charge forms at the hydrogen ends of the water molecule although it remains as electrically neutral molecule.
Opposites attract each other. Since the hydrogen and oxygen atoms in the molecule carry partial opposite charges, nearby water molecules are attracted to each other. This attraction (called electrostatic interaction) between the partially positive hydrogen and partially negative oxygen in adjacent molecules is called hydrogen bonding. Hydrogen bond is directional.

Each water molecule can interact with other four water molecules - two hydrogen atoms interact with oxygen atoms of two other water molecules and oxygen atom interacts with one hydrogen atom each from two other water molecules.

So water is basically a network of water molecules bonded with hydrogen bonds. But, the hydrogen bonds in water are constantly being broken and reformed as it has an average energy of 20 kJ/mol, which is less than an O-H covalent bond, which is 460 kJ/mol. The average hydrogen bond lasts only picosecond.

The special density of water arises because of the presence of large number of hydrogen bonds in water pulling the molecules together even though an individual hydrogen bond is relatively weak.

Scientists are trying to solve hydrogen-bonded network structure to understand its unusual properties. Traditional scientific picture of solid ice state is that every individual water molecule forms four hydrogen bonds making a network of tetrahedrons. Ice has well defined structure where water molecules are further apart from one another than they are in the liquid state making it less dense than liquid water. This is the reason for floating of icebergs in ocean and ice cubes in a beverage. When ice melts to liquid water, these bonds may become distorted and up to 20-percent of them broken although liquid water still retains its tetrahedral network. For long time, this tetrahedral network structure, coupled with hydrogen bonds has been considered for unusual properties of water.

However, in 2004, a team of scientists from Stanford Synchrotron Radiation Laboratory (SSRL) and Stockholm University reported that in the liquid state, more than 80 percent of the hydrogen bonds between water molecules were broken. On the average, they found each liquid water molecule formed only two hydrogen bonds - one electron donor and one electron acceptor. From this they concluded that in the liquid state, water molecules form a network of large rings or chains, rather than tetrahedrons(1).

In 2006, a University of California San Francisco /Berkeley Joint Graduate Group in Bioengineering provided further evidence to their findings in 2002 that the alternative model of liquid water, the "rings and chains", may exist for briefest time but the tetrahedral network is the average structure that persists at time scales of picosecond or beyond(2).

Prof. Martin Chaplin of the London South Bank University has also proposed an icosahedral clustering model in which 280 H2O units form an icosahedron containing twenty 14-molecule tetrahedral units to explain all of the unusual properties of water(3).

No matter whatever the microscopic structure is, water is present as a chain of hydrogen bonds, and stayed among us as liquid at atmospheric pressure and room temperature to create or serve creatures in this earth.

1. http://news-service.stanford.edu/pr/2004/water47.html
2. http://www.lbl.gov/Science-Articles/Archive/PBD-water-controversy.html
3. http://www.lsbu.ac.uk/water/clusters.html

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