Chemical Properties of Water
Water is a special chemical substance consisting of two atoms of hydrogen and one atom of
oxygen.
O--H bond length = 95.7 picometers
H--O---H angle = 104.5°
O-H bond energy = 450 kJ/mol
Dipole moment = 1.83 debyes
The hydrogen atoms are "attached" to one side of the oxygen atom, resulting in a water molecule
having a positive charge on the side where the hydrogen atoms are and a negative charge on the
other side, where the oxygen atom is. Since opposite electrical charges attract, water molecules tend to attract each other,
making water kind of "sticky." The side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge)
of a different water molecule.
The water molecule maintains a bent shape because of two considerations:
1. The tetrahedral arrangement around the oxygen.
2. The presence of lone pair electrons on the oxygen.
Two electrons not involved in the covalent bonds are called lone pair electrons. The pairs of electrons are left alone.
These lone pairs are very negative - containing two negative electrons each - and want to
stay away from each other as much as possible.
These repulsive forces act to push the hydrogens closer together. The net result is a
terahedral arrangement. Tetrahedral means "four-sided". It is the arrangement of four atoms around a central atom
such that the distance between them is maximized.
The arrangement adopted will be the form of a regular tetrahedron. It has regular bond angles of 109.5°.
If we do a similar arrangement of water, putting oxygen in the center, and using the
two hydrogens and two lone pairs at the corners, we also come up with a tetrahedral
arrangement.
However, there is one important difference - the bond angles for water are
not 109.5°. Because of the presence of the very negative lone pair electrons,
the two hydrogens are squeezed together as the two lone pairs try to get away
from each other as far as possible. The resulting angle gives water a 104.5 bond angle.
Because we don't "see" the electrons, the resulting tetrahedron "looks" BENT
Links between Chemical Structure and Physical Properties
Two other features of the water molecule are also important for its properties:
- the small size of molecule
- the molecule is strongly dipolar.
This property makes water an effective solvent, particularly for crystalline salts.
The small size of hydrogen atoms makes it possible for molecules of water to effectively
bond together or chemically associate, particularly at lower temperatures.
This gives water its surface tension and liquid properties and also gives water it's unique
physical properties [link here].
However, water also partially dissociates into very minute concentrations of acid
[H3O+] and base [OH-] ions, a characteristic which leads to the use of the pH scale
to measure relative acidity or alkalinity. This also helps in dissolving ions and transporting H+ ions.
Major Chemical Properties of Water
Polarity
Two atoms, connected by a covalent bond, may exert different attractions for the electrons of the
bond. In such cases the bond is polar, with one end slightly negatively charged (-) and the other
slightly positively charged (+).
Although a water molecule has an overall neutral charge (having the same number of electrons and
protons), the electrons are asymmetrically distributed, which makes the molecule polar.
The oxygen nucleus draws electrons away from the hydrogen nuclei, leaving these nuclei with a
small net positive charge. The excess of electron density on the oxygen atom creates weakly
negative regions at the other two corners of an imaginary tetrahedron.
Water structure - hydrogen bonds
Because they are polarized, two adjacent H2O molecules can form a linkage known as a hydrogen
bond. Hydrogen bonds have only about 1/20 the strength of a covalent bond.
A hydrogen bond is therefore a weak chemical bond between a hydrogen atom in one polar
molecule and a very electronegative atom of a second polar molecule.
The hydrogen of one water molecule will be attracted to the oxygen of another water molecule.
The are usually 4-8 molecules per group in liquid water.
The surface tension of water is due to the hydrogen bonding in the associated groups of
water molecules.
Hydrogen bonds are strongest when the three atoms lie in a straight line.
The cohesive nature of water, through the hydrogen bonding and the small size of the molecule,
allowing the molecules to pack together, is responsible for many of its unusual properties,
such as high surface tension, specific heat, and heat of vaporization.
Molecules of water join together transiently in a hydrogen-bonded lattice.
Even at 37 degrees C, 15% of the water molecules are joined to four others in a short-lived assembly
known as a "flickering cluster."
Hydrophylic ('Water Loving') and Hydrophobic ('Water Hating') Molecules
Hydrophylic Molecules
Substances that dissolve readily in water are termed hydrophilic.
They are composed of ions or polar molecules that attract water molecules through electrical
charge effects.
Water molecules surround each ion or polar molecule on the surface of a solid substance and
carry it into solution.
Ionic substances such as sodium chloride dissolve because water molecules are attracted to
the positive (Na+) or negative (Cl_) charge of each ion. Polar substances such as urea dissolve because their molecules form hydrogen bonds with the
surrounding water molecules.
Hydrophobic Molecules
Molecules that contain a preponderance of non-polar bonds are usually insoluble in water
and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain
many C-H bonds. Water molecules are not attracted to such molecules as much as they are to other water
molecules and so have little tendency to surround them and carry them into solution.
But the so-called 'Hydrophobic Effect' does not mean that non-polar molecules are not
attracted to water!
We have all seen what happens after vinegar and oil salad dressing are vigorously shaken;
one does get a mixture of sorts, but after a little time the ingredients separate with
the lighter oil on top and a denser vinegar/water solution on bottom.
This is an illustration of an important chemistry principle expressed by the rule
that 'like dissolves like.' This refers to the phenomena that when two liquids made
of molecules of similar size and polarities are mixed, they will usually form a single
phase solution, no matter what the relative number of moles of each species.
This is expressed by the jargon that the two substances are miscible in all proportions.
In contrast, when a highly polar substance, such as water, is mixed with a non-polar or
weakly polar substance, such as most oils, the substances will separate into two phases.
This phenomenon is usually rationalized in introductory chemistry text books by saying
that oil is hydrophobic, and thus does not make solutions with water, while polar small
organic acids (such as acetic acid from which house vinegar is made) are hydrophilic,
and thus are miscible with water.
This explanation almost universally leads people to believe that individual water and oil
repel each other, or at least attract each other very weakly.
Nothing can be further from the case!
An individual oil molecule is attracted to a water molecule by a force that is much
greater than the attraction of two oil molecules to each other.
We can observe the consequence of this greater attraction when we put a drop of oil on a
clean surface of water.
Before hitting the surface, the oil will be in the shape of a spherical droplet.
This is because the oil molecules are attracted to one another and a spherical shape
minimizes the number of oil molecules that are not surrounded by other molecules.
When the oil hits the surface of the water, it spreads out to form a thin layer.
This happens because the attractions between the oil and water molecules gained by spreading
over the surface is larger than the oil-oil attraction lost in making a large oil surface on
top of the water.
If a sufficiently small drop of oil is put on the surface, it will spread to form a single
molecular layer of oil.
By measuring the area produced, one can get a simple estimate for the size of each oil
molecule and thus Avogadro's number.
Given these strong interactions, why does not each oil molecule dive into the water solution?
and surround itself with the favorable water attractions?
The reason is that to do so, it must come between water molecules that are already
attracting each other!
The strength of water-water attraction is much higher than water-oil interactions, and
thus there is a net cost of energy in putting the oil molecules into a water solution.
Thus the vast majority of oil molecules stay out of the water, though as many as will
fit will hang on to the surface water molecules that do not have a full complement of partners.
The meniscus is the curved surface of a liquid in a graduated cylinder or any other
small diameter glassware. Water adheres to the sides of any container creating a "cup"
of surface tension.
This induced structure is very important is it related to the structure and function
of membranes which are very characteristic of life as we know it.
Membranes in bacteria are composed of phospholipids and proteins. Phospholipids contain a
charged or polar group (often phosphate, hence the name) attached to a 3 carbon glycerol
back bone. There are also two fatty acid chains dangling from the other carbons of glycerol.
The phosphate end of the molecule is hydrophilic and is attracted to water. The fatty
acids are hydrophobic and are driven away from water.
Because phospholipids have hydrophobic and hydrophilic portions, they do remarkable things.
When placed in an aqueous environment, the hydrophobic portions stick together, as do
the hydrophilic. A very stable form of this arrangement is the lipid bilayer.
This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic.
Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment.
The cytoplasmic membrane is stabilized by hydrophobic interactions (i.e. water induced)
between neighboring lipids and by hydrogen bonds between neighboring lipids.
Hydrogen bonds can also form between membrane proteins and lipids. These are known as membrane
vesicles and are used to study membrane properties experimentally. There is some
evidence that these structures may form abiotically and may occur on particles that
rain down on earth from space.
Water as a Solvent - Acids & Bases - pH - Hydration
Water as a Solvent
Many substances, such as household sugar, dissolve in water. That is, their molecules separate
from each other, each becoming surrounded by water molecules.
When a substance dissolves in a liquid, the mixture is termed a solution.
The dissolved substance (in this case sugar) is the solute, and the liquid that does the
dissolving (in this case water) is the solvent.
Water is an excellent solvent for many substances because of its polar bonds.
Acids
Substances that release hydrogen ions into solution are called acids.
Many of the acids important in the cell are only partially dissociated, and they are
therefore weak acids-for example, the carboxyl group (-COOH), which dissociates to give
a hydrogen ion in solution. Note that this is a reversible reaction.
Bases
Substances that reduce the number of hydrogen ions in solution are called bases.
Some bases, such as ammonia, combine directly with hydrogen ions.
Other bases, such as sodium hydroxide, reduce the number of H+ ions indirectly, by
making OH- ions that then combine directly with H+ ions to make H2O.
Many bases found in cells are partially dissociated and are termed weak bases.
This is true of compounds that contain an amino group (-NH2), which has a weak
tendency to reversibly accept an H+ ion from water, increasing the quantity of free OH- ions.
Hydrogen Ion exchange
Positively charged hydrogen ions (H+) can spontaneously move from one water molecule to
another, thereby creating two ionic species.
Since the process is rapidly reversible, hydrogen ions are continually shuttling between
water molecules. Pure water contains a steady state concentration of hydrogen ions and
hydroxyl ions (both 10-7 M).
pH
The acidity of a solution is defined by the concentration of H+ ions it possesses.
For convenience we use the pH scale, where pH = log10[H+].
For pure water [H+] = 10_7 moles/liter
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