Water structure and properties

98% of living mass is water.

Water is a the nearest thing known to a universal solvent; it dissolves more substances than anything else.

Molecular structure

Oxygen has six electrons in the outer shell, so it needs two more to complete the shell. It gets them by sharing two electron pairs with hydrogen.

Oxygen is more electronegative than hydrogen, so the covalent bond is polarized.

Water is a nonlinear molecule; partial negative charges point in one direction, partial positive charges point in the other direction yielding a tetrahedral arrangement of charges.

This geometry of partial negative and partial positive charges results in hydrogen bonding to other water molecules.

Hydrogen bonds, which are short-lived, lasting 10-11 sec, make a lattice-work reminiscent of a crystal.

Solvation layer

Water neutralizes ionic charges, both positive and negative.

A solvation layer reduces strength of ionic bonds by an order of magnitude.

Water physical properties

Cohesion-Adhesion: capillary action

The network of hydrogen bonds results in the high cohesion of water: water molecules want to stick together.

The attraction of water to charged surfaces results in adhesion to polar surfaces.

By adhesion and cohesion combined, water can rise without the expenditure of energy, by capillary action.

Thermal conductance

Heat exchange through water is fast, because water's high thermal conductance is ideal for heat transfer.

Specific heat

Water's high specific heat ensures slow temperature changes.

Boiling point

Water remains liquid even at relatively high temperatures.

Density

Water expands on crystallization, i.e. on freezing, so ice is lighter and floats to the top. Ice is a very poor thermal conductor and insulates the remaining water from the temperature above, preventing it from freezing.

Hydrophobic interactions

In order to dissolve a nonpolar molecule like butane, water must disrupt its intermolecular lattice of hydrogen bonds. It forms a "cage-like" clathrate around the butane.

Because of this property, water tends to exclude butane from solution, forming an interface between compounds.

The tendency of nonpolar solutes to be excluded from water solution is called hydrophobicity. Note that hydrophobic interaction is not a feature of nonpolar molecules per se.

The interface between water and nonpolar molecules can be a barrier (as in membranes) or a means of concentrating reactant (cytochromes c of mitochondria).

Water chemistry

Water acts as both an acid and a base

Acid

An acid is a compound which releases protons (H+) (or accepts OH-) when dissociating e.g. HCl

Base

A base is a compound which accepts protons or releases OH- when dissociating e.g. NaOH.

Salts

A salt is a compound in which H+ has replaced by a +vely charged ion, e.g. NaCl

pH scale

pH is defined in terms of the concentration of H+ in solution.
pH =-log[H+]

pH 3 means that [protons] is 10-3 M

Examples: gastric juice pH 1; Coca Cola 2; coffee 5; blood 7.5; soap pH 11

Water's ion product is 10-14 = [H+][OH-] thus it's pH is 7.0

Buffers

Think of buffers as ambivalent molecules: both acid and basic in character.

Example: bicarbonate ion:
Base: proton (H+) acceptor:
H+ + HCO3- ® H2CO3 ® H2O + CO2
Acid: hydroxide (OH-) acceptor:
OH- + HCO3- ® CO32- + H2O

A cell needs a sink of protons or hydroxide ions to prevent pH changes generated by chemical reactions.

P. Fajer Lecture 4: Water structure 5/4 WATER.DOC 9/5/95 9:50 PM 5