3.4 PHYSICOCHEMICAL INTERACTIONS
Atoms on a molecule can also be attracted to atoms on another molecule, producing inter-
molecular forces. These forces are primarily electrostatic. The strength of these forces
range from that of hydrogen bonds, the strongest, to the van der Waals forces, the weakest.
However, all of them are weaker than the forces involved in chemical bonding.
Hydrogen bonds are intermolecular attractions that occur between electronegative
atoms on one molecule (e.g., O, N, o r Cl) and with hydrogen atoms in another molecule.
They will occur only if the hydrogen is bonded to other atoms more electronegative than
carbon (e.g., O or N). The latter atoms pull the electrons away from the hydrogen, creating
a strong local positive charge. This charge consequently has a strong attraction to electro-
negative atoms on the other molecules.
Even in the absence of polarity-producing asymmetry, a molecule can exhibit a tem-
porary polarity as electrons shift from one side to another, or shift in response to the pre-
sence of the electron cloud of another molecule nearby. This produces a weaker
electrostatic attraction, the van der Waals force.
Attractions due to hydrogen bonds, van der Waals forces, or polarity produce several
important effects:
The strength of these attractions allow chemicals to form liquids and solids. In more
precise terms: They tend to raise the boiling point and melting point, increase the
heat capacity and heat of vaporization, and decrease the vapor pressure.
Dissimilar molecules that have sufficient attraction to each other can dissolve or
mix, whereas those that do not attract tend to form separate phases.
They contribute to the shape of large molecules because of attractions between
different parts of the same molecule, modified by competition with other attractions,
such as with solvent molecules or other dissolved species.
They affect the rate of chemical reactions between like and unlike molecules by
affecting the proximity and orientation of molecules to each other.
Water illustrates several physicochemical interactions and also plays a key role in
biochemistry. Water is the most abundant chemical in living things. Living things are
composed of between 50 and 90% water by weight. In humans, 99 of 100 molecules
are water. The water molecule is bent; that is, rather than being directly on opposite sides
of the oxygen, the two HO bonds form a 104.5
angle with each other. The
electronegativity of the oxygen pulls the electrons from the hydrogen. This leaves the
molecule with a strong dipole, and each atom can form hydrogen bonds with other
water molecules. These properties give it strong self-attraction. This results in the
highest boiling point and heat of vaporization of any chemical of similar size or molar
mass (i.e., molecular weight). Water’s heat capacity and latent heat helps it regulate the
temperature of living things, especially animals, which can generate heat at a rapid rate. Eva-
poration of water carries away a large amount of heat. If a human consuming 2000 Cal-
ories per day (note that 1 dietary Calorie ¼ 1000 calories, distinguished in wr iting by their
capitalization) were to release all that energy as heat, it could be consumed by evaporation
of 3418 grams of water, less than 1 gallon. The actual evaporation is less because not all
dietary calories are released as heat, and some heat is lost by convection.
Water’s polarity also enables it to reduce the repulsive forces between charged particles
such as ions, because it can align itself with the ion’s electric field. The reduction in
40 THESUBSTANCESOFLIFE