Covalent Bond Polarity: In a covalent bond, the shared electrons are not necessarily
shared evenly. If there are two identical atoms in a covalent bond, such as in hydrogen
gas, nitrogen gas, or oxygen gas, then the sharing of electrons is even. If, however, the
atoms are not identical, then the shared electrons will cluster more towards the atom with
the greater nuclear charge. For example, we have hydrochloric acid, HCl. In this
compound, the electron shared by hydrogen tends to be closer to the chlorine atom; also
the electron shared by the chlorine atom doesn't stray too far from the chlorine atom
itself. Thus, on the side of the molecule with the hydrogen, there's a slight positive
charge and on the side of the molecule with the negative charge, there's an equally slight
negative charge. This creates what is called an "electric dipole". An electric dipole is a
single system in which one side is positively charged and the other side is negatively
charged (these charges don't have to be equal in magnitude, but they almost always are in
natural systems).
Different atoms in polar molecules tug on electrons differently. For instance, in HCl the
shared electrons aren't as tightly clustered around the non-hydrogen atom as they are in
HF, hydrofluoric acid. Since different atoms tug on these shared electrons differently, a
measurable quantity called "electronegativity" has been devised. In Figure 17-22, the
electronegativities of all the elements are shown. Only the noble gases have zero
electronegativity--they basically don't pull on electrons in order to bond with them. We
see that the highest electronegativities tend to lie in the halogen group and in the other
columns on the right-hand side of the periodic table. In determining the strength of the
dipole of a molecule, it's the difference in electronegativity between the two molecules
that determines how polarized the molecule is. The greater the difference, the more
strongly attracted the covalent electrons are attracted to the atom with the greater
electronegativity and the greater the electric polarity of the molecule.
We can see from the table that ionic bonds tend to have the greatest polarity. For
instance, with francium fluoride, the difference in electronegativity is 3.98 - 0.70 = 3.28;
francium fluoride is the most ionic of the ionic bonds. Compare that with, say, lithium
astatide, LiAt: the difference in electronegativities is 2.20 - 0.98 = 1.22. Lithium astatide
is among the least ionic of the ionic solids. This means that the lithium has completely
given up its electron to astatine. Instead, the electron does hang around lithium a little
bit. There's no such thing as a perfect ionic compound. Some are just closer than others.
Smaller differences inherently occur in covalent bonds which, if you'll recall, can occur
only among non-metals. Restricting ourselves to the non-metals, we find that the largest
covalent dipole occurs between phosphorus and fluorine: 3.98 - 2.19 = 1.79. The
smallest difference occurs between two identical atoms bonding covalently. Between
different elements, the smallest covalent bond occurs with hydrogen and astatine, where
the difference is virtually zero. We will find that the polarity in the molecules plays a big
role in the chemical behavior of the molecule.
Molecular Polarity: For non-dipole molecules, determination of the polarity of the
molecule is most easily determined through the Electron Dot Structures. If we look at
water, we see an oxygen and two hydrogen molecules. The Electron Dot Structure of
water looks something like
Note the existence of two pairs of electrons from the oxygen atom that aren’t associated
with any hydrogen atom. Electrons can coexist in pairs despite their mutual electrical
repulsion because of magnetic effects. Each pair of electrons, though, will try to place as
much distance between themselves as any other pair of electrons. The configuration of
electron pairs that allows them to do this is a pyramidal structure or tetrahedron.
The Chemical Equation: We are now going to start discussing the nature of chemical
reactions. First, some terminology. When two or more substances are interacting with
each other, we call the initial substances the reactants and the resulting substance(s) the
product(s). Molecular formulas are usually used to represent the reactants and products
and the phase of each substance is sometimes represented by an s, g, or l for solid, gas or
liquid; materials that are dissolved in water are designated as aq for aqueous.
Coefficients are also placed in front of each substance to indicate relative quantity used in
the reaction (no coefficient means 1).
An absolutely vital principle in chemical reactions is the conservation of mass. In fact, in
a chemical reaction, no atom will have its fundamental nature changed, i.e., there must be
as many carbon atoms before an interaction as after an interaction, ditto for oxygen, ditto
for any other element. Chemical equations with an unequal number of any particular
element on each side is considered unbalanced and, hence, inaccurate. Chemical
equations must be balanced. When balancing equations to understand the reaction, one is
never allowed to adjust a subscript in a molecular formula--doing so changes the
substance that is undergoing the chemical reaction. The only thing that can be adjusted is
the coefficient in front of each substance in the interaction. Balancing chemical
equations is an important part of "stoichiometry". For instance, suppose we wish to break
up potassium chlorate into potassium chloride and oxygen gas. The first thing we need to
do is come up with the molecular formulas for potassium chlorate and oxygen gas.
Potassium can be ionized to +1 and chlorate (ClO3) can be ionized to -1, so potassium
chlorate is KClO3. Oxygen gas is O2 and potassium chloride is KCl. So we need to
figure out the coefficients in the reaction KClO3  KCl + O2. This is unbalanced, as we
can see by comparing the number of oxygen atoms in each side, 3 on the left and 2 on the
right. We can start by multiplying O2 by 3 and KClO3 by two, which then gives 6 oxygen
atoms on each side. But this also gives us two potassium atoms and two chlorine atoms
on the left and only one of each on the right. However, multiplying KCl by 2 gives us
two of each on the right and our balanced equation has become
2KClO3  2KCl + 3O2,
meaning that two potassium chlorate molecules yield two potassium chloride molecules
and 3 oxygen gas molecules.
Energy and Chemical Reactions: Why would one direction of any chemical equation
be favored over the other? The answer comes from two sources: energy and entropy.
The direction in which a chemical reaction occurs if allowed to proceed spontaneously is
determined by the “Gibbs Free Energy”, named after an American chemist from the mid19th century. The Gibbs Free Energy of a substance is determined through the formula
G = U + PV – TS
where G is the Gibbs Free Energy, U is the internal (or thermal) energy, P is the pressure
exerted by the substance, V is the volume occupied by the substance, T is the temperature
and S is the entropy. In figuring out the direction of the reaction, the change in the Gibbs
Free Energy must be determined:
Gfinal – Ginitial = (Ufinal - Uinitial) + (PfinalVfinal – PinitialVinitial) – (TfinalSfinal - TinitialSinitial).
If the reaction is allowed to proceed open to the atmosphere (i.e., not in an enclosed
container) and if we assume that the change in temperature is very small, then the
formula simplifies somewhat to
Gfinal – Ginitial = (Ufinal - Uinitial) + P(Vfinal – Vinitial) – T(Sfinal - Sinitial)
 G = U + PV – TS.
We will see that the reaction that is favored is the one in which G < 0.
In examining this equation, we see that the change in entropy is not the only determinant
with regards to in which direction the reaction spontaneously goes. Certainly, if the
entropy were to increase, then S > 0 and –TS < 0. But the presence of U and PV
tells us we don’t have the whole story. PV turns out to be the work done by the system.
If V > 0, then the system expands and does work; this would decrease the energy that is
available to make the reaction work, so this acts against the reaction. On the other hand,
if V < 0, then the system is compressed, meaning something has come along and done
work on the system, adding that much energy to the system to allow it to proceed. Then
there’s the change in the internal (or thermal) energy, U. Since we’re assuming the
reaction is taking place at a constant temperature, the only thing that can affect the
internal energy is the number of particles, N (recall that U = (3/2)NkT). If the number of
particles increase, then since each of those particles has roughly the same energy as each
particle did before the reaction, energy is used up on the extra numbers of particles. This
energy has to come from the energy of the reaction, so that causes it to be less favored. If
U < 0, though, then more energy is available for the reaction to proceed.
Rate of Reaction: The chemical reaction of trinitrotoluene (TNT) occurs in a very short
period of time, resulting in an explosion. The rusting of iron acts over a long period of
time, so slowly that changes on a day-to-day basis aren’t noticed. What is there such a
big difference between these chemical reactions? This question is answered in the field
of Chemical Kinetics. There are some relatively obvious things we can do to increase the
rate at which a reaction takes place. One would be to increase the surface area of
interaction between the reactants. Another would be to increase the temperature, which
would give the individual atoms and molecules more energy to initiate a reaction. We
can also increase the concentration of the reactants, giving us more atoms and molecules
in close proximity and allowing for a faster reaction. Finally, we can do something to
give the system a push, like lighting a match; technically, this is adding a “catalyst.”
In order to have a better understanding of reaction rates, we need to examine two topics,
“concentration” and “activation energy.” The concentration of a substance is defined to
be the number of moles per liter of the substance, i.e., quantity/volume. So, if I have 0.3
moles of, say, hydrogen gas (H2) crammed into 20 mL, then the concentration is
expressed as [H2] = 0.3 moles/0.020 L = 15 moles/L = 15 M (brackets around the element
or compound formula indicates we’re examining the concentration). The idea behind the
activation energy is that it is the minimum energy needed to break apart the chemical
bonds in the reactants. It’s similar to trying to push a boulder over a hill and letting the
boulder roll down to the other side of the hill.
It’s a concept that’s relatively simple, but determining it is actually rather difficult. By
and large, the activation energy is experimentally determined. Discussing it in more
detail would likely be counterproductive.