All chemical reactions involve orbital interactions. The orbital
description of a reaction can help you understand how chemical
reactions occur. As you study the various reactions presented in this
book, think about the orbitals involved in the reactions. Figure 5.1 is a
molecular orbital picture of ammonia reacting with boron trifluoride to
form a new bond. Ammonia is a base with a pair of nonbonding
electrons. The nitrogen of ammonia is sp3 hybridized. Boron
trifluoride is an acid with an incomplete octet of electrons. The boron
is sp2 hybridized with an empty p orbital. The reaction occurs when an
sp3 orbital of ammonia overlaps with the empty p orbital of boron
trifluoride. In the process, the boron becomes sp3 hybridized. With this
overlap the two molecules form a new bond.
Exercise 5.2
Show the orbitals involved in the acid-base reaction of a hydrogen ion
with a hydroxide ion.
Being able to identify an acid or base is important. Of equal
importance is the ability to recognize how the structure of that acid or
base affects its strength. The rest of this chapter is devoted to helping
you acquire the tools to do so. With these tools, you can predict the
outcome of chemical reactions. Much of the rest of the material in this
book depends on your ability to recognize acids and bases and their
relative strengths.
For this reaction, the amount of autoionization is extremely slight—at
25oC, it is 10–7 M (moles/liter). The concentrations of H3O⊕ and c- OH
are equal; that is, both measure 10–7 M. Chemists call this a neutral
solution. If you add a compound that is more acidic than water, you
increase the concentration of H3O⊕ ions and make the solution acidic.
If you add a compound that is more basic than water, you increase the
concentration of c- OH ions and make the solution basic.
The product of the H3O⊕ and c- OH concentrations in water is
equal to 10–14 and is a constant, Kw. Chemists define Kw with the
following equation.
Kw = [H3O⊕][ c- OH] = 1.00 x 10–14
Because the concentrations of H3O⊕ and c- OH are equal in a neutral
solution, you can easily calculate the concentration of both:
[H3O⊕] = [c- OH] = 1.00 x 10–7 M
Because the product of the two concentrations is a constant, Kw, when
one concentration increases, the other must decrease. For example, if
you add c- OH ions to water the concentration of the H3O⊕ decreases
by whatever amount is necessary for the product of the two
concentrations to still equal 10–14.
Because the hydronium ion concentrations can span a very
wide range of values, from greater than 1 M down to less than 10–14
M, chemists measure the concentration of H3O⊕ on a logarithmic scale
called pH. The pH values give the hydronium ion concentration of a
solution. Therefore, measuring the pH of a solution is a means of
quantifying the acidity of that solution. Chemists define this