Module 28 — Equilibrium
Module 28 ± Equilibrium
In this module, if you have had a prior chemistry course, the initial lessons may
If you think you are familiar with the topic of a lesson, try a few problems at the
end of the last problem set in the lesson.
If you get those right, move to the next lesson.
Chemical reactions can be divided into three types.
Reactions that go nearly 100% to completion.
Burning paper is one such reaction.
Once the reaction begins, the reaction goes until one of the reactants (the paper or
oxygen) is essentially used up.
Reactions that don±t go.
Trying to convert carbon dioxide and water into paper is very
difficult to do in a chemist±s laboratory (though plants are able to accomplish most steps
in this reaction by the remarkable process of photosynthesis).
Reactions that are reversible and go partially to completion.
In reversible reactions, as
the reaction proceeds, the reactants are gradually used up.
As a result, the forward
reaction slows down.
As product concentrations increase, they more frequently collide
and react to re-form the reactants.
Finally, both the forward and reverse reactions are
going at the same rate.
As long as no substances or energy are added to or removed
from the reaction system, the two rates will remain equal and no further reaction seems
to take place. The system is said to be at
For equilibrium to exist,
all reactants and products must be present in at least small quantities, and
the reaction must be in a
system: no particles or energy can be entering or
leaving the reaction vessel.
At equilibrium, no reaction seems to be occurring, but this appearance is deceiving.
the forward and reverse reactions continue.
the rates of reaction of the forward and reverse reactions are the same, there is no
In theory, all reactions that occur are reversible, and
reactions go to equilibrium.
practice, many equilibria favor the products so much that nearly all of the limiting reactant
is used up, and the reaction is considered to go ²to completion.³
For those reactions, if the
limiting reactant is known, calculation of the amounts of reactants used up and products
formed can be done using conversion stoichiometry.
When reactions go only partially to completion, there is no limiting reactant completely
used up that decides how much of the products form.
Reactions that go to equilibrium
require an accounting system to track the particles used up and formed.