Chemical Equilibrium | AP Chemistry
Irreversible Reactions
• Technically, all chemical
reactions are reversible
o But some reverse
reactions are so
energetically unfavorable
that we consider them
impossible
Such reactions are
said to be
irreversible
• For our studies of equilibrium,
we will work with reactions that
are reversible
Achieving Equilibrium
• Take a look at a reversible,
elementary reaction in which red
balls (A) react to become blue
balls (B):
Simple Reaction
•
•
•
•
Since this is an elementary
reaction, we can easily deduce
the forward and reverse rates
o rateforward = kf[A]
o ratereverse = kr[B]
If we start with a lot of [A] and
no [B], the forward reaction rate
will be very high, and the reverse
rate will be 0
But as A is converted to B, [A]
decreases and [B] increases
o So the forward rate
decreases and the reverse
rate increases
The forward and reverse rates
will “fight” back-and-forth until
they reach equilibrium
o At equilibrium
rateforward = ratereverse
•
Equilibrium doesn’t tell us that
[A] and [B] are equal
o Rather, the rates of their
formation are equal
Equilibrium
At equilibrium, the rates of formation of A and B
are equal. For each A that’s converted to B,
there’s a B converted to A, such that neither
concentration changes. Note that at equilibrium,
[A] and [B] need not be equal.
•
Since we have more B than A at
equilibrium, the above reaction is
said to be product-favored
o Increasing kforward would
make the reaction even
more product-favored
Running to Completion
• To run the above reaction to
completion, all of the A would
have to be converted to B
• A reaction can be run to
completion if the product is
actively removed as it forms
o Leaving no reactant for
the reverse reaction
• Suppose the above reaction is
occurring in a beaker, and that B
is a gas
Running the Reaction to Completion
Product B is a gas, and as it forms, it bubbles out
of the beaker. With no B remaining in the
beaker, there’s no reactant for the reverse
reaction. All the A will be converted to B.
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Chemical Equilibrium | AP Chemistry
The Equilibrium Constant
• Take a look at the following
elementary reaction:
•
•
•
•
Since the reaction is elementary,
its forward and reverse rates are:
o rateforward = kf[A]a[B]b
o ratereverse = kr[C]c[D]d
And at equilibrium, those rates
will equal one another:
o rateforward = ratereverse
o kf[A]a[B]b = kr[C]c[D]d
Rearranging algebraically, we
can see that
k f [C]c [D]d
=
k r [A]a [B]b
We can group kf and kr into a
single constant, K or Keq, known
as the equilibrium constant
Reciprocal of the Equilibrium Constant
The reciprocal of the equilibrium constant is the
equilibrium constant of the reverse reaction.
After all, the reactants and products switch
places.
•
Given the equilibrium constants
for the 3 steps of a multistep
reaction, the equilibrium constant
of the overall reaction will be
their product
The Equilibrium Constant
•
The equilibrium constant is
always equal to a reaction’s
products raised to their
coefficients divided by reactants
raised to their coefficients
o This even applies for nonelementary reactions!
Properties of the Equilibrium
Constant
• The reciprocal of the equilibrium
constant gives the equilibrium
constant of the reverse reaction
o The reactants and
products switch places
Equilibrium Constant of a Multistep Reaction
Keq of a 3-step reaction will equal the product of
the equilibrium constants for all 3 steps.
•
When a multistep reaction is at
equilibrium, all of its individual
steps are at equilibrium
Equilibrium of a Complex Reaction
If all 3 steps of this complex reaction are at
equilibrium, the overall reaction is at
equilibrium.
•
Also, keep in mind that since kf
and kr are temperature-
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Chemical Equilibrium | AP Chemistry
•
dependent, Keq will also be
temperature dependent
Lastly, solids & pure liquids are
not included in the Keq
expression
o So, given our reaction
from before, suppose we
are told that product C is
a solid:
Product C as a Solid
•
Since C is a solid, we omit it
from the Keq expression:
Solids are Omitted from the Keq Expression
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