Transcript Equilibrium

Equilibrium
Equilibrium
Some reactions (theoretically
all) are reversible reactions,
in which the products take
part in a separate reaction to
reform the reactants.
Equilibrium
We show equilibrium in a
reaction using an arrow pointing
both ways (  ).
For example:
2NO2 (g)  N2O4 (g)
Equilibrium
►Chemical
equilibrium is when
the rate of the forward reaction
is equal to the rate of the
reverse reaction.
Equilibrium
►At
equilibrium, the
concentrations of reactants and
products remain constant over
time.
Equilibrium
The equilibrium constant
expresses the relative
concentrations of reactants and
products at equilibrium. It is
Keq.
Equilibrium
For the reaction
aA + bB  cC + dD
[C]c[D]d
Keq =
[A]a[B]b
Equilibrium
Keq =
► [A],[B],[C]
[C]c[D]d
[A]a[B]b
and [D] are molar concentrations
► a, b, c and d are the coefficients in the balanced
equation
Equilibrium
Keq =
► When
[C]c[D]d
[A]a[B]b
writing equilibrium constant expressions, it
is customary to omit units
Equilibrium
For example: Write the
equilibrium expression for this
reaction:
2CO(g) + O2 (g)  2CO2(g)
[CO2]2
Keq =
2
[CO] [O2]
Equilibrium
By plugging in values for
concentration, we can
determine whether a system is
at equilibrium and how it might
change as it heads toward
equilibrium.
Equilibrium
Consider this reaction:
COCl2 (g)  CO(g) + Cl2(g)
Keq = 170
If the concentrations of CO and Cl2 are each
0.15 M and the concentration of COCl2 is 1.1
x 10-3 M, is the reaction at equilibrium? If
not, in which direction will it proceed?
Equilibrium
Set up the equilibrium expression
and plug in values:
Keq =
[CO][Cl2]
[COCl2]
= (0.15) (0.15)
(1.1 x 10-3)
Equilibrium
Q = 20, which is not the same as
170, so the system is not at
equilibrium.
To be at equilibrium, we would
like to have it get bigger.
Equilibrium
If Q is to get bigger, we would
need more CO and Cl2, so the
reaction needs to shift to the
right.