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CHEM 120 Chapter 17: Equilibrium The Extent of Chemical Reactions -…
CHEM 120 Chapter 17: Equilibrium
The Extent of Chemical Reactions
Equilibrium State & Equilibrium Constant
Most reactions
do not go to completion
but stop when there is a
mixture
of reactants and products.
In a
reversible
process, the products of the reaction can react to
form reactants
.
The system is in
equilibrium
when the rates of
forward and reverse
reactions are the same.
Reaction Quotient & Equilibrium Constant
aA + bB ⇌ cC + dD
At equilibrium,
Q
=
K
.
The reaction quotient,
Q
, gives the ratio of product to reactant concentrations
at any point
in a reaction.
A
heterogeneous
equilibrium involves reactants and / or products in
different phases
.
A pure
solid
or
liquid
always has the same number of
moles per liter
of solid or liquid.
The expressions for
Q
and
K
include only species whose
concentrations change
as the reaction
approaches equilibrium
.
Equilibrium Constant K
K
reflects a particular ratio of
product to reactant
equilibrium concentrations
for a reaction.
K
indicates the
extent
of a reaction,
how far it proceeds
toward the products at the given temperature.
A
small
value for
K
indicates that the reaction yields little product before reaching equilibrium. The reaction
favors the reactants
.
A
large
value for
K
indicates that the reaction reaches equilibrium with very little reactant remaining. The reaction
favors the reactants
.
Manipulating
K
Original equation is
reversed
.
New constant is the
reciprocal
of the original.
Original equation is
multiplied
by a number.
New constant is the original
raised
to the same number.
Original equation is
divided in half
.
New contant is the
square root
of the original.
Two equations are
added
.
New constant is the
product
of the two originals.
Expressing Equilibria with Pressure Terms
K
for a reaction may be expressed using
partial pressures
of
gaseous reactants
instead of molarity.
The partial pressure of each gas is
directly proportional
to its
molarity
.
K
p
= K
c (RT)
^ ∆
n (gas)
If the moles of gas
does not change
in the reaction,
∆
n
= 0 and
K
p =
K
c.
Comparing Q & K to Determine Reaction Direction
Q
<
K
Reactants Decrease
Products Increase
Forward Toward Products
Q
>
K
Reactants Increase
Products Decrease
Reverse Toward Reactants
Q
=
K
At Equilibrium
No Net Change
Reaction Conditions & Equilibrium
Le Châtelier's Principle
When a chemical system at equilibrium is disturbed, it
reattains equilibrium
by undergoing a net reaction that
reduces the effect
of the disturbance.
A system is disturbed when a
change in conditions
forces it
temporarily
out of equilibrium.
Disturbances
Change in Concentration
Change in Pressure
Caused by Change in Volume
Affect Equilibrium Systems Containing Gaseous Components
Causes Equilibrium to Shift Accordingly
Adding an
inert gas
has
no effect
on the equilibrium position as long as the volume does not change.
Changing the
volume
of the reaction vessel will cause equilibrium to shift if
∆
n
≠ 0
.
Changes in
pressure (volume)
have
no effect
on
K
.
Change in Temperature
Heat is considered a component of the system.
Heat is a
product
in an
exothermic
reaction.
Heat is a
reactant
in an
endothermic
reaction.
An
increase
in temperature adds heat and results in the system shifting in the
endothermic (heat-absorbing)
direction to consume some of the added heat.
A
decrease
in temperature removes heat and results in the system shifting in the
exothermic (heat-releasing)
direction to produce more heat.
Van't Hoff
For a reaction with ∆
H
°rxn > 0, an increase in temperature will cause
K
to
increase
.
For a reaction with ∆
H
°rxn < 0, an increase in temperature will cause
K
to
decrease
.
ln
K
2 /
K
1 = ∆
H
°rxn /
R
(1 /
T
1 - 1 /
T
2)
Equilibrium Position
A shift to the
left
is a net reaction from
product to reactant
.
A shift to the
right
is a net reaction from
reactant to product
.
Only substances that appear in the expression for
Q
can have an effect, so changes in the amounts of
pure liquids and solids
cannot
.
A change in
concentration
has
no effect
on the value of
K
.
Catalysts
A catalyst
speeds u
p a reaction by lowering its activation energy, speeding up the forward and reverse reactions to the
same extent
.
A catalyst causes a reaction to reach equilibrium more
quickly
but has
no effect
on equilibrium position.
How to Solve Equilibrium Problems
If equilibrium quantities are
given
, we simply
substitute
them into the expression for
K
c to calculate its value.
If only
some
equilibrium quantities are given, use a
reaction table
to calculate them and find
K
c.
Reaction Table
Balanced
Equation
Initial
Quantities of Reactants & Products
Changes
in Quantities of Reactants & Products
Equilibrium
Quantities of Reactants & Products
To solve for
x
, rearrange the expression into the form of a
quadratic expression
, and eliminate the value that does not make sense
chemically
.
5% Rule
If a reaction has a relatively small
K
and a relatively large initial reactant concentration, the concentration change (
x
) can often be
neglected
.