CHEMICAL EQUILIBRIUM

1
CHEMICAL EQUILIBRIUM
2
Chemical Equilibrium
Reversible Reactions
A chemical reaction in which the products can
react to re-form the reactants
Chemical Equilibrium
When the rate of the forward reaction equals the
rate of the reverse reaction and the
concentration of products and reactants remains
unchanged
2HgO(s) ? 2Hg(l) O2(g)
Arrows going both directions ( ? ) indicates
equilibrium in a chemical equation
3
2NO2(g) ? 2NO(g) O2(g)
Remember this? Why was it so important to
measure reaction rate at the start of the
reaction (method of initial rates?)
4
Law of Mass Action
For the reaction
jA kB ? lC mD
Where K is the equilibrium constant, and is
unitless
5
Product Favored Equilibrium
Large values for K signify the reaction is
product favored k gt 1
When equilibrium is achieved, most reactant has
been converted to product
6
Reactant Favored Equilibrium
Small values for K signify the reaction is
reactant favored k lt 1
When equilibrium is achieved, very little
reactant has been converted to product
7
Writing an Equilibrium Expression
Write the equilibrium expression for the reaction
2NO2(g) ? 2NO(g) O2(g)
K ???
8
Conclusions about Equilibrium Expressions

• The equilibrium expression for a reaction is the
  reciprocal for a reaction written in reverse

2NO2(g) ? 2NO(g) O2(g)
2NO(g) O2(g) ? 2NO2(g)
9
Conclusions about Equilibrium Expressions

• When the balanced equation for a reaction is
  multiplied by a factor n, the equilibrium
  expression for the new reaction is the original
  expression, raised to the nth power.

2NO2(g) ? 2NO(g) O2(g)
NO2(g) ? NO(g) ½O2(g)
10
Equilibrium Expressions Involving Pressure
For the gas phase reaction 3H2(g) N2(g) ?
2NH3(g)
K and Kp are NOT interchangeable!
11
Heterogeneous Equilibria

• The position of a heterogeneous equilibrium does
  not depend on the amounts of pure solids or
  liquids present

Write the equilibrium expression for the
reaction PCl5(s) ? PCl3(l) Cl2(g)
Pure solid
Pure liquid
12
CAN you..

• Write an Keq expression?
• Tell how Keq changes as the stoichiometry
  changes?
• Convert Keq in terms of Kc vs Kp?
• Explain what K means for the rxn?

13
The Reaction Quotient
When the reaction is NOT at equilibrium for a
given time, the reaction quotient, Q takes the
place of K, the equilibrium constant, in the law
of mass action.
jA kB ? lC mD
Useful in determining what will happen under
special conditions.
14
Significance of the Reaction Quotient

• If Q K, the system is at equilibrium

• If Q gt K, the system shifts to the left,
  consuming products and forming reactants until
  equilibrium is achieved

• If Q lt K, the system shifts to the right,
  consuming reactants and forming products until
  equilibrium is achieved

15
Solving for Equilibrium Concentration
Consider this reaction at some temperature H2O(g)
CO(g) ? H2(g) CO2(g) K 2.0
Assume you start with 8 molecules of H2O and 6
molecules of CO. How many molecules of H2O, CO,
H2, and CO2 are present at equilibrium?
Here, we learn about RICE the most important
problem solving technique in the second semester.
You will use it for the next 4 chapters!
16
Solving for Equilibrium Concentration
H2O(g) CO(g) ? H2(g) CO2(g) K 2.0
Step 1 We write the law of mass action for the
reaction
17
Solving for Equilibrium Concentration
Step 2 We RICE the problem, beginning with
the R for balanced reaction
H2O(g) CO(g) ? H2(g) CO2(g)
Reaction
Initial
Change
Equilibrium
8
6
0
0
-x
-x
x
x
8-x
6-x
x
x
18
Solving for Equilibrium Concentration
Step 3 We plug equilibrium concentrations into
our equilibrium expression, and solve for x
H2O(g) CO(g) ? H2(g) CO2(g)
Equilibrium 8-x 6-x x x
19
Solving for Equilibrium Concentration
Step 4 Substitute x into our equilibrium
concentrations to find the actual concentrations
H2O(g) CO(g) ? H2(g) CO2(g)
Equilibrium 8-x 6-x x x
Equilibrium 8-44 6-42 4 4
20
LeChateliers Principle

• When a system at equilibrium is placed
• under stress, the system will undergo a
• change in such a way as to relieve that
• stress.

Translated The system undergoes a temporary
shift in order to restore equilibrium.
21
LeChatelier Example 1
A closed container of ice and water is at
equilibrium. Then, the temperature is raised.
Ice Energy ? Water
The system temporarily shifts to the _______ to
restore equilibrium.
right
22
LeChatelier Example 2
A closed container of N2O4 and NO2 is at
equilibrium. NO2 is added to the container.
N2O4 (g) Energy ? 2 NO2 (g)
The system temporarily shifts to the _______ to
restore equilibrium.
left
23
LeChatelier Example 3
A closed container of water and its vapor is at
equilibrium. Vapor is removed from the system.
water Energy ? vapor
The system temporarily shifts to the _______ to
restore equilibrium.
right
24
LeChatelier Example 4
A closed container of N2O4 and NO2 is at
equilibrium. The pressure is increased.
N2O4 (g) Energy ? 2 NO2 (g)
The system temporarily shifts to the _______ to
restore equilibrium, because there are fewer
moles of gas on that side of the equation.
left