Equilibrium

1
Equilibrium

• Chapter 13

2
What Is It?

• The state where concentrations of all
  reactants and products remain constant with time.

At the molecular level, the reaction continues.
Macroscopically, the reaction appears static.
3
Why Is It?

• Reaction rate depends on concentration.
• Collision Theory says more collisions faster
  reactions.
• As the reaction progresses, the concentration of
  products begins to increase.

4
Equilibrium Position is Determined By

• Initial Concentrations
• Energies of reactants and products
• Organization (disorder) of reactants and products

The Haber Process
N2(g) 3H2(g) 2NH3(g)
Nothing appears to change because the rates of
reaction are so naturally slow. A catalyst is
used to begin the process for commercial
production of ammonia. Haber applied Le
Chateliers Principle to maximize the forward
reaction.
5
Le Chateliers Principle

• When a stress is applied to a system at
  equilibrium, the reaction shifts to relieve the
  stress.

6
What stresses out a Reaction?

• Heat (heat is measured in kJ or kcal)
• Pressure (related to of moles on each side of
  the reaction)
• Concentration (how much of each component is
  added)
• When any of these increase or decrease from
  original conditions, that is stress.

7
The Equilibrium Constant

• The equilibrium constant is represented by K

Where wA xB yC zD
8
Try Me!

• Write the equilibrium expression for the
  following reaction
• 4 NH3 7O2 4NO2 6H2O

Calculate the value of the equilibrium constant
if the concentrations of the reactants and
products are as follows NH3 3.1 x 10-2
mol/L O2 5.4 x 10-2 mol/L NO2 3.1 x 10-2
mol/L H2O 4.7 x 10-2 mol/L
9
Equilibrium Position

• A set of concentrations that indicate whether
  products or reactants dominate while the rxn is
  at equilibrium.

Number of equilibrium positions available
Number of values for K
1
10
Equilibrium and Pressures

• Pressure and concentration are interchangeable

PV nRT P (n/V)RT n/V concentration
K
Kp
vs
In General Kp K(RT)Dn
Do Now!
Figure out the relationship between K and Kp for
the Haber Process.
Dn Sproducts -Sreactants
11
Heterogeneous Equilibria

• Pure liquids and pure solids are not included in
  the equilibrium expression for a reaction.
• Concentrations of PURE liquids and solids cannot
  change.

12
Applications of the Equilibrium Constant

• The Equilibrium constant gives information that
  will allow us to
• Decide how likely it is that the reaction will
  occur.
• Determine if a reaction is at equilibrium given a
  set of concentrations.
• Determine which direction the reaction must shift
  in order to reach an equilibrium position.

If K is greater than 1 The reaction is much More
likely to occur Spontaneously (the equilibrium
lies farthest to the right)
If K is very small The reaction is not likely to
be spontaneous (the equilibrium is near the
reactants)
Spontaneous Does Not Mean Fast!
13
The Reaction Quotient
Q

• Obtained by replacing initial concentrations into
  the concentrations of the equilibrium expression.
• Three possible cases

Q gt K Shift Left
Q K No Shift
Q lt K Shift Right
14
Try Me!!

• For the synthesis of ammonia at 500oC, the
  equilibrium constant is 6.0 x 10-2. Predict the
  direction in which the system will shift to reach
  equilibrium
• NH3o 1.0 x 10-3 M
• N2o 1.0 x 10-5 M
• H2o 2.0 x 10-3 M

Q 1.3 x 107 Shift to the Left
15
Try Me Again and Again!

• Same problem, different conditions
• NH3o 2.0 x 10-4 M
• N2o 1.5 x 10-5 M
• H2o 3.54 x 10-1 M
• NH3o 1.0 x 10-4 M
• N2o 5.0 M
• H2o 1.0 x 10-2 M

Q 6.01 x 10-2 No Shift
Q 2.0 x 10-3 Shift to the Right
16
Solving for Concentrations and Pressures

• Several Types of problems and Solving methods.
• Plug and Chug
• ICE
• ICE with Stoichiometry
• ICE with the quadratic equation

17
Type 1 Plug and Chug

• Consider an experiment in which gaseous N2O4 was
  placed in a flask and allowed to reach
  equilibrium at a temperature where Kp 0.133.
  At equilibrium, the pressure of N2O4 was found to
  be 2.71 atm. Calculate the equilibrium pressure
  of NO2.

N2O4(g) 2NO2(g)
18
Type 2 Initial, Change,Equilibrium

• At a certain temperature a 1.00L flask initially
  contained 0.298 mol PCl3(g) and 8.70 x10-3 mol
  PCl5(g). After the system had reached
  equilibrium, 2.00 x10-3 mol Cl2(g) was found in
  the flask. Gaseous PCl5 decomposes according to
  the reaction
• PCl5(g)
  PCl3(g) Cl2(g)
• Calculate the concentrations of all species and
  the value of k.

19
Type 3 ICE with Stoichiometry

• Carbon Monoxide reacts with steam to produce
  carbon dioxide and hydrogen. At 700K the
  equilibrium constant is 5.10. Calculate the
  equilibrium concentrations of all species if 1.00
  mol of each component is mixed in a 1.00L flask.

20
Type 4 ICE with Quadratic

• A 1.00L flask is filled with 1.0 mol H2 gas and
  2.0 mol I2 gas at 448oC. The value of the
  equilibrium constant Kc for the reaction
• H2 I2 2HI
• at 448oC is 50.5. What are the equilibrium
  concentrations of H2, I2 and HI in mol/L?