AP Chemistry: Chapter 13 Student Notes

Objectives

þ  What is Equilibrium
þ  The Equilibrium Constant
þ  Equilibrium and Pressure
þ  Calculations involving equilibrium
þ  Le Chatelier’s Principle

What is Equilibrium

An Analogy

A Dance

Who is present at a dance?

______

______

What happens at a high school dance?

Lets Graph this:

So we can summarize this by saying

______+ ______ ______

What is the purpose of the

Definition of Equilibrium

Equilibrium is Dynamic

=

The Equilibrium Constant

Law of Mass Action

For the reaction:

jA + kB nC + mD

Kc=

This value is constant (hence the name) at a specified ______

The Meaning of K

K>1

K<1

K≈1

Equilibrium and Pressure

Many reactions are easier to measure using partial pressures. So for the reaction:

N2(g) + 3H2(g) 2NH3(g)

Kp=

Relationship between Kc and Kp


Calculations involving equilibrium

How to solve Equilibrium Problems:

1.  Start with a balanced Chemical Equation

2.  Write down the amounts (either concentration or pressure units) in an ICE table.

3.  Shift the equilibrium by subtracting and adding x to either side to the equation.

4.  Solve for x using your solver on your calculator

5.  Sometimes the equations must be solved in reverse….

Examples

Example 1

The equilibrium constant for the following reaction is 0.0900 at 25ْC:

H2O(g) + Cl2O (g) 2HOCl (g)

For which of the following situations is the system at equilibrium? For those which are not at equilibrium, in which direction will they shift?

a.

b.

c.


Example 2:

The value of the equilibrium constant for the following reaction is 1.5 x 10-5 at 298K.

2HF(g) H2(g) + F2(g)

If the [HF] = 2.0M. What is the final concentrations of all species once equilibrium is established?

Example 3:

The value of the equilibrium constant for the following reaction at 500K is 4.35:

N2(g) + H2 (g) 2NH3(g)

If the initial [N2] = 2.3 M and the [H2]=1.25M. What are the total concentrations of all species once equilibrium is established?


Example 4:

Phosphorus pentachloride, at initial partial pressure of 189.2 torr was placed into an empty flask and allowed to decomplse to PCl3 and Cl2 by the following reaction at 557K.

PCl5(g) PCl3(g) + Cl2(g)

At equilibrium the total pressure inside the flask was observed to be 358.7 torr. Calculate the partial pressure of each gas at equilibrium and the value of Kp at 557K


Le Chatelier’s Principle

What is Le Chtelier’s Principle?

When a system at equilibrium is placed under stress it will ______to relive that stress. Stressors of equilibrium are:

·  ______

·  ______

·  ______

Stressors do not include adding an ______or a ______

The only way to change the value of the equilibrium constant is to change the ______

Adding and Removing Chemicals in the reaction

·  Adding chemicals in the reaction will shift the reaction ______from that side of the reaction

·  Removing chemicals in the reaction will shift the reaction ______from that side of the reaction

Adding Chemicals other than those in the reaction

·  Adding chemicals other than those in the reaction will usually have ______affect on the equilibrium.

·  However, sometimes adding a chemical will ______a chemical in the reaction and thus taking that chemical ______from the reaction.

Temperature shifts:

When heat is added to a system the reaction will shift ______from the side of the equation where energy is:

·  For an exothermic reaction: Increasing temperature will shift the reaction to the ______

·  For an endothermic reaction: Increasing temperature will shift the reaction to the ______

Pressure Shifts

·  When pressure is increased, the reaction will shift toward the side of the reaction with the ______number of ______of gas

·  When pressure is decreased, the reaction will shift toward the side of the reaction with the ______number of ______of gas

Adding Inert Gases

·  Adding inert gases shifts the equilibrium to the ______

Adding a Catalyst:

·  Adding a catalyst will shift the equilibrium to the ______

Example

Hydrogen for use in ammonia production is produced by the reaction:

CH4(g) + H2O(g) CO(g) + 3H2(g)

What will be happen to the reaction mixture if:

a.  H2O(g) was removed?

b.  The temperature is increased?

c.  Helium is added?

d.  CO(g) is removed?

e.  The volume of the container is tripled?

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