1. Mr. Kinton Honors Chemistrry Enloe High School
Chemical Equilibrium
Mr. Kinton
Honors Chemistrry
Enloe High School

2. Chemical Equilibrium
Condition which the concentrations of all reactants and products in a closed system cease to change with time
Occurs when opposing reactions are proceeding at equal rates
Equal rates DOES NOT mean equal amounts
With gaseous reactions, we will use partial pressures in aqueous reactions we will examine concentration
When a reaction reaches equilibrium reactants do not stop reacting

3. Chemical Equilibrium Vocabulary
Law of mass action: rules by which the equilibrium constant is expressed in terms of the concentrations of reactants and products, based on a balanced chemical reaction
Equilibrium-Constant Expression: expression that describes the relationship among the concentrations (partial pressures) of the substances present in a system at equilibrium
Equilibrium Constant: The numerical value of the equilibrium- constant expression for a system at equilibrium, denoted by Keq

4. Equilibrium Constant Expression
Let’s examine 2 different general expressions
aA + bB cC + dD
If the reactants and products are gases the equilibrium expression is written as follows:
Keq= 𝑃𝐶 ^𝑐 𝑃𝐷 ^𝑑 𝑃𝐴 ^𝑎 𝑃𝐵 ^𝑏
If the reactants and products are aqueous solutions:
Keq= [𝐶]^𝑐[𝐷]^𝑑 [𝐴]^𝑎[𝐵]^𝑏

5. Equilibrium-Constant Expressions
Given the following reactions write an equilibrium expression:
N2O4(g) NO2(g)
Ag+(aq) + 2NH3(aq) Ag(NH3)2+(aq)

6. Magnitude of Equilibrium Constants
Equilibrium constants can have very large or very small values
The magnitude of the equilibrium constant will tell us which side will be most prevalent
Keq » 1: equilibrium lies to the right; products predominate
Keq « 1: equilibrium lies to the left; reactants predominate

7. Other ways to Manipulate an Equilibrium Expression
The equilibrium expression for a reaction written in one direction is the reciprocal of the one written in reverse
The direction of the reaction is what is most important
The equilibrium constant of a reaction can be multiplied by a number
An equilibrium of a net reaction made up of multiple steps is the product of the equilibrium constant for the individual steps
Go over examples that are on page 584 and 585 in the Brown and LeMay textbook

8. Homogeneous and Heterogeneous Equlibrium
All of the examples that we have examined have had the reactants and products all in the same phase (Homogenous)
In other cases, the substances in equilibrium will occur in different phases (Heterogeneous)
Partial pressures of gases are substituted into the equilibrium expression
Molar concentrations of dissolved substances are placed in the equilibrium expression
Pure solids, liquids, and solvents are not included in the equilibrium expression

9. Heterogeneous Equilibrium
Given the following chemical reactions, give the correct equilibrium expression:
CO2 (g) + H2 (g) CO (g) + H2O (l)
SnO2 (s) + 2CO (g) Sn (s) + 2CO2 (g)
Sn (s) + 2H+ (aq) Sn2+ (aq) + H2 (g)

10. Calculating Equilibrium Constants
For the scope of our class if you are asked to calculate the value of Keq all of the concentrations/pressures will be given to you
All you will need to do is write the equilibrium expression and then perform the calculation
Example: A mixture of H2 and N2 in a reaction vessel is allowed to attain equilibrium at 472oC. At equilibrium the partial pressure of each gas was measured and found to be atm H2, 2.46 atm N2 and atm NH3. Calculate Keq.

11. Le ChÂtlier’s Principle
If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its equilibrium to counteract the disturbance
Le Châtlier’s Principle can be used to make qualitative predictions about a reaction
Adding or removing a reactant or product
Changing the pressure
Changing the temperature

12. Change in Reactant/Product Concentration
At equilibrium a reaction is in a state of balance, if a change is made that balance is disrupted
In order to re-establish equilibrium the reaction will shift to counteract that change
Adding reactant/product will cause the reaction to shift to the opposite side of where substance was added
Removing reactant/product will cause the reaction to shift toward the side that had a substance removed
Adding or removing a pure liquid or solid will not alter the equilibrium

13. Changing Volume and Pressure
This change will only influence our reactions that contain gases in their equilibrium
Reducing the volume (increasing pressure) will cause the equilibrium to shift to the side with the fewest moles of gas
Increasing the volume (decreasing pressure) will cause the equilibrium to shift to the side with the most moles of gas
Additionally, these changes are assumed to be made at a constant temperature
Changing the concentrations or pressure DOES NOT change the value of Keq

14. Changing Temperature
Changing the temperature on a reaction at equilibrium will change the value of the equilibrium constant
For endothermic reactions, increasing T will increase Keq
For exothermic reactions, increasing T will decrease Keq
When temperature is being changed, we will treat heat like a reactant or product
Endothermic: Reactants + Heat Products
Exothermic: Reactants Products + Heat
Do example problems on page 600 to practice predicting shifts in equilibrium

15. Entropy and Equilibrium
Entropy is a measure of the amount of disorder in a system
When entropy increases it has a positive value, when entropy decreases it has a negative value
Entropy increases when:
Phase changes occur resulting in a less organized state (solid to liquid)
The number of particles increases (solid ground to a powder)
The number of product molecules is greater than the number of reactant molecules
When the temperature increases