1. Chemical Equilibrium
This video is an intro. To chemical equilibrium.

2. What is Chemical Equilibrium?
Reversible Reaction
-a chemical reaction in which the products can react to re-form the reactants.
Chemical Equilibrium
the forward reaction equals the rate of its reverse reaction and the concentrations of its products and reactants remain unchanged
How is it Written?
use double arrows
2HgO(s)  2Hg(l) + O2(g)
So far all the reactions we have dealt with have gone to completion. For example when NaCl(aq) + AgNO3(aq) are mixed, a precipitate, AgCl, is produced with NaNO3.
Some reactions do not run to completion. Instead the products that formed in the reaction may re-form the original reactants.This is called a reversible reaction.
Since it is reversible the reaction can proceed in two directions, forward and reverse.
Let’s look at an example.
Mercury (II) oxide decomposes when heated
2HgO(s)  2Hg(l) + O2(g)
Mercury and Oxygen combine to form mercury (II) oxide when heated gently
2Hg(l) + O2(g)  2HgO(s)
If we had this chemicals in a test tube and heated the mercury(II) oxide would decompose into the mercury and oxygen. After enough product has formed then the mercury and oxygen would recombine to form mercury (II) oxide.
When the forward rate equals the reverse rate we now have chemical reaction that is in chemical equilibrium

3. The Equilibrium Expression
Equilibrium Constant
-ratio of the concentrations of the products to the concentrations of the reactants
there is no unit and it is dependent on temperature
Equilibrium Expression
K= [C]x [D]y
[A]n [B]m
n,m,x,y are the coefficients and are raised to the power equal to the coefficient of the substance
The [ ] represent the concentrations of the substances
Lets look at a generic equation.
nA +mB  xC +yD
The capital letters are the substances and the italic letters are the coefficients.
Initially the concentrations of C and D are zero and those of A and B are maximum. Over time A and B are used up and C and D are formed. When the forward and reverse rates become equal equilibrium is established. The individual concentrations of A,B,C, and D undergo no further change if conditions remain the same. After equilibrium is attained the concentrations of products and reactants remain constant, so a ratio of their concentrations should also remain constant. This ratio is called the Equilibrium constant.
The equation for K is sometimes referred to as the equilibrium expression

4. Equilibrium Constant Example
An equilibrium mixture of N2, O2, and NO gases at 1500 K is determined to consist of 6.4 x 10-3 M of N2, 1.7 x 10 –3 M of O2, and 1.1 x 10-5 M of NO. What is the equilibrium constant for the system at this temperature?
N2(g) + O2(g)  2 NO(g)
K= [NO]2
[N2] [O2]
K=[1.1 x10-5]2
[6.4 x 10-3][1.7 x10-3]
K= 1.1 x 10-5
Homework p. 474, practice problems 1-4

5. Shifting Equilibrium What is Le Chatelier’s Principle?
If a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress.

6. What stresses or changes affect equilibrium?
Changes in Concentration
a. Increase(add) a reactant or product will shift away from the added reactant or product
b. Decrease ( remove) a reactant or product will shift toward the removed reactant or product
An increase or decrease in the concentration of a reactant or product would produce a stress on an equilibrium system.

7. Change in concentration cont.
A + B  C + D
Which way will it shift if we:
Add B
Shift right
Add C
Shift left
Remove A
Remove D

8. Equilibrium stresses cont.
Changes in pressure
only affects equilibrium systems in which gases are involved
a. If the pressure is increased,it will shift towards the side with the smaller number of molecules
b. If the pressure is decreased, it will shift towards the side with the larger number of molecules.
Remember Boyle’s Law that pressure and volume have an inverse relationship. If one goes up the other goes down and vice versa.
If pressure is increased( goes up), what happens to the volume of the container. It gets smaller.
If pressure is decreases ( goes down) what happens to the volume of the container. It gets larger.

9. Change in pressure cont.
N2(g) + 3H2 2NH3
What would happen if we increased the pressure?
Shift right
What would happen if we decreased the pressure?
Shift left

10. Equilibrium stresses cont.
Changes in Temperature
a. Increase ( add) temperature will cause a shift away from the added temperature
b. Decrease (removal) temperature will cause a shift toward the removed temperature
Reversible reactions are exothermic in one direction and endothermic in the other.
The effect of changing temperature depends on which of the opposing reactions is endothermic and which is exothermic.
If a reaction is endothermic, heat energy is treated as a reactant.
If a reaction is exothermic, heat energy is treated as a product.

11. Changes in temperature cont.
556 kJ + CaCO3(s)  CaO(s) + CO2(g)
What would happen if we increased the temperature?
Shift right
What would happen if we decreased the temperature?
Shift left

12. States of matter in Equilibrium problems
Have you noticed that all the examples that I have used have only included the gas or aqueous state? Why?
The concentrations of pure substances in solids and liquids phases are not changed by adding or removing quantities of the substance
They are not included in the equilibrium expression.
Write this equation on the board:
CaCO3(s) CaO(s) + CO2(g)
What is the equilibrium expression for this equation
K= CO2

13. Solubility Product Constant
Is the product of the molar concentrations of its ions each raised to the power of its coefficients
Example:
BaSO4(s)  Ba2+(aq) + SO42-(aq)
Ksp= [Ba2+] [SO42-]
Ksp is an equilibrium constant for slightly soluble ionic substance
AgCl is slightly soluble. When put into water it will slightly dissociates into its ions of Ag+ and Cl-
If we look at its equilibrium expression we get
Ksp= [Ag+] [Cl-]

14. Example of Ksp Consider the dissociation of the salt CaF2.
CaF2(s)  Ca2+(aq) + 2F-(aq)
At a temperature of 298 K, the concentration of Ca2+ ions is 2.2 x10-4 M, and Ksp for the salt is 1.46 x What is the concentration of F- in the solution?