1. Basic Equilibrium Principles 18.1

3. Dynamic Equilibrium in Reactions

4. Dynamic Equilibrium with Molecules

5. Reversible Reaction Model

6. Reversible Reaction Graph

7. “Favored” products and reactants
Chapter 18
“Favored” products and reactants
products of the forward reaction favored, lies to the right
products of the reverse reaction favored, lies to the left
Neither reaction is favored

8. Equilibrium Constant (KC)
After equilibrium is reached, the individual concentrations of A, B, C, and D undergo no further change if conditions remain the same.
A ratio of their concentrations should also remain constant.
The equilibrium constant is designated by the letter K.

9. Equilibrium Constants
K = [Products]
[Reactants]

10. K Values in relation to products & reactants
Favors reactants
(little product formed)
Favors products
(more products exist than reactant)

11. Shifting Equilibrium & Le Châtelier’s Principle (18.2)

12. Reaction Quotient (Q)
K describes an equation that is at equilibrium, whereas Q describes a reaction that is not at equilibrium. Q helps you to predict which way a stressed system will shift.
Stress
applied
Stress
applied

13. Henri Louis Le Châtelier 1888
Le Châtelier’s principle states that if a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress.
this is true for chemical & physical equilibria

14. Shift due to change in pressure
This “stress” only affects systems involving gases.
To have a change in pressure, the number of moles of gas on the right side of a chemical equation must not equal the number of moles of gas on the left side of the chemical equation.
An increase or decrease in pressure is due to the change (adding or subtracting) in the number of moles of gas.
Equilibrium favors the side with the least moles.
Ex. 2NO N2O4

15. Shift due to change in pressure
Increase in pressure = products are favored (increase) shift right
(increase in product)
Decrease in pressure = reactants are favored (increase) shift left
(increase in reactant)
Gas leaks out = products are favored (increase) shift right, the rxn runs to completion
Ratios are constant so K does not change
If you add a gas that is not a reactant or a product, K will not change, but pressure will rise.

16. Shift due to change in concentration
Ratios are constant so K does not change
Pure substances (ex. H2O) do not change in concentration and would not be shown in K calculation

17. Shift due to change in concentration
Increase in reactant = products are favored (increase) shift right
Increase in product = reactants are favored (increase) shift left
Decrease in product = products are favored (increase) shift right
If products are insoluble precipitate or water, the rxn runs to completion

18. Shift due to change in temperature
All reactions that are considered reversible have opposing thermic reactions.
In one direction, the reaction is exothermic and in the opposite reverse reaction, endothermic.
The addition of energy in the form of heat shifts the equilibrium so that energy is absorbed. This favors the endothermic reaction.
heat + A B A B + heat
(endothermic) (exothermic)
The removal of energy in the form of heat shifts the equilibrium so that energy is released. This favors the exothermic reaction.
A B + heat (exothermic)
heat + A B (endothermic)
Excess heat is shifted to the other side to relieve stress

19. Shift due to change in temperature
The Equilibrium Constant (K) is affected by temperature changes.
Adding heat to the exothermic reaction decreases K.
Adding heat to the endothermic reaction increases K.
Catalysts are independent of the chemical reaction/equation and therefore have no effect on K.
Catalysts increase the rates of forward and reverse reactions in a system by equal factors. Therefore, they do not affect K.