1. EQUILIBRIUM

2. How Chemical Reactions Occur
Collision model – molecules must collide in order for a reaction to occur
Rate depends on concentrations of reactants and temperature.

3. Conditions That Affect Reaction Rates
Concentration – increases rate because more molecules lead to more collisions
Temperature – increases rate
Why?

4. Conditions That Affect Reaction Rates
Activation energy – minimum energy required for a reaction to occur

5. Conditions That Affect Reaction Rates
Catalyst – a substance that speeds up a reaction without being consumed
Enzyme – catalyst in a biological system

6. Reactions
Homogeneous reaction – all reactants and products are in one phase
Gas
Solution
Heterogeneous reaction – reactants in two phases

7. Reactions

8. The Equilibrium Condition
Equilibrium – the exact balancing of two processes, one of which is the opposite of the other

9. The Equilibrium Condition
Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant

10. Chemical Equilibrium: A Dynamic Condition
Equal numbers of moles of H2O and CO are mixed in a closed container.
The reaction begins to occur, and some products (H2 and CO2) are formed.
The reaction continues as time passes and more reactants are changed to products.
Although time continues to pass, the numbers of reactant and product molecules are the same as in (c). No further changes are seen as time continues to pass. The system has reached equilibrium.

11. Chemical Equilibrium: A Dynamic Condition
Why does equilibrium occur?

12. The Equilibrium Constant: An Introduction
Law of chemical equilibrium
For a reaction of the type
aA + bB  cC + dD
Equilibrium expression
Each set of equilibrium concentrations is called an equilibrium position.

13. Applications Involving the Equilibrium Constant
The Meaning of K
K > 1  the equilibrium position is far to the right
*Favors products
K < 1  the equilibrium position is far to the left
*Favors reactants

14. Heterogeneous equilibria – an equilibrium system where the products and reactants are not all in the same state
*Their value is 1

15. Le Chatelier’s Principle
Le Chatelier’s Principle – when a change is imposed on a system at equilibrium the position of the equilibrium shifts in a direction that tends to reduce the effect of that change

16. Effect of a Change in Concentration

17. Effect of a Change in Concentration
When a reactant or product is added the system shifts away from that added component.
If a reactant or product is removed, the system shifts toward the removed component.

18. Effect of a Change in Volume
The system is initially at equilibrium.
The piston is pushed in, decreasing the volume and increasing the pressure. The system shifts in the direction that consumes CO2 molecules, lowering the pressure again.

19. Effect of a Change in Volume
Decreasing the volume
The system shifts in the direction that gives the fewest number of gas molecules.

20. Effect of a Change in Volume
Increasing the volume
The system shifts in the direction that increases its pressure.

21. Effect of a Change in Temperature
The value of K changes with temperature. We can use this to predict the direction of this change.
Exothermic reaction – (heat is a product)
Adding energy shifts the equilibrium to the left (away from the heat term).
Endothermic reaction – (heat is a reactant)
Adding energy shifts the equilibrium to the right (away from the heat term).

22. The value of K for a system can be calculated from a known set of equilibrium concentrations.
Unknown equilibrium concentrations can be calculated if the value of K and the remaining equilibrium concentrations are known.