1. or How Chemical Reactions Occur
REACTION MECHANISM
or How Chemical Reactions Occur

2. Conditions for reactants to be converted to products:
1. Reactant molecules should collide with sufficient force and proper orientation
2. The energy of collision must be strong enough to overcome the bond energy between the atoms.

4. ACTIVATION ENERGY
- Minimum amount of energy that must be supplied for a reaction to take place

5. ENDOTHERMIC REACTION

6. EXOTHERMIC REACTION

7. EFFECT OF THE PRESENCE OF A CATALYST ON THE ACTIVATION ENERGY

9. CHEMICAL EQUILIBRIUM
- Rate of forward reaction (reactant-to-product formation) is equal to the rate of backward formation (product-to-reactant formation)

10. CHEMICAL EQUILIBRIUM IN A REVERSIBLE REACTION

11. EQUILIBRIUM CONSTANT (Keq)
Given the reversible reaction:
aA + bB <=> cC + dD
A and B are the formulas of the reactants
C and D are the formulas of the products
a, b, c, and d are the coefficients in the balanced equation

12. Example:

13. Given the reaction : CO + H2 ⇆ CH3OH What is the Keq for the reaction?

14. A large equilibrium constant means that, at equilibrium, there will be a high concentration of products and a low concentration of reactants.

15. A small equilibrium constant means that, at equilibrium, there will be a high concentration of reactants and a low concentration of products.

16. Changing concentration affects equilibrium
When a system at equilibrium is disturbed, it shifts to minimize the disturbance. In this case, adding NO2 (the disturbance) causes the reaction to shift left, consuming NO2 (minimizing the disturbance).

17. Addition of N2O4 (the disturbance) causes the reaction to shift right, producing NO2 (minimizing the disturbance).

18. Increased pressure can affect equilibrium
When the volume of an equilibrium mixture is decreased, the pressure increases. This system responds (to bring the pressure back down) by shifting to the right, the side of the reaction with the fewest moles of gas particles.

19. When the volume of an equilibrium mixture is increased, the pressure decreases. This system responds (to raise the pressure) by shifting to the left, the side of the reaction with the most moles of gas particles.

20. Heat affects equilibrium
If the reaction is exothermic, heat can be considered a product; adding heat to the system would be like increasing the concentration of a product species: the equilibrium would shift left.

21. If the reaction is exothermic, heat can be considered a product; removing heat from the system would be like decreasing the concentration of a product species: the equilibrium would shift right.

22. Equilibrium as a function of temperature
N2O4(g) ⇆ 2 NO2(g)
Since the reaction is endothermic, cool temperatures cause a shift to the left to colorless N2O4. Warm temperatures cause a shift to the right to brown NO2