1. Unit 12: Reaction Rates & Equilibrium
Chemistry
Unit 12: Reaction Rates & Equilibrium
Lecture #1

2. Objectives Define Reaction Rates Describe the collision theory
Interpret Reaction Profile (Coordinate) Graphs
List the factors that affect reaction rates and explain HOW they do so.

3. Big Idea Not all reactions happen at the same rate!
Some are really fast…
Explosion of dynamite
(completely finished in a fraction of a second!!)
Some are really slow…
Oxidation of copper (patina)
(not completely reacted for many years!!)

4. Reaction Rate Two Ways to Define:
The rate at which the concentration of the reactants decreases (disappear)
The rate at which the concentration of the products increases (appears)

5. Controlling Reaction Rates
Often chemists try to control the rate of a chemical reaction! Sometimes we want to speed it up… Like a paint formula that will dry quickly Sometimes we want to slow it down… Like a paint formula that will resist fading

6. Collision Theory Molecules must COLLIDE to react
In effective collisions existing bonds are broken as new bonds are formed, thus transforming reactants into products.

7. Requirements for Effective Collisions
Must have frequent collisions
Collisions must have sufficient energy
Collisions must have proper orientation (geometry)

8. Behind-the-Scenes of a Chem. Rxn.
For a chemical reaction to occur, the reactants must collide with sufficient energy to react.
This energy is required to achieve the transition state (activated complex) required to form the products (b)
Without sufficient
energy, the reaction
does not occur (a)

9. Reaction Profiles
Shows the energy of the reactants and the products during a chemical reaction. The highest point is the transition state (activated complex) Activation Energy Heat of Reaction (ΔH or q)

10. Endothermic Reactions
An endothermic reaction absorbs heat as the reaction proceeds: 2H2O + heat  O2(g) + 2H2(g)
For endothermic
reactions, the
energy of the
reactants is lower
than the energy
of the products

11. Exothermic Reactions
An exothermic reaction releases heat as the reaction proceeds: NO2 + CO  NO + CO2 + heat
For exothermic reactions, the energy of the reactants is higher than the
energy of the
products

12. Catalysts
A catalyst is a substance that allows a reaction to go faster by lowering the activation energy. It does this by fining an alternate pathway for the reaction to occur. Catalysts sometimes also help with orientation Do not change ΔH Speed up both fwd. and reverse rxns.
An inhibitor is the opposite of a catalyst…it slows down a chemical reaction

13. Factors That Affect Reaction Rate
Temperature: An increase in temp. means faster moving particles. This means more collisions, AND collisions with greater energy.  FASTER Rxn.
Concentration: An increase in concentration means more particles. This means more collisions  FASTER Rxn
Pressure: If reactants OR products are gases, then increasing the pressure crowds the particles. This means more collisions.  FASTER Rxn.
Surface Area: Rxns only happen on the surface. More surface area  FASTER Rxn.
Catalysts: Aid in orientation, or provide alternate pathways.  FASTER Rxn.

14. Write Summary & Questions