1. Reaction Rates and Activation Energy

2. Kinetics
The study of
how fast chemical reactions take place, changing reactants into products
how much energy is used and produced in the reaction

3. Reversible Reactions
All reactions are reversible, under the right circumstances
Most reactions proceed in one direction (“forward”, reactants  products) easily, and it is much more difficult to get them to proceed the other way (“backward”, products  reactants)

4. Reversible Reactions
Some reactions easily reverse. For example, in the presence of
heat
water
salt
Iron will continuously form rust, and reform iron:

5. Collision Theory
Reactants must collide with enough energy to form products. The rate at which this occurs depends on temperature, nature of reactants, and concentration.

6. Collision Theory, cont.
Effective Collision: A collision between two reactants where a new substance results
Reaction Rate: How fast effective collisions occur among reactants to form products
Activation Energy (Ea) : The minimum energy required to have an effective collision

7. Collision Theory, cont.
Activated Complex: A transitional structure that results from an effective collision and that exists while old bonds are breaking and new bonds are forming; this is VERY short-lived and only exists until the product is formed.
Reactants
Activated
Complex
Products

8. Enthalpy: Heat of a reaction
Heat and Reactions
Exothermic:
Heat is released:
Enthalpy is negative, ∆H = -
ex. Heat pack
Endothermic:
Heat is absorbed
Enthalpy is positive, ∆H = +
ex. Cold pack
Enthalpy: Heat of a reaction
∆H = Hfinal - Hinitial
(∆G = total energy in reaction)

9. Another reversible reaction
HgO, mercury (II) oxide, when heated produces mercury and oxygen.
Also, the mercury and oxygen reform HgO.

10. Cinnabar with mercury

11. Exothermic Reactions (∆H = -)
Potential Energy
Activated Complex
Activation Energy (Ea)
CH4 + 2O2
H of reactants ∆H
CO2 + 2H2O
H of products

12. Endothermic Reactions (∆H = +)
Activated Complex
Activation Energy (Ea)
CH4 + 2O2 ∆H
H of products
Potential Energy
CO2 + 2H2O
H of reactants

13. Two Factors Affecting Rxn. Rate
Temperature: As temp. rises, the rxn. can proceed faster because there is more kinetic energy -- particles can collide with more energy and produce more effective collisions.
Concentration: As conc. rises, there are more particles packed into the same volume so more effective collisions can occur.

14. Two More Factors Affecting Rxn. Rate
Surface Area: Smaller particles have more surface area per given mass and have a better chance of having an effective collision; crushing increases surface area.
Catalyst: A catalyst is a substance that increases the rate of the rxn. without being used up at the end of the rxn. It lowers the activation energy required so more products are easily formed.

15. Energy and Reactions
Energy: It takes energy to cause a reaction. If there is not enough energy, reactants will not become products.
Activation Energy: Minimum amount of energy required to turn reactants into products.
Molecules collide, but in order for valence electrons of one substance to bond with valence electrons of another, bonds need to be broken and reformed -- this takes energy!

16. Activation Energy ex. Paper + O2 NR (No Reaction)
ex. Paper + O CO2 + H2O

17. Catalysts and Activation Energy
What does a catalyst do to the activation energy?
P E ∆H
Activation
Energy (Ea)
WITHOUT Catalyst
P E ∆H
Activation
Energy (Ea)
WITH Catalyst

18. Review/Tutorial
We are going to review what you just learned, using an online tutorial. (Collision Theory)