1. Mini-Unit 12: Reaction Rates and Equilibrium
Introduction to Rates of Reaction

2. Reaction Rates Rxn speed: Examples Very fast Moderate Very slow
Mg and hydrochloric acid, sodium and water, burning a match, combustion of fuel, etc.
Food spoiling, rusting of iron, baking a cake, food digestion, cooking, etc.
Forming diamonds, weathering of rocks, setting concrete, silver tarnishing, etc.

3. Rates
A rate measures changes that occur within an interval of time.

4. In chemistry, the rate of a reaction is expressed as the amount of reactant changing per unit time. (Example: mol/sec)

5. Collision Theory
Collision theory: atoms, ions, and molecules can react to form products when they collide with one another, provided they have enough kinetic energy.
If they do not have enough kinetic energy they will collide then bounce apart.
Similarly, bonds holding the molecules together can break apart if supplied with enough energy.

6. Activation Energy (Ea)
The minimum energy colliding particles must have in order to react.
During a reaction there is an in-between state called the activated complex or transition state which is the unstable arrangement of atoms at the peak of the activation energy barrier. (Exists for a very short amount of time, ~10-13 seconds.)

7. Energy Changes in a Reaction
Ea of forward rxn
Energy of products
Enthalpy (ΔH) of reaction
Energy of reactants
Time

9. Factors That Affect Reaction Rates
The rate of a chemical reaction depends on:
Temperature
Concentration
Particle size
Adding a catalyst

10. Temperature
Increasing the temperature raises the frequency of collisions and the number of particles that have enough kinetic energy to get over the energy barrier.

11. Concentration
Increasing the concentration causes an increase in the frequency of collisions.

12. Particle Size
The smaller the particle size, the larger the surface area for a given mass of particles.
The activation energy is lower in the catalyzed reaction vs. the uncatalyzed reaction.

13. Adding a Catalyst
The addition of a catalyst lowers the energy barrier of the activated complex. Example: 2H2(g) + O2(g)  2H2O(l) Pt

14. Catalyst: Increases the rate of a reaction by lowering the Ea
Catalyst: Increases the rate of a reaction by lowering the Ea. Inhibitor: A substance that decreases the rate of a reaction by increasing Ea.