1. Unit 11: Energy of Reactions
Exam: Next Thursday, March 26

2. Today’s Learning Objective
You should be able to explain to me what the Collision Theory means and how it works.
You should be able to describe what increases or decreases reaction rates.

3. Review Chemical vs Physical changes and properties
Chemical change & property – change in formula composition
Ex: CO becoming CO2
Physical change & property – describes appearance and does not change formula composition
Ex: NaCl dissolving in H2O is still Na + Cl and H2O

4. Review Extensive vs Intensive properties
Extensive – changes depending on the amount of substance
Ex: mass of a person, volume of a balloon
Intensive – does not change depending on the amount of substance
Ex: density of gold, the atomic mass of helium

5. Review (cont.)
A new substance is formed when e- are transferred (ionic compound) or shared (covalent compound)
Remember the rules for naming both the covalent and ionic formulas!
Exothermic vs Endothermic rxns
Exothermic – rxn releases E
Endothermic – rxn absorbs E

6. Definitions
Chemical change = an intensive property that occurs when bonds break or form
Ex: 2H2 + O2  2H2O
Chemical reaction = shows the direction of reactions  products
There are 4 types of substances: solid (s), liquid (l), gas (g), and aqueous (aq)
Law of Conservation of Energy – the total amount of E in a system cannot change but its form can
Lightbulbs convert E into light E and heat E

7. Definitions (cont.) Breaking vs forming bonds
Breakings bonds requires E (so ______thermic)
Forming bonds releases E (so ______thermic)
Enthalpy (H) change – the difference in energies between the reactants and products
Endothermic = the products E > reactants E
Exothermic = the products E < reactants E

8. Definitions (cont.)
Collision Theory –in order to make or break bonds, reactant particles have to collide with sufficient E and correct orientation

9. Reaction Rate
Reaction Rate is the number of particles that react in a given time to form products
We can measure this by how fast the reactants disappear or how fast the products appear 9

10. Reaction Rate Graph For the reaction H2 + I2 → 2HI,
What is happening to the concentration of the reactants?
What is happening to the concentration of the products? 10

11. Factors that Affect Reaction Rate
(Hint: Think of Gas Laws, PV = nRT and yesterday’s lab)

12. Factors that Effect Reaction Rate
Concentration of Reactants
The more particles, the more collisions
As concentration increases, rate increases 12

13. Factors that Effect Reaction Rate
Temperature of Reactants
As temperature increases, particles move faster; there are more high-energy collisions 13

14. Factors that Effect Reaction Rate
Particle Size (surface area)
For solids, breaking up big pieces increases surface area, increasing rate by having more places for the molecules to interact
Ionic compounds have more surface area than covalent 14

15. Factors that Effect Reaction Rate
Pressure – Gases only!
As pressure increases the concentration increases, so you will have more collisions 15

16. Factors that Effect Reaction Rate
Catalysts – compounds added to speed up reaction but not used in the reaction.
Lowers activation energy but does not change ΔH
Examples:
Enzymes for the body (digestion, metabolism, contact cleaner)
Catalytic converters (platinum, palladium, rhodium, etc. plates that convert toxic gases to less harmful products like NO → N2 + O2 and CO → CO2) 16

17. Catalyst 17

18. Factors that Effect Reaction Rate
Inhibitors
Compounds added to a reaction that slow it down
Examples: Lead in diesel, preservatives in food. 18

19. Reaction Rates Discussion
Why do you add lighter fluid to charcoal in a grill?
How does opening and closing air vents on a charcoal grill effect how fast the coals burn?
How can an explosion occur in a grain elevator when grain is not considered flammable? 19