1. Energy Changes & Reactions Rates
Unit 7 - Chapter 7.3 & 7.4

2. Chemical Bonds and Energy
Chemical reactions:
Breaking chemical bonds in the reactants
Forming new chemical bonds in the products.
Chemical energy: Energy stored in the chemical bonds of a substance.
True for both reactants and products

3. Chemical Bonds and Energy
Energy changes in reactions  determined by changes in chemical bonding.
Breaking chemical bonds requires energy
Forming of chemical bonds releases energy.
For combustion reactions  Heat and light are given off because new chemical bonds formed

4. Chemical Bonds and Energy

5. Chemical Bonds and Energy
Exothermic and Endothermic Reactions
During a chemical reaction, energy is either released or absorbed.
Exothermic
Releases energy to its surroundings
“Exo” means Energy “exits” from the reaction
Energy required to break reactant bonds  less than energy released as products formed

6. Chemical Bonds and Energy
Exothermic reaction: chemical energy of reactants is greater than chemical energy of products.

7. Chemical Bonds and Energy
Chemical energy peaks before reactants change into products.
Peak Height = energy required to break the chemical bonds of the reactants.
Particles must collide with enough energy to break these bonds, or the reaction will not occur.

8. Chemical Bonds and Energy
Endothermic - Absorbs energy from surroundings
“Endo” means Energy goes “into” the reaction
Example: Emergency Cold Packs
Energy required to break reactant bonds  greater than energy released by formation of products.

9. Chemical Bonds and Energy
Endothermic reaction - energy of products is greater than energy of reactants.

10. Chemical Bonds and Energy
Conservation of Energy:
Total Energy is same before and after a reaction
Exothermic reaction:
Chemical energy of reactants = heat + chemical energy of products.
Reactants  Heat + Products
Endothermic reaction:
Chemical energy of reactants + heat = chemical energy of products.
Reactants + Heat  Products

11. Reaction Rates Reaction Rates tell you:
 the rate that reactants change into products
Fast or slow, controlled or uncontrolled - total amount of energy released is the same in all cases

12. Reaction Rates What is a Rate? Reaction rates are often expressed as:
Changes over time are expressed as rates.
Reaction rates are often expressed as:
How fast reactants are consumed <or>
How fast products are formed <or>
How fast energy is absorbed or released

13. Some Factors Affecting Chemical Reaction Rates:
Temperature
Surface area
Concentration
Stirring
Catalysts

14. Factors Affecting Reaction Rates: General
Reaction rate depends on how often reactant particles collide.
If collisions are more frequent reaction rate increases.
If collisions are less frequent reaction rate decreases.
Reaction rates can be changed
Vary conditions so collisions increase or decrease

15. Factors Affecting Reaction Rates: Temperature
Increasing temperature increases reaction rate
Decreasing temperature decreases reaction rate
Example: Milk stored in a refrigerator slows down the reactions that cause the milk to spoil.
WHY?
Increased temperature  particles moving faster
Collide more often
 Collide with more energy
 faster rate of reaction

16. Factors Affecting Reaction Rates: Surface Area
For a given mass of material:
smaller particle sizes  greater surface area
Increased surface  increased collisions 
More particles reacting  Faster reaction rate
Examples:
Crushed ice melts much more quickly than large block of ice
Small twigs and branches burn much more quickly than big logs

17. Factors Affecting Reaction Rates: Stirring
Stirring reactants:
Increases particles exposure to each other (moving faster – colliding more often)
More exposure  faster reaction rate
Example: Clothes in washing machine

18. Factors Affecting Reaction Rates: Concentration
Concentration = # of particles in a given volume.
More particles in a given volume 
more opportunities for collisions 
reaction rate is faster
Greater the concentration  faster the reaction rate

19. Factors Affecting Reaction Rates: Concentration
Dye solution in left beaker is more concentrated than solution in right.
Increasing concentration of dye increases rate of color change in the material.

20. Factors Affecting Reaction Rates: Gas Pressure
For gases, concentration changes with pressure.
Greater the pressure of a gaseous reactant 
Greater is its concentration
Faster reaction rate.

21. Factors Affecting Reaction Rates: Catalysts
Catalyst: substance that affects the reaction rate without being used up in the reaction.
Catalysts can:
Speed up a reaction <or>
Enable a reaction to occur at lower temperature.

22. Factors Affecting Reaction Rates: Catalysts
Ex: One of the ways to make sulfuric acid is to react sulfur dioxide with oxygen to form sulfur trioxide.
This reaction happens very slowly without a catalyst
[vanadium(V) oxide is a good catalyst for this reaction]
The catalyst is neither a reactant nor a product, so it is written over the arrow.

23. Factors Affecting Reaction Rates: Catalysts
Catalyst lowers energy required for effective collisions

24. Exit Slip Questions
1. Label the following graphs as Endo or Exothermic and mark the change in energy: 24

25. Exit Slip Questions
2. A reaction rate is the rate at which reactants change into products over time. True False

26. Exit Slip Questions
3. What are two things that might happen to a reaction if a catalyst is added to the reaction? 26

27. Exit Slip Questions
4. What impact will each of the following changes have on the rate of most chemical reactions (increase or decrease)?
decreasing the temperature
grinding a reactant into a fine powder
stirring the reaction mixture
decreasing the concentration of one of the reactants