1. Can a chemical reaction go backwards?
Target 6-1.

2. Quick Review
What happens to PARTICLES to make a chemical reaction happen?
What is ACTIVATION ENERGY?
What is the difference between ENDOTHERMIC and EXOTHERMIC reactions?
What are the five things that can change the RATE of a chemical reaction?
What is the difference between REACTANTS and PRODUCTS in a chemical reaction?

3. Quick Review
What happens to PARTICLES to make a chemical reaction happen?
Collision Theory
Particles must collide – break bonds and make new bonds
Particles need kinetic energy
Particles need to be in the correct orientation

4. Quick Review What is ACTIVATION ENERGY?
Energy needed to break bonds and start the reaction

5. Quick Review
What is the difference between ENDOTHERMIC and EXOTHERMIC reactions?
Endothermic – absorb energy
Exothermic – release energy

6. Quick Review
What are the five things that affect the RATE of a reaction?
Temperature (kinetic energy)
Concentration (amount)
Pressure (squeezing)
Surface Area (powders)
Catalysts (activation energy)

7. Quick Review
What is the difference between REACTANTS and PRODUCTS in a chemical reaction?
Reactants = starting substance(s)
Products = ending substance(s)

8. Can a chemical reaction go backwards?
What We Think
Our Questions

9. Why do substances WANT to react?
Energy
Chemical processes naturally want to RELEASE energy (exothermic)
Once the activation energy allows bonds to break, they re-form very very quickly
Examples
Reaction of hydrogen + oxygen (exploding balloon)
Burning phosphorous (“lightbulb” flask)
So…why do endothermic reactions happen?

10. Why do substances WANT to react?
Entropy
Chemical processes naturally want to INCREASE entropy
Remember: entropy = disorder/chaos in a system
How to increase entropy:
More molecules = more entropy
Solid  Liquid  Gas
Examples:
Barium hydroxide and ammonium chloride powders
Melting ice

11. Energy vs. Entropy
Basically, substances want to have a reaction if it decreases energy or increases entropy.
If it does both, they will definitely react.
But what if energy and entropy disagree?

12. Does the reaction WANT to happen?
Energy vs. Entropy
Does the reaction WANT to happen?
Energy Increases
Energy Decreases
Entropy Increases
???
YES
Entropy Decreases NO

13. Energy vs. Entropy
Sometimes energy wants the reaction to happen, but entropy does not (or vice versa)
These reactions will go both ways. They are called reversible reactions.

14. Reversible Reactions
Reversible Reaction: a reaction that can occur in both directions at the same time
Forward and Reverse reaction are OPPOSITES
One direction favored by entropy
One direction favored by energy

15. Reversible Reactions Example: N2 + 3 H2 ↔ 2 NH3 + heat
Forward Reaction:
N2 + 3 H2  2 NH3 + heat
Reverse Reaction:
2 NH3 + heat  N2 + 3 H2
Reactants
Products
Endothermic or Exothermic?
Favored by entropy or energy?

16. Reversible Reactions Example: N2 + 3 H2 ↔ 2 NH3 + heat
Forward Reaction:
N2 + 3 H2  2 NH3 + heat
Reverse Reaction:
2 NH3 + heat  N2 + 3 H2
Reactants
N2 + 3 H2
2 NH3 + heat
Products
Endothermic or Exothermic?
Exothermic
(heat is a product!)
Endothermic
(heat is a reactant!)
Favored by entropy or energy?
Energy
Entropy