1. Equilibrium and collision theory
Chemistry
Chapter 17
Equilibrium and collision theory
Chemistry
Zuhumdal
Last revision Fall 2014

2. Collision Theory
Rates of chemical reactions are related to the properties of atoms, ions, and molecules through a model called collision theory.
According to collision theory, atoms, ions, and molecules can react to form products when they collide, provided they have enough kinetic energy.

3. Think clay
If you throw 2 clay balls together gently, they do not stick together, kinda like the lack of reaction between colliding particles with low energy.
If you throw the same balls of clay together with great force, they stick together.

4. Activation Energy
Activation energy is a barrier that reactants must cross to be converted to products.

5. Activated Complex
An activated complex is the arrangement of atoms at the peak of the activation-energy barrier
Also called transition state.

6. Factors Affecting Reaction Rates
Temperature
Concentration
Particle Size
Catalysts
Pressure

7. Temperature Concentration
Usually, raising the temperature speeds up the reactions, while lowering the temperature slows down the reactions.
This will increase the number of particles that have enough kinetic energy when they collide.
Concentration
Cramming more particles into a fixed volume increases the concentration of reactants.

8. Particle Size Catalyst
The smaller the particle size, the larger the surface area for a given mass of particles.
Which burns faster, a piece of wood or a kindle? Why?
Catalyst
A catalyst is a substance that increases the rate of a reaction without being used itself.
Catalyst permit reactions to proceed at a lower energy than required.

9. Reversible Reaction
Do reactions only go one way or do reactions go 2 ways?
Let’s look at this equation:
2SO2(g) + O2 ---- 2SO3
What happens when sulfur dioxide and oxygen gas are mixed in a sealed container? 

10. Reversible Reactions
As the SO3 concentration increases, a small amount collides and reverts to SO2 and O2 by the reverse direction!
As the concentration of SO3 becomes higher and higher, the reverse reaction speeds up!
Eventually SO3 decomposes to SO2 and O2 as fast as it forms SO3.

11. Dynamic Chemical Equilibrium
Equilibrium occurs when opposing reactions occur at equal rates and the concentrations of the products and reactants stop changing.
Reactants and products are not necessarily equal in amount.
And the forward and reverse reactions have not stopped.

12. Le Chatelier’s Principle
If a stress is applied to a system in dynamic equilibrium, the system changes to relieve stress.
Stresses include changes in the concentration of reactant or products, changes in temperature, and changes in pressure

13. Le Chatelier's Principle >> Concentration Changes
The concentration can be changed by adding or removing a substance.
If added, the equilibrium shifts to remove the substance.
If removed, the equilibrium shifts to replace it.
Adding or removing (provided some remains) solid, pure liquid, or solvent does not affect the equilibrium, since the concentration of these substances is constant.

14. Le Chatelier's Principle >> Concentration Changes
Example 15
For the reaction,
CH3OH(g) + O2(g)  HCOOH(g) + H2O(g)
What direction does the equilibrium shift if more oxygen is added?
What direction does the equilibrium shift if water is removed?
How does the concentration of methanol (CH3OH) change if more oxygen is added?
How does the concentration of methanol change if more water is added?
How does the concentration of methanol change when more methanol is added?

15. Le Chatelier's Principle >> Pressure Changes
Changing pressure only affects gases.
Higher pressures favor fewer moles of gas
Lower pressures favor more moles of gas.
Only consider moles of gas (not other physical states) in determining equilibrium shifts.

16. Le Chatelier's Principle >> Pressure Changes
Example 16
How does an increase in pressure affect the concentration of the reactants in the following reactions?
C2H4(g) + H2(g)  C2H6(g)
Xe(g) + 3F2(g)  XeF6(g)
C(s) + 2F2(g)  CF4(g)
H2S(g) + Hg(l)  HgS(s) + H2(g)

17. Le Chatelier's Principle >> Temperature Changes
The easiest way to predict equilibrium shifts is to consider energy (or heat) as a product or a reactant.
Energy is a product in exothermic reaction (– H)
Energy is a reactant in endothermic reactions (+ H).
Increasing temperature increases the "concentration of energy."

18. Le Chatelier's Principle >> Temperature Changes
Example 17
How does the concentration of the product change if the temperature increases?
Fe2S3  2Fe S 2–       + H
Si + 2F2  SiF4         – H