1. Reaction rates and equilibrium
Chapter 18

2. Rate and Collision Theory
Rate: Measure of the speed of any change that occurs within an interval of time.
For a chemical reaction rate: Amount of reactant changing per unit of time.
Collision Theory: Atoms, molecules, and ions can react to form products when they collide with one another, provided that the collided particles have enough kinetic energy.
Particles without enough energy with bounce away from each other unchanged.
Activation Energy: The minimum energy that colliding particles must have in order to react.

3. Transition State
The transition state, which can result in the formation of new products or the re-formation of reactants, is also known as the activated complex.
Activated Complex: The unstable arrangement of atoms that form momentarily at the peak of the activation energy barrier.
(lifetime of s)

4. Factors Affecting Reaction Rates
Temperature – increase of KE leads to particle collisions of greater frequency and force.
Concentration- more particles within the same space results in more collisions.
Particle Size- more surface area is impacted with small particle sizes. (more reactant exposed)
Catalysts – increases rate without being consumed.
Inhibitors – Interferes with the action of a catalyst (reacts with or poisons catalyst)

5. Reversible Reactions
One in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

6. Equilibrium
When the forward and reverse reactions are equal, and the reaction has reached a state of balance.
No net change occurs in the actual amounts of the components of the system.
Relative concentration of reactants and products constitute the equilibrium position and the longer arrow in a chemical equation represents that the formation of those reactants or products are favored.

7. La Chatelier’s Principle
If stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.
“Stresses” include: Concentration, Temperature, and Pressure

8. Adding “Stress”
Determine effects of adding concentration of reactants or products, temperature, or increase/decrease of pressure.

9. Equilibrium Constant
The ratio of the product concentrations to the reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced equation.
Keq > 1, products favored
Keq < 1, reactants favored
Concentration in mol/L

10. Practice
The colorless gas dinitrogen tetroxide (N2O4) and the dark brown gas nitrogen dioxide (NO2) exist in equilibrium with each other.
N2O4(g) NO2(g)
A liter of a gas mixture at equilibrium at 10°C contains mol of N2O4 and mol of NO2. Write the expression for the equilibrium constant and calculate the equilibrium constant (Keq) for the reaction.

11. Practice