Reaction Rates Ch 18 Notes

Collision Theory 1.Reacting substances (atoms, ions, molecules, etc.) must collide in order to react 2.Collisions have to happen with the correct orientation 3.Reacting substances must have enough energy to form the activation complex ~this amount of energy is the ‘activation energy’ 1.Reacting substances (atoms, ions, molecules, etc.) must collide in order to react 2.Collisions have to happen with the correct orientation 3.Reacting substances must have enough energy to form the activation complex ~this amount of energy is the ‘activation energy’

sToP & tHinK  Space shuttles use liquid H 2 and O 2 in a chemical reaction to get enough energy for propulsion.  Why do they use these substances in liquid phase and not gaseous phase?  Space shuttles use liquid H 2 and O 2 in a chemical reaction to get enough energy for propulsion.  Why do they use these substances in liquid phase and not gaseous phase?

1. Reacting substances (atoms, ions, molecules, etc.) must collide in order to react

2. Collisions have to happen with the correct orientation

3. Reacting substances must have enough energy to form the activation complex

Collision Theory - Pictoral Summary

Activation Energy difference between starting energy and activated complex.

Reactions happen in steps…  Most reactions are complex, meaning they happen in two or more ‘elementary’ steps  example:  Most reactions are complex, meaning they happen in two or more ‘elementary’ steps  example:

<- single step complex reaction -> Different Energy diagrams

sToP & tHinK  Assume that this reaction is reversible, meaning that products can react to reform reactants. Which ‘direction’ of the reactants would have a larger activation energy?  reactants to products OR products to reactants  Assume that this reaction is reversible, meaning that products can react to reform reactants. Which ‘direction’ of the reactants would have a larger activation energy?  reactants to products OR products to reactants

Factors that affect Reaction Rate 1.Nature of Reactants 2.Concentration 3.Surface Area 4.Temperature 5.Catalysis 1.Nature of Reactants 2.Concentration 3.Surface Area 4.Temperature 5.Catalysis

1. Nature of Reactants  Some elements/substances are more reactive than others  Ionization energy  Electronegativity  Solid, Liquid, or Gas  Single, Double, or Triple Bonds  Some elements/substances are more reactive than others  Ionization energy  Electronegativity  Solid, Liquid, or Gas  Single, Double, or Triple Bonds

2. Concentration  Higher reactant concentrations means…  more particle collisions and, therefore, a faster reaction rate because more collisions increases the chance of having collisions with enough energy and correct orientation.  Higher reactant concentrations means…  more particle collisions and, therefore, a faster reaction rate because more collisions increases the chance of having collisions with enough energy and correct orientation.

3. Surface Area  Greater surface area creates a larger interface for particle collisions, so increases reaction rate.

4. Temperature  Higher temperature means higher kinetic energy, more particle collisions, and faster reaction rate.

5. Catalysts  Very different from the other factors that affect reaction rate  Instead of increasing/decreasing collisions, catalysts lower the activation energy needed for the reaction to occur  activation energy - amount of energy needed for a reaction to take place  inhibitor - slows down or even blocks a reaction  Very different from the other factors that affect reaction rate  Instead of increasing/decreasing collisions, catalysts lower the activation energy needed for the reaction to occur  activation energy - amount of energy needed for a reaction to take place  inhibitor - slows down or even blocks a reaction