Section 18.1 Collision Theory, Activation Energy, & Rates of Reactions Adapted from Kelly Deters “Chemistry”
Collision Theory Collision Theory – Defines 3 criteria to be met for a reaction to occur. 1 Reactants must collide 2 Collision must be at the correct orientation Collision must have minimum amount of energy for reaction to occur 3 Only a small number of collisions meet the requirements and result in a reaction
Collisions Must Occur In order for two molecules to react, they must come in contact with one another N O F There’s no way they’ll ever react if they don’t run into one another!
Collision with Incorrect Orientation For a collision to result in a chemical reaction, it must occur with the correct orientation N O F This is not the correct orientation. The reaction will not happen.
Collision with Correct Orientation For a collision to result in a chemical reaction, it must occur with the correct orientation F N O N O F F But not every collision produces a reaction—they must collide in the correct orientation to react This is the correct orientation. The reaction will happen.
Collision with Not Enough Energy For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction N O F The collision does not have enough energy to produce a reaction
Collision with Enough Energy For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction F N O N O F F This collision had more energy (faster moving molecules). A reaction will occur..
To Review 3 criteria must be met for a reaction to occur. 1 2 3 Reactants must collide 2 Collision must be at the correct orientation Collision must have minimum amount of energy for reaction to occur 3
Activation Energy Minimum amount of energy for reaction to occur during a collision
Reaction Coordinate Diagram Reaction coordinate diagrams show the energy changes throughout the reaction Activated complex (Also called the transition state) Activation Energy Products Energy Energy change for reaction Reactants Reaction proceeds
Activated Complex What is an “activated complex”? F N O N O F F Reaction proceeds Energy Activated Complex Reactants F2NO2 F2 + NO2 Products F + FNO2 F N O N O F F
Factors Affecting Reaction Rates
Surface Area of Reactants How does the surface area of the reactants affect the reaction rate? More reactants can collide at the same time and a fraction of those will result in reaction Larger surface area means more particles can come in contact with each other at the same time Reactants must collide in order to react As surface area increases, reaction rate increases
Concentrations of Reactants How does the concentration of reactants affect the reaction rate? Only a small fraction of the collisions meet the requirements and result in a reaction If more collisions occur, more will meet the requirements and result in a reaction More reactants mean more collisions will occur As reaction concentration increases, reaction rate increases
For most reactions, as temperature increases, reaction rate increases How does temperature affect the reaction rate? With higher energy molecules, collisions will have higher energy and more often result in reaction If molecules are at a higher temperature, they have a higher average kinetic energy Reactants must collide with at least energy equal to the activation energy For most reactions, as temperature increases, reaction rate increases
Catalysts Catalysts – Substance that increases the rate of reaction without being used up A + B + C D + C “C” is the catalyst…it is present in the beginning and in the end Enzymes are catalysts in the body
Catalysts How do catalysts help speed up the reaction without being used? They increase the chances that a collision will successfully produce a reaction For example, catalysts hold one or more of the reactants in place to allow collisions to occur with the correct orientation Catalysts make a successful collision more likely. They lower the activation energy by allowing reactants to react in a different way, or by holding reactants in place for the reaction. Once they’ve done their job, they are released and available to “help” another reactant. Once the reaction has occurred, the catalyst releases the molecule(s) and finds another one to help
Catalysts & Reaction Diagrams Reaction Path without catalyst Reaction Path with catalyst Energy Reaction proceeds Catalysts lower the activation energy of the reaction by letting it proceed in a different way. With lower activation energy, a higher percentage of collisions will be successful (they don’t need to collide with as much energy to be successful)