Section 2.8—Speeding Up A Reaction Objectives: explain Collision Theory and relate theory to variables that effect the rate of a reaction define and explain catalysts role in a reaction

Kinetics & Reaction Rates Kinetics – Study of the rates of reactions Reaction Rate – Rate (or speed) at which reactants produce products

Collision Theory Collision Theory – Defines 3 circumstances to be met for a reaction to occur. 1 Reactants must collide 2 Collision must be at the correct orientation Collision must have minimum energy for reaction to occur 3

Collisions Must Occur In order for two molecules to react, the “correct atoms” must come in contact with one another. The “correct atoms” are the atoms that will have a bond formed between them during the reaction.. N O F There’s no way they’ll ever react if they don’t run into one another in the correct orientation!!

Collision with Enough Energy Not only must the molecules “come in contact” with one another, they must collide or hit each other. In addition, the collision has to have a certain minimum amount energy for a reaction to occur. N O F The collision does not have enough energy to produce a reaction!

Activation Energy Activation energy is the minimum amount of energy required for a reaction to occur during a collision. The greater the activation energy, the less likely a collision will occur with the energy necessary to start the reaction.

Reaction Coordinate Diagram A reaction coordinate diagram shows the energy of the reactants and products and the activation energy.

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 F2NO2 F2 + NO2 Products F + FNO2 F N O N O F F

Activated complex An activated complex is defined as the short-lived, unstable arrangement of atoms that breaks apart to form the products. It is also referred to as the transition state.

Factors Affecting Reaction Rates

Surface Area of Reactants How does the surface area of the reactants affect the reaction rate? More collisions means more particles have the opportunity to react. Larger surface area means more particles can collide with each other. 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? If more collisions occur, there is a greater possibility that more will meet the requirements for a reaction to occur. Only a small fraction of collisions meet the requirements that result in a reaction. If there are more reactant particles, 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 (that is, MORE ENERGY) Reactants must collide with at least energy that is equal to the activation energy For most reactions, as temperature increases, reaction rate increases.

Catalysts Catalysts are 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? Catalysts increase the chances that a collision will successfully produce a reaction by holding one or more of the reactants in 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 reactant(s) and finds another one to help.

Catalysts & Reaction Diagrams Reaction Path without catalyst Reaction Path with catalyst Energy Reaction proceeds The starting and ending energies of the reaction are unchanged. Activation energy is lowered, however.

Catalysts As a result of their presence, 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).

What did you learn about antacids?

Balanced Chemical Equations Antacids Are a Chemical Formulas Mixture Used in Symbolized with Balanced Chemical Equations Of different Compounds Symbolized with undergo Some are Chemical Reactions Acids & Bases Collision Theory Speed governed by