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.

Reactants must collide

Collision must be at the correct orientation

Collision must have minimum energy for reaction to occur

1

2

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..

FF

FFNN

OO

OO

There’s no way they’ll ever react if they don’t run into one another in the correct orientation!!

FF

FF

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.

NNOO

OO

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

Products

Activated complex(Also called the transition state)

Activation Energy

ReactantsEnergy change for reaction

Reaction coordinate diagrams show the energy changes throughout the reaction

Reaction proceeds

Ene

rgy

F + FNO2

FF

FF

Activated Complex

What is an “activated complex”?

Reaction proceeds

Ene

rgy

NNOO

OOFF FF

NNOO

OO

ReactantsActivated Complex

ProductsF2 + NO2

F2NO2

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?

Reactants must collide in order to react.

Larger surface area means more particles can collide with each other.

More collisions means more particles have the opportunity 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 collisions meet the requirements that result in a reaction.

If there are more reactant particles, more collisions will occur.

If more collisions occur, there is a greater possibility that more will meet the requirements for a reaction to occur.

As reaction concentration increases, reaction rate increases.

Temperature

How does temperature affect the reaction rate?

Reactants must collide with at least energy that is equal to the activation energy

If molecules are at a higher temperature, they have a higher average kinetic energy (that is, MORE ENERGY)

With higher energy molecules, collisions will have higher energy and more often result in reaction

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.

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

Reaction proceeds

Ene

rgy

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?

Antacids

MixtureMixture

Are a

CompoundsCompounds

ChemicalFormulas

ChemicalFormulas

Collision Theory

Collision Theory

Acids & Bases

Acids & Bases

Chemical Reactions

Chemical Reactions

Of differentSymbolizedwith

Some are

Balanced Chemical Equations

Balanced Chemical Equations

undergo

Symbolizedwith

Used in

Speed governed by