Section 2.8—Speeding Up A Reaction

How can we make those antacid reactions occur faster?

Kinetics & Reaction Rates

Kinetics – Study of the rates of reactions

Reaction Rate – Rate 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

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

FF

FFNN

OO

OO

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

Collision with Correct Orientation

For a collision to result in a chemical reaction, it must occur with the correct orientation

FF

FFNN

OO

OO

This is not the correct orientation. The reaction will not happen.

NNOO

OO

FF

FF

Collision with Correct Orientation

For a collision to result in a chemical reaction, it must occur with the correct orientation

FF FF

NNOO

OO

This is the correct orientation. The reaction will happen.

FF

FF

Collision with Enough Energy

For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction

NNOO

OO

The collision does not have enough energy to produce a reaction

FF

FF

This collision had more energy (faster moving molecules). A reaction will occur..

NNOO

OO

Collision with Enough Energy

For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction

FF FF

NNOO

OO

Activation Energy

Minimum energy for reaction to occur during a collision

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

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 come in contact with each other at the same time

More reactants can collide at the same time and a fraction of those will result in reaction

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

More reactants mean more collisions will occur

If more collisions occur, more will meet the requirements and result in a reaction

As reaction concentration increases, reaction rate increases

Temperature

How does temperature affect the reaction rate?

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

If molecules are at a higher temperature, they have a higher average kinetic 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 – 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

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

Reaction proceeds

Ene

rgy

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)