Thermochemistry and Kinetics

Kinetics

• The branch of chemistry that is concerned with reaction rates and reaction mechanisms

• Reaction rate: change in concentration of reactants per unit of time as a reaction proceeds

• Reaction mechanisms: the step-by-step sequence of reactions by which the overall chemical change occurs

Collision Theory

• In order for reactions to occur between substances, their particles (molecules, atoms, or ions) must collide.

• These collisions result in interactions between particles.

• The set of assumptions regarding collisions and reactions is known as collision theory.

Activation Energy

• When a collision is violent enough, old bonds can be broken and new bonds can form, but only collisions that occur with sufficient energy can be effective.

• Activation Energy is the minimum energy needed for a reaction to occur.

Collision Theory

Activated complex

• Endothermic Reaction

• +ΔH (heat of reaction) value; enthalpy

• Products less stable than reactants

• Exothermic Reaction

• -ΔH (heat of reaction) value; enthalpy

• Products more stable than reactants

Chemical Kinetics

The activation energy often acts like a barrier preventing a reaction from happening easily.

Are there ways of overcoming this barrier without the addition of the required energy?

Activation Energy

Ea Diagram with catalyst

reaction coordinate

no catalyst present

catalyst present

A + B

C + D

Catalyst = Decreases Activation Energy

Rate-Influencing Factors

• Nature of reactants (Increases Rxn.)

• Surface area (Increases Rxn.)

• Concentration(Increases Rxn.)

• Temperature (Increases Rxn.)

• Presence of catalysts (Decreases activation energy)

Factors Affecting Reaction Rate

Temperature Two Things Happen:(1)The collisions are

more forceful resulting in more collisions having the minimum energy (Ea) to be successful. This is shown on the graph at the left.

Factors Affecting Reaction Rate

Temperature Two Things Happen:(2) Since the particles

have more energy, they are moving faster and the collision frequency is increased. This results in more collisions per time unit thus increasing

the odds of a successful collision.

©Mr. D. Scott; CHS

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

Let’s start with a simple idea:

Building a camp fire.

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

First, These must be split into smaller pieces.

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

Next,Some of these need to be

placed under the larger logs Before these are used to

start the fire.

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

Smaller pieces of material

react faster than larger

pieces.

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

Higher concentration

of material reacts faster than lower

concentration

Factors Affecting Reaction Rate

ConcentrationThis factor is mainly a surface area issue.

Gases react faster than liquids which react faster than

solids.

EXOTHERMIC & ENDOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases heat.

A release of heat corresponds to a decrease in enthalpy

Exothermic process: H < 0 (at constant pressure)

Burning fossil fuels is an exothermic

reaction

Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.

An input of heat corresponds to an increase in enthalpy

Endothermic process: H > 0 (at constant pressure)

Photosynthesis is an Endothermic reaction (requires energy input from

sun)

Cellular Respiration is an Exothermic

reaction (releases energy)