Kinetics and Equilibrium

Kinetics The branch of chemistry known as chemical kinetics is concerned with the rates of chemical reactions and the mechanisms by which they occur.

The rate of a chemical reaction is measured in terms of the # of moles of a reactant consumed (or produced) in a unit of time.

The mechanism of a chem rxn attempts to describe the order of events in which the reaction takes place.

Energy and Reaction Rates Energy is required to initiate a rxn. Activation energy is defined as the min energy required to make a rxn occur.

The heat of reaction illustrates the difference in heat content before and after the rxn.

It is also the difference in potential energy between the prod and reactants.

If the products have less energy than the reactants, the reaction is considered exothermic.

Endothermic reactions occur when the products have more energy than the reactants. They are given by a positive heat of rxn.

Potential Energy Diagram We can show the relationship between activation energy and heat of reaction for a given rxn on a graph.

We place PE on the vertical axis, and the “reaction coordinate” on the horizontal axis.

In order for any rxn to occur, an activated complex must be formed.

This is an intermediate product, and may be considered to be a temporary association of atoms or molecules which when finished, will give us our products.

The highest point on the curve of a PE diagram shows the PE of the intermediate product (activated complex).

Once the formation of an activated complex has occurred, the rxn can continue to give the products.

The PE of the products will be lower than the peak energy of the activated complex.

If the PE is higher for the products, the rxn was endothermic, and if the PE is higher for the reactants, the rxn was exothermic.

The heat of reaction is strictly the difference between the PE’s of the products and reactants, and is the same regardless of the amounts of each involved.

Factors that Affect Reaction Rates Chem rxns depend on collisions between reacting species - atoms, molecules, ions, etc.

A rxn only occurs when adequate energy has been given to form an activated complex.

Generally speaking, anything that causes more collisions to occur will increase the rate of reaction.

Nature of the Reactants The nature of the reactants, with respect to the types of bonds involved, contributes to determining the rate of rxn.

If few bond rearrangements are necessary, the rxn will occur quickly, such as with ionic substances in water.

If bonds need to be broken or newly created, more time is necessary, such as oxygen and hydrogen bonding to form water.

Concentration If the concentration of one or more reactants is increased, the rate of rxn increases. Compressing a gas has the same effect.

Temperature An increase in temperature will increase the rate of reaction for two reasons - increased frequency of collisions and increased energy values in each collision.

Reaction Mechanism Most reactions do not occur in a single step, but in a series of steps called the reaction mechanism.

Each individual step is generally a two- particle collision, and the number of steps and their individual complexity affects the rate of the overall rxn. The more steps, the slower the rxn.

Catalysts Fast reactions have low activation energies, while slow reactions have high activation energies.

The addition of a catalyst to a system changes its activation energy, thereby changing its rate of rxn.

A catalyzed rxn requires less activation energy, without changing the overall rxn.

Even though the activated complexes in a catalyzed rxn have less PE than a non- catalyzed rxn, the heats of rxn are the same.