Chemical Kinetics

Reaction rate - the change in concentration of reactant or product per unit time.

Factors Affecting Rates

Temperature Concentration of

reactants Particle size Presence of a catalyst

Rates Average rates - the

difference in concentration over a set amount of time. Table 12.2

Instantaneous rates - calculated from the slope of the line tangent to the curve at a certain point.

Reaction rates

Rates are not constant. Rates vary with time

because concentrations vary with time.

Rate Laws

Rate = k[A]n[B]m[C]p

k = rate constant n,m,p = order ; must be

determined by experiment.

Differential rate law

Expresses how a rate depends on concentration.

Often this is just called the rate law.

Integrated rate law

Expresses how the concentration depends on time.

Reaction Mechanism The series of elementary

steps by which a reaction occurs.

An elementary step is a step whose rate law can be determined from its molecularity.

Molecularity The number of species that

must collide to produce the reaction in that step.

Unimolecular involves one molecule.

Bimolecular and termolecular involve 2 and 3 species respectively

Intermediate

A species that is neither a reactant or a product. This species is produced and consumed in the reaction.

Rate Determining Step

This is the slowest step of the mechanism. The reaction can only proceed as fast as this rate determining step will allow.

Requirements of Mechanisms

The sum of the elementary steps must give the overall balanced equation.

The mechanism must agree with the experimentally determined rate law. The rate determining step.

Collision model for kinetics Reactant molecules must

collide in order to react. Concentration Particle size Temperature Molecular orientation

Requirements for reaction The collision energy must

equal or exceed the activation energy.

The orientation of the reactants must allow the formation of new bonds.