Lecture21222

Kinetics: the Rate of Chemical

ReactionLecture 21

It is now time to learnHow fast the reaction is proceeding at a given moment;What the reactant and product concentrations will be when the reaction is complete;Whether the reaction will proceed by itself and release energy or it will require energy to proceed.

Chemical equilibrium

in a chemical process is the state in which the chemical activities or

concentrations of the reactants and products have no net change

over time.

Chemical thermodynamics

studies the effects of changes in temperature, pressure, and volume on chemical systems at

the macroscopic scale by analyzing the collective motion

of their particles.

Chemical kinetics

is the study of rates of chemical processes, the

changes in concentrations of reactants (or products)

as a function of time.

Key factor of reaction rate: concentrationMolecules must collide to react.Reaction rate is proportional to the concentration of reactants.Rate ~ collision frequency ~ concentration.

Key factor of reaction rate: physical stateMolecules must mix to collide.The more finely divided a solid or liquid reactant, the greater its surface area per unit volume, the more contact it makes with the other reactant, and the faster the reaction occurs.

Key factor of reaction rate: temperatureMolecules must collide with enough energy to react.At a higher temperature, more collisions occur in a given time.Raising the temperature increases the reaction rate by increasing the number and, especially, the energy of the collisions.Rate ~ collision energy ~ temperature.

Key factor of reaction rate: catalysisA catalyst effects a lower activation energy, which in turn makes the rate constant larger and the rate higher.In more detail - on Wednesday.

Rate

is a change in some variable per unit of

time.

Rate of motion =

change in position x2 — x1

∆x

------------------------- = -------------- = --------change in time t2 — t1 ∆t

Rate of growth =

change in height l2 — l1 ∆l

------------------------- = -------------- = --------change in time t2 — t1 ∆t

Reactant concentrations

decreasewhile product concentrations

increase.Consider A B

The change in concentration

of reactant A in A Bis always negative.

The change in concentration

of product B in A Bis always positive.

Rate of concentration =

change in concentration of A= — --------------------------------------- = change in time concentration A2 — concentration A1

= — ----------------------------------------------- = t2 — t1

∆(conc A) = — --------------

∆t .

Use square brackets

to express concentration in moles per liter:

∆ [A] ∆ [B]

Rate = — ------------ or -------------

∆ t ∆ t

The rate of the reaction itself

varies with time as the reaction proceeds.

The rate decreases during the course of the reaction.

Reaction rates: different approachesAverage rate;Instantaneous rate;Initial rate.

A sample problem

on expressing rate in terms of changes in

concentration with time.

THE END