Chemical kinetics

(1) Define

Chemical (Reaction) Kinetics in terms of Collision Theory and Transition Theory.

(2)Define the factors that affect the

rate of reaction.

ď‚— In 1864, Peter Waage and Cato Guldberg pioneered

the development of chemical kinetics.

ď‚— They formulated the law of mass action, which states

that the speed of a chemical reaction is proportional to the quantity of the reacting substances.

Chemical Kinetics  Chemical kinetics, also known as reaction kinetics,

is the study of rates of chemical processes.

 Simply put how fast can the REACTION go!

 Chemical kinetics includes investigations of how

different experimental conditions:

 can influence the speed of a chemical reaction  yield information about the reaction's mechanism and

transition states  can lead to the construction of mathematical models that can describe the characteristics of a chemical reaction.

Chemical Kinetics ď‚— Chemical Kinetics can be looked at from TWO points

of view.

Collision Theory

Transition State Theory

Collision Theory

Imagine many of these happening‌.

Collision Theory

 It is based on kinetic theory and supposes that

particles must collide with both the correct orientation and with sufficient kinetic energy if the reactants are to be converted into products.

 Let’s look at the following reaction:

A+B

products

 In this theory, once particles (reactants) have

sufficient kinetic energy these particles will collide and form products in a ONE-STEP process.

Collision Theory ď‚— Not all collisions are successful, because not all

particles have the activation energy. ď‚— The activation energy is the minimum energy required for a reaction to occur.

Transition State Theory  This theory supposes that as reactants approach each

other a transitory activated complex (transition state) is formed at a potential energy maximum.

 At the activated complex original bonds are

weakened and new bonds are partially formed.

 Hence collisions are not necessary for a successful

reaction.

A+B

[T.S.]

products

Transition State Theory ď‚— In the transition state theory the activation energy

is the energy difference between the reactants and the potential energy maximum.

Factors affecting Rate of Reaction Nature of the reactants Physical state Concentration Temperature Catalysts Pressure

Nature of the reactants  Depending upon what substances are reacting, the

time varies.

 Acid reactions, the formation of salts, and ion

exchange are fast reactions.

 When covalent bond formation takes place between

the molecules and when large molecules are formed, the reactions tend to be very slow.

Physical State  Reactants in the same phase react faster than those in

different phases.

 This is so because when they are in different phases, the

reaction is limited to the interface between the reactants.

 Vigorous shaking and stirring can also increase the

rate of reaction by finely dividing the reactant particles.  This results in a greater surface area per unit volume,

and the more contact it makes with the other reactant, thus the faster the reaction.

Concentration

 Concentration plays a very important role in reactions

according to the collision theory of chemical reactions, because molecules must collide in order to react together.

 As the concentration of the reactants increases, the

frequency of the molecules colliding increases, striking each other more frequently by being in closer contact at any given point in time.

 By increasing the amount of one or more of the reactants

it causes these collisions to happen more often, thus increasing the reaction rate.

Temperature ď‚— Molecules at a higher temperature have more thermal

energy.

ď‚— Although collision frequency is greater at higher

temperatures, this alone contributes only a very small proportion to the increase in rate of reaction.

ď‚— Much more important is the fact that the proportion

of reactant molecules with sufficient energy to react (energy greater than activation energy: E > Ea) is significantly greater.

Temperature

Catalyst  A catalyst is a substance that accelerates the rate of a

chemical reaction but remains chemically unchanged afterwards.

 The catalyst increases reaction rate by providing a

different reaction mechanism to occur with a lower activation energy.

 A catalyst does not affect the position of the

equilibria, as the catalyst speeds up the backward and forward reactions equally.

Pressure ď‚— Increasing the pressure in a gaseous reaction will

increase the number of collisions between reactants, thus increasing the rate of reaction.