[Chemistry Class Notes] on Chemical Kinetics Pdf for Exam

Chemical kinetics is the study of chemical reactions with respect to the rate of reaction, formation of intermediates, rearrangement of atoms, and the effect of different variables. There are certain factors that affect the rate of reaction. They are the catalyst concentration of reactants, and temperature. The prediction of the rate of reaction is not possible and hence it has to be determined experimentally. The mathematical representation of the rate of reaction is given by the rate law. Measurement of the rate of reaction of a chemical is done by measuring either:

Collision

The chemical reactions occur only if the reactant molecules collide with each other, this is called the collision theory of Chemical Kinetics. The collision states that In order to have a chemical reaction the reactants must collide with adequate energy that is greater than the activation energy (which is represented as Ea).

Rate of a Chemical Reaction 

Rate of a chemical reaction depends on the concentrations of reactants or products and the time required to complete the chemical change. 

 

Rate of a chemical reaction can be defined as the change in concentration of a reactant or product in unit time.

 

Factors Influencing the Rate of Reaction

There are certain factors that influence the rate of reaction, such as increasing the fraction of molecules with energies greater than activation energy Ea. The factors that influence the rate of a reaction are,

• The Nature of the Reactants

• The Concentration of the Reactants

• The Temperature of the Reactants

• The Presence of a Catalyst

The Nature of the Reactants

Chemical reactions in an aqueous solution take place immediately due to the breakage of the molecular bonds in the molecules of reactants. The attractive forces of the ions are broken and become hydrated by the water molecules.

Additionally, attractive forces are equally distributed in all directions by the ions. Generally, no covalent bonds are broken, in contrast, reactions between molecules that require covalent bonds are broken very slowly. Therefore, the rate of reaction is influenced by certain structural characteristics of the reactant molecules, such as bond polarity, geometry, overall size, and orientation influence.

The Concentration of the Reactants

For most of the reactions, if the concentration of the reactants increases the rate of reaction also increases. Increasing the concentration of a reactant is nothing but increasing the number of reactant molecules of the same volume.

For most of the reactions, the concentration and rate of reaction are directly related. Therefore, if the concentration is doubled, the rate of reaction also gets doubled. It can be explained using the collision theory that if the number of reactant molecules gets doubled, there will be an occurrence of twice as many collisions at the same time.

The Temperature of the Reactants

In every chemical reaction, the increase in the rate of a reaction increases the temperature. If there is an increase in temperature by 10°, the rate of reaction gets doubled. This is an extremely powerful effect. For instance, If the temperature is increased from 20° to 80°, the rate of reaction will be 26 = 64 times faster. 

It can also be explained with the Collision theory: every molecule’s average kinetic energy is a direct function of temperature. The molecules collide with greater energy, therefore their activation energy decreases.

The dependence of the rate of reaction on temperature can be explained using the Arrhenius equation.

Which is, k = A [frac{e -Ea}{RT}] 

where, 

[frac{e -Ea}{RT}] is the number of molecules with energies greater than the activation energy.

The Presence of a Catalyst

A Catalyst is a substance that does not participate in the reaction but somehow increases the rate of reaction. Catalysts provide alternate pathways for the reaction. It requires a small amount of energy for the reactants to convert into products. Some catalysts increase the rate of reaction in multiple types of reaction, whereas some catalysts like enzymes, which are basically in our bodies, are specific to only one reaction or single type of reactant molecule. Thus, a catalyst increases the rate of reaction of a chemical.

How to Find Order of Reaction?

Order of reaction is determined by experiment. Although if we know rate law expression determined experimentally then we can determine order of reaction using rate law. Order of reaction can be an integer or fractional value. Following orders of reactions are possible – 

• Order of reaction can be zero – In zero order reaction the concentration of reactant/s doesn’t affect the rate of a reaction. 

• Order of reaction can be negative integer – Negative integer value of order of reaction indicates that the concentration of the reactants inversely affects the rate of a reaction. 

• Order of reaction can be positive integer – Positive integer value of order of reaction indicates that the concentration of the reactants directly affects the rate of a reaction. 

• Order of reaction can be fractional value – Fractional value of order of reaction indicates a more intricate relationship between concentration of reactants and rate of reaction. Generally, complex reactions possess fractional values of order of reaction.

 

Following Methods can be Used for Determination of Order of Reaction

• Differential Method – It is also called the initial rates method. In this method concentration of one reactant varies while others are kept in constant concentration and initial rate of reaction is determined. Suppose if three reactants A, B and C are taking part in the reaction then in this method we keep varying the concentration of one reactant (for example reactant A) while concentration of other reactants such B and C constant. 

• Graphical Method – This method is used when only one reactant takes part in the reaction. In this method if we draw a graph between log A

• A (where A is a reactant and A is concentration of reactant A) and t (time) and it’s a straight line then reaction follows a first order.  In the same way if we draw a graph between 1/[A] and t and get a straight line then reaction follows second order. While if we draw a graph between 1/[A]2 and t and get a straight line then the reaction i
  s a third order reaction. 

• Integral Method – In this method concentrations of the reactants are compared with the integral form of the rate law. It is used for verification of the initial rate method. 

Molecularity of a Reaction 

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

Zero Order Reaction 

In these reactions the rate of reaction doesn’t depend upon the concentration of reactants. It means change in concentration of reactants doesn’t affect the rate of reaction. 

 

Example – 2NH₃(g) → N₂(g) + 3H₂(g) [Fe or W as catalyst]

 

In zero order reactions, the rate of reaction is proportional to zero power of the concentration of reactants. Suppose the reaction is –

R ? P 

Then, Rate = k

Ro = – d[R]dt

As, Ro  = 1,

So, Rate = k 1 =  -d[R]dt

dR = -k dt

on integration -R = -k t + I

where, I = constant of integration 

At t = 0,

R = R0

R0 =   I

On substituting the value of 1 in the above equation –

R = -k t + R0

Above equation is similar to the equation of a straight line (y = mx + c). So, if we plot R against t, we get a straight line. 

k = [frac{R_0 – R}{t}]