XI Chemistry - Chapter 8 - Introduction to Chemical Kinetics

Chemical Kinetic
“It is the branch of chemistry which deals with the study of rate of reactions and factors affecting the rate.”

Rate of Reaction
Substance that are react in a chemical reaction are called Reactant and those that are produced are called Product.

During the course of a reaction, the molar concentration of reactants gradually decreases, whereas that of product increases. Consider a general chemical reaction.

A(Reactant) ® B(Product)


Rate of reaction is defined as,

“The quantity of a reactant consumed or the quantity of a product formed per unit time.”


The concentration of a reactant or a product is usually expressed in tem of mole/dm3. The time taken for the change is expressed in seconds. The rate so expressed in mole/dm3/sec.

The rate of reaction is not uniform throughout it decreases with the passage of time as the reactant are gradually consumed during chemical reaction.

Velocity of Reaction
It is defined as:

“Rate of reaction at a specific time.”

If in a small interval of time ‘dt’ amount of substance changed is ‘dx’ and during this interval, the rate of reaction remain constant then:


Velocity of reaction may also be termed as instantaneous rate of reaction. Velocity of reaction is the slope of the tangent. Consider the reaction between Mg and HCl.

Mg + 2HCl ® MgCl2 + H2


Velocity of reaction at 48s = Slope of Tangent

= = 0.83c.c/s

Rate Constant
Consider the reaction:

Reactant ® Product

According to the law of mass action, the rate of a chemical reaction is directly proportional to the active mass or molar concentration of the reactant.

Rate of reaction µ [R]

But we know that:


Þ Þ Where K is the rate constant specified. The rate constant is called specific rate constant. If the concentration of reactant is 1 mole/dm3 at given temperature then:

= V

Þ K = V / [R]

Since, R = 1mole/dm3, therefore,

K = V

So rate constant is the ratio between the rate of reaction and concentration of reactant.

Energy of Activation
“The minimum amount of energy required in addition to the average energy, just to convert the reactants into the products is called the Activation Energy.”

It is represented by EA.


According to the collision theory of reaction rate, the reaction takes place when molecules of reacting substances collide together.

It has been found that only those collisions are effective in which the colliding molecules possess a minimum amount of energy called ‘Threshold Energy’. Thus,

Activation Energy = Threshold Energy – Average Internal Energy

Every reaction has its own energy of activation. A reaction with low activation energy takes place at low temperature and a reaction with higher activation energy will take place at high temperature.


Types of Reaction
The chemical reactions are classified into three types on the basis of reaction rule:

1. Fast Reaction
They are ionic in nature and takes place instantaneously. Such reactions have very low energy of activation. Since these reactions involve ions, hence are also called ionic reaction. It is possible to determine rates of such reaction.

Ag+NO3-+ Na+Cl- ® AgCl + Na+NO3-

2. Slow Reaction
These are the reactions, which proceed at extremely slow speed and take very long time for completion. Such reactions have very high energy of activation. It is difficult to determine experimentally velocity of such reactions for example rusting of Iron, radioactive decay of elements.

3. Moderate Reaction
These are the reactions, which proceed at experimentally measurable rate. Generally reactions of organic and covalent compounds are such type.

CH3COOH + C2H5OH ® CH3COOC2H5 + H2O

Determination of Rate of Reaction
Two methods are employed for the determination of rate of chemical reaction.

1. Physical Method
In this method, rate of a reaction is determined by observing the change in some physical property of either reactant or product.

Physical properties involved may be:

(i) Change in colour of reactant or product.

(ii) Evolution of gas its volume is measure.

(iii) Change in pH

(iv) Change in Optical rotation of plane polarized light.

2. Chemical Method
In this method, samples are drawn from the reacting vessel at regular intervals of time. Reaction is stopped at that particular moment by suddenly chilling the sample adding it to a suitable chemical.

The amount of a reactant or product present at that time is generally found by titrating the sample against a proper reagent.

For example, acidic hydrolysis of ethyl acetate is followed by determining the amount of CH3COOH formed at different time by titration against standard NaOH.

H+

CH3COOC2H5 + H2O Û CH3COOH + C2H5OH

Rate is determined from the graph.


Rate = Slop= Factors Affecting Rate of Reaction

The rate of a reaction involving collisions of molecules is influenced by many factors.

1. Concentration of reactants

2. Surface area of reactants

3. Nature of reactants

4. Temperature

5. Catalyst

6. Radiation







1. Concentration of Reactants
Concentration is defined as,

“Number of molecules of a substance per unit volume.”

Rate of reaction varies with the concentration of reactants. According to law of mass action, the rate of a chemical reaction is directly proportional to the concentration of reactants. By increasing the concentration of reactants, more crowding the molecules so frequency of collisions between them increases, resulting in the increase in rate of formation of products. Consider a general reaction:

XA + YB ® Product

Rate of Reaction µ [A]X[B]Y


In this rate of expression the sum of exponents of concentration (X + Y) is called Order of Reaction.

The order of reaction is defined as,

“Sum of all the exponents of the concentration in terms of the reactants involved in the rate equation.”

Thus,

(i) A + 2B ® Product


Order of reaction = 1 + 2 = 3 (Third order reaction)

(ii) 2A ® Product

(Second order reaction)

(iii) A ® Product

(First order reaction)

2. Surface Area of Reactant.
In heterogeneous reactions, the reactants are present in different physical states. In such reactions the rate depends upon the surface area of solid reactant greater the surface area, highe is the rate of reaction.

For example, finely divided marble reacts more quickly with HCl than marble chips, because powdered marble offers greater area to the reacting acid.

CaCO3 + 2HCl ® CO2 + CaCl2 + H2O

3. Nature of Reactant
The nature of the reactants also affects the rate because activation energy for effective collision is different for different reacting substance.

2NO + O2 ® 2NO2 (Fast)

2CO + O2 ® 2CO2 (Slow)

Reaction between NO and O2 is fast while that for CO and O2 is slow. This is because activation energy for CO is higher than that of NO.

4. Temperature
The rate of a chemical reaction generally increases with the rise in temperature. As a general rate of thumb, the rate of reaction doubles for every 10°K rise in temperature. The reasons are:

(i) As temperature increases the velocity of molecules also increases, and increases the frequency of collisions.

(ii) The rise in temperature raises the kinetic energy of each molecule. It has been found by raising the temperature by 10°K, the fraction of molecules possessing threshold or activation energy becomes double as a result number of effective collision is also doubled, hence rate is doubled.


The graph shows the kinetic energy of molecules at different temperatures. The shaded area shows the fraction of molecules having threshold energies. Shaded area o T2 is doubled of T1.

5. Catalyst
A catalyst is a substance, which alters the rate of a reaction without itself undergoing a permanent chemical change.

In other words, a catalyst may take part during chemical reaction, but it remains unchanged at the end of the reaction. Catalyst may be of two types

(a) Positive Catalysts
Positive catalyst increases the rate of reaction by lowering the energy of activation. As a result more reactants (molecules) possess the energy required for a successful collision. The total number of effective collisions per unit time increases and thus the rate of reaction increases.

2KClO3 2KCl + 3O2

(b) Negative Catalysts
Negative catalyst decrease the rate of reaction by increasing the energy of activation. This decrease the effective collisions, hence rate of reaction decreases.

CHCl3 + ½ O2 COCl2 + HCl




6. Radiation
Some chemical reactions proceed only under the influence of light. These are called photochemical reaction. The concentration of reactants does not influence rate of such reactions, so they are zero order reaction.

H2 + Cl2 2HCl

CH4 + Cl2 CH3Cl + HCl

Radiation or light consists of Photons. When Photons strike the reactant molecule, they provide the necessary activation energy to the reactant molecules to react.