Chemical Kinetics | 1st year– Class Notes

Chemical Kinetics

Introduction

The branch of physical chemistry which deals with the speed or rate at which a reaction occurs is called chemical kinetics.

The study of chemical kinetics, therefore includes the rate of a chemical reaction and also the rate of chemical reaction and also the factors which influence its rate.

Slow and Fast Reaction

Those reactions for which short time is required to convert a reactant into product are called fast reaction but if more time is required for the formation of a product then the reactions are called slow reactions.

Usually ionic reactions which involve oppositely charged ions in aqueous medium are very fast. For example, reaction between aqueous solution of NaCl and AgNO3 gives white precipitates of AgCl instantaneously.
AgNO3 + NaCl —-> AgCl + NaNO3

Such reactions are very fast and these are completed in fractions of seconds.
But those reactions which involve covalent molecules take place very slowly. For example, conversion of SO2 into SO3

2 SO2 + O2 —-> 2 SO3

It is a slow reaction and required more time for the formation of a product.

Rate Or Velocity of a Reaction

Definition

It is the change in concentration of a reactant or product per unit time.

Mathematically it is represented as

Rate of reaction = Change in concentration of reactant or product / Time taken for the change

The determination of the rate of a reaction is not so simple because the rate of a given reaction is never uniform. It falls off gradually with time as the reactants are used up. Hence we can not get the velocity or rate of reaction simply by dividing the amount of substance transformed by the time taken for such transformation. For this reason we take a very small interval of time “dt” during which it is assumed that velocity of reaction remains constant. If “dx” is the amount of substance transformed during that small interval of time “dt” then the velocity of reaction is expressed as

Velocity of a reaction = dx / dt

Thus with the velocity of a chemical reaction we mean the velocity at the given moment or given instant.

The Rate Constant

Definition

The proportionality constant present in the rate equation is called rate constant.

According to law of mass action we know that the rate of chemical reaction is directly proportional to the molar concentration of the reactants. For example
R —-> P

The rate of reaction ∞ [R]

Or

dx / dt = K [R]

Where K is known as rate constant.

Specific Rate Constant

When the concentration and temperature both are specified, the rate

constant is known as specific rate constant.

When the concentration of each reactant is 1 mole per dm3 at given temperature, the specific rate constant numerically equals to the velocity of the reaction.
dx / dt = V = K [R]

Or

K = V / [R]

When R = 1 mole/dm3

K = V

But when different reactant are reacting with different number of moles then

the value of K may be calculated as

2 SO2 + O2 —-> 2 SO3

= dx / dt = K [SO2]2 [O2]

Or

K = V / [SO2]2 [O2]

Determination of Rate of Reaction

There are two method for the determination of rate of a chemical reaction.

1. Physical Method

When the rate of a chemical reaction is determined by using physical properties such as colour change, volume change, state change the method known as physical method.

2. Chemical Method

In the method the change in concentration of reactant or product is noted and with the help of this change rate of reaction is determined e.g.,

For the reaction R —-> P

Velocity of reaction = – d[R] / dt = + d[P] / dt

The negative sign indicates a decrease in concentration of the reactant while positive sign indicates an increase in the concentration of product.
Ionization is thus a reversible process. To this process, the law of mass action can be applied as

K(C) = [Na+] [Cl-] / [NaCl]

3. The number of positive and negative charges on the ions must be equal so that the solution as a whole remains neutral.

4. The degree of ionization of an electrolyte depends upon (a) the nature of electrolyte, (b) dilution of the solution (c) the temperature

5. When an electric current passes through the solution of an electrolyte the positive ions i.e., the cations move towards the cathode and the anions move towards the anode. This movement of ions is responsible for the conductance of electric current through the solution.

6. The electrical conductivity of the solution of an electrolyte depends upon the number of ions present in the solution. On reaching the electrodes, the ions lose their charge and change into neutral atoms or molecules by the gain or loss of electrons.

Applications of Arrhenius Theory

This theory explain many peculiarities in the behavior of electrolytic solutions.

For example, the elevation in boiling point of 1 molal solution of glucose is 0.52ºC while this elevation in 1 molal solution of NaCl is 1.04ºC. This difference in elevation of boiling point can be explained on the basis of Arrhenius theory.

In one molal solution of glucose the number of (molecules) particles are 6.02 x 10(23) per dm3 of solution while in 1 molal solution of NaCl 6.02 x 10(23) ions of Na+ and 6.02 x 10(23) ions of Cl- are present because NaCl is an ionic compound. Since the number of particle are double in NaCl solution, therefore the elevation in boiling point is also double than the solution of glucose.

Similarly the other colleative properties such as lowering in vapour pressure, depression in freezing point and osmosis are explained on the basis of this theory.

Note
Collegative properties are those properties which depends upon the number of particles.

Conductance of Electric Current Through Solutions

The ability of a solution to conduct electric current depends upon the ions present in the solution. The conductance of a solution is increased when
1. The solution is diluted

2. The degree of dissociation of the electrolyte is high

3. The temperature of the solution is high

4. The velocity of the ions is high

But in a concentrated solution, the number of ions per unit volume of solution increases and the distance between ions decreases causing strong interionic attraction. As a result, migration of ions becomes more difficult and the conductance decreases with increase in concentration. As the conductance is related with the movement of ions, so conductance increase with the increase of absolute velocity of ions in the solution.

The conductance of an electrolyte also depends upon the degree of ionization. The degree of ionization is denoted by α and calculated as
α = No. of dissociated molecules / Total molecules dissovled

Electrolysis

Electrolyte
A chemical substance which can conduct electric current in molten form or in its aqueous solution with a chemical change is called electrolyte.

Electrolysis
The movement of anions and cations towards their respective electrodes with all accompanying chemical changes in an electrolytic solution under the influence of electric current is known as electrolysis.

Explanation
To explain the phenomenon of electrolysis consider the example of CuCl2 solution. the ionization of CuCl2 in the solution may be represented as
CuCl2 <—-> Cu+2 + 2 Cl-

When electric current is passed through this solution, the movement of these ions begins to take place Cu+2 ions migrate towards cathode and Cl- ions towards anode. At cathode Cu+2 ions are discharged as copper atoms by the gain of electrons (reduction)

Cu+2 + 2 e- —-> Cu(M) …….. Reduction at Cathode

At anode Cl- ions are discharged as Cl2 by the loss of electrons (oxidation)
2 Cl- – 2 e- —-> Cl2(g0 …… Oxidation at Anode

The overall reaction of the electrolysis may be written as
Cu+2 + 2 e- —-> Cu(M)

2 Cl- – 2 e- —-> Cl2(g)

Cu+2 + 2 Cl- —-> Cu(M) + Cl2(g)

OR

CuCl2 —-> Cu(M) + Cl2(g)

When all the ions present in the solution have been changed to neutral particles, the flow of current is stopped.

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