CHAPTER # 8

Chemical Kinetic:

            The word Kinetic is derived from a Greek word “Kinetic” which means “moving”. Hence chemical kinetics is that branch of chemistry which deals with the study of:

1)      Rates of Chemical reactions

2)      Factor affecting rate

3)      The mechanism through which the reaction proceeds

4)      Optimum conditions for the maximum yield of a product

Rate of Chemical Reactions:

            The change in the concentration of the reacting substances per unit second is called the rate of reaction

Formula:

            Rate = Change in Concentration / Change in time(s)

OR!

            Increase in the concentration of product or the decrease in the concentration of reactant per unit second is called rate of reaction.

Its unit is,

            Mol/dm_­³.s    or        mol.dm^-3.s^-1   or        MS^-1

Rate Law:

            The experimental relationship between the rate of reaction and the concentration of reactant is known as rate law.

Velocity of Rate of Reaction:

            The change in concentration in the reacting substances at a particular moment is known as velocity of reaction. It is also called the instantaneous rate.

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

Determination of Velocity of Reaction:

            To determine the velocity of reaction, let us consider the formation of hydrogen gas in the following reaction.

            Mg + H₂SO₄ à MgSO₄ + H_2­

            Now, we calculate the change in concentration after regular intervals. Then, we plot a graph b/w the time and the concentration of hydrogen.

(Fig 8.2 from Book pg227)

            A curve is obtained in the graph. To this curve, we draw a tangent at a fixed time. To this tangent, we draw a perpendicular and horizontal line. To find out, the change in conc. And time.

            Let, the change in conc.

dx = 40

and change in time,

dt = 40

.:. V= dx / dt

V= 40/40

V=1 MS^-1

Rate Expression:

            A mathematical equation, that gives the reaction b/w the rate of reaction and the conc. Of the reactant is called rate expression.

Derivation of Rate Expression:

            Consider a general reaction in which ‘A’ combine with ‘B’ to form ‘AB’.

A + B à AB

According to the law off mass action

R ∞ [A][B]

Or,

R = K [A][B] ----- (1)

Eq (1) is considered as rate expression, In eq(1) “K” is proportionality constant and it is known as the rate constant. It can define as,

            The ration of rate of reaction the conc. Of reactant is called rate constant.

K = R / [A][B] ----- (2)

In eq (2), if;

[A]=[B] = 1mol/dm³ (unit molar conc.)

then;

K = R/(1)(1)

Or

K = R

Now, ‘K’ is called the specific rate constant and it is always equal to the reaction.

            The specific rate constant is defined as,

            “The rate constant for the unit molar conc. of the reactant is called the specific rate constant”.

Rate Constant = K = 3.02 x 10^-2 dm³/mole.s

Characteristics of K:

1)      It has a fixed value at a particular temperature.

2)      Its value varies with temperature.

3)      Its value at a temperature remains unchanged when the concentration of either or of all reactants is changed.

Types of Reaction based on Reaction Velocity:

ð     Reaction at very slow speed:         These are the reactions which proceed at extremely slow speed and take very long time for completion. It is difficult to determine experimentally velocity of such reactions. Example: Rusting of Iron, radioactive decay of elements and formation of diamond from carbon in earth crust.

ð     Reaction at very fast speed:           These reactions are instantaneous reaction i.e. they are so fast that they completed in very small time of the order of 10^-6s. All ionic reactions are of this type. It is impossible to determine of such reactions. Example: HCL + NaOH à NaCl + H₂0

ð     Reaction at moderate speed:          These are the reactions which proceed at experimentally measurable rate i.e. they have limited speed and are completed at most in few hours. Generally reactions of organic and covalent Compounds are of such type. Example:

C₁₂H₂₂O₁₁ + H₂0 à C₆H₁₂O₆ + C₆H₁₂O₆

Determination of Rate of Reaction:

            Two methods are employed fpr the determination of rate of chemical reaction.

ð     Physical Method

ð     Chemical Mehtod

Physical Method:

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.

Factor Affecting Rate of Reaction:

ð     Concentration of reactants

ð     Nature of reactants

ð     Temperature

ð     Pressure of catalyst

ð     Surface area of reactants (heterogeneous reactions)

ð     Radiation

Concentration of Reactants:

            Rate of reaction is directly proportional to the conc. of the reacting substances because by increasing the conc the distance b/w the molecules decreases. Hence the affective collisions are increased.

Temperature:

            By increasing the temperature, the rate of reaction increases because of high temperature, the average Kinetic Energy of the molecules is increased due to which they frequently collide with one another.

            It is also absorb that for every 10 Degree in rise of temperature the rate of reaction become double. It is called the Rule of Thumb.

Surface Area of Reactants:

            With the increase in exposed surface area, the rate of reaction increases. For example, the reaction of Zinc dust is faster the chunck zinc. Similarly, the marble powder reacts with HCL acid faster than the marble chips.

Nature of Reactants:

            The rate of reaction is also affected by the nature of reactant. For example: A substance having low activation energy will react faster than the substance having high activation energy.

Pressure of Catalyst:

            A chemical substance which can change the rate of chemical reaction but itself remains unchanged is called catalyst.

            The catalyst are of two types.

Positive Catalyst:

            It increases the rate of reaction

Negative Catalyst:

            It decreases the rate of reaction

Radiation OR Intensity of Light:

            The rate of photo-chemical reaction increase with the increase in intensity of light or Radiation.

Order of Reaction:

1^st Order Reaction:

            HCOOH à H₂O = CO

R = K[HCOOH]

Sum of exp (n) = 1

.:. It is the 1^st order reaction

2^nd Order Reaction:

            CH₃COOH + C₂H₅OH ßà CH₃COOC₂H₅ + H₂O

R = K[CH₃COOH][ C₂H₅OH]

Sum of Exp = 1+1=2

.:. It is second order reaction

3^rd Order Reaction:

            2NO + O₂ à 2NO₂

R = K[NO]²[O₂]

Sum of exp: 2+1=3

.:. It is 3^rd order reaction

Zero Order Reaction:

            H₂ + Cl₂ à_(sunlight)        2HCl

                        A reaction which doesn’t depends on the conc. is always the zero order reaction.