Chemistry Podcast

Saturday, 26 April 2014

12 Class Chapter 5- Surface chemistry

                            Surface Chemistry

Adsorption: -  

                 The accumulation of molecular species at the surface rather than bulk of solid or liquid is called Adsorption.

Adsorbate: -

                 The molecular species or substance which accumulate at the surface.

Adsorbent: -

                 The surface of material on which Adsorption takes place.

Desorption: -

                  The process of removing of adsorbed substance from the surface on which it is adsorbed.

Absorption: - 

                    When the molecular species or substance enters in the bulk phase in solid or liquid is called as Absorption.

Sorption: - 

              When both adsorption and absorption takes place simultaneously is called as sorption process.

Mechanism of Adsorption

Inside the Adsorbent (in bulk) the force acting between the particles are mutually balanced but on the surface, the particles are not surrounded by atoms or molecules of their kind on all sides and hence they posses attraction force so particle stick on the surface of the Adsorbent.
The extent of adsorption increases with increase in surface area per unit mass of the adsorbent at a given temperature and pressure.
Heat of adsorption: - With increase in heat Adsorption process decreases.
Adsorption equilibrium: - As the molecules of the adsorb ate are held on the surface of the solid adsorbent.
Entropy decreases, i.e. S is negative
For the process of adsorption to occur, G must be negative which is possible only when, S keeps on decreasing and TS keeps on increasing till ultimately H becomes equal.
To TS so that G = 0, this state is called adsorption equilibrium.

Types of adsorption

There are two types of adsorption
        i.            Physical Adsorption or physisorption: - If accumulation of gas on the surface of solid occurs on account of weak vanderwalls forces is called physical Adsorption.
      ii.            Chemical Adsorption or chemosorption: - When gas molecules or atoms are held to the surface (solid) by chemical bonds, the Adsorption is called Chemical Adsorption.

Characteristics of physical Adsorption or physisorption

1)      Lack of specificity: - A given surface of an Adsorbent does not show any preference for a particular gas as the vanderwalls forces are universal.
2)      Nature of Adsorbate: - The amount of gas Adsorbed by a solid depends on the nature of the gas.
3)      Reversible nature: - Physisorption is reversible because adsorbate may be removed by decreasing pressure.
4)      Surface area of Adsorbent: - Physisorption increases with increase in surface area.
5)      Enthalpy of Adsorption: - Physical Adsorption is exothermic process but its enthalpy of adsorption is low (20-40 KJ mol-1).

Characteristics of Chemical Adsorption or chemosorption

1)      High specificity: - It is high specific because it occurs if there is some possibility of chemical bonding.
2)      Irreversibility: - As chemisorptions involve compound formation, so it is usually irreversible process.
3)      Temperature: - Chemisorptions increases with increase in temperature after saturation starts decreasing.
4)      Pressure: - it is also increases with increase in pressure.
5)      Surface area: - chemisorptions increases with increase in surface area.
6)      Enthalpy of Adsorption: - Enthalpy of chemisorptions is high (80-240 KJ mol-1) as it involves chemical bond formation.

Adsorption isotherm:

                                  The variation in the amount of gas Adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as Adsorption isotherm.

Freundlich Adsorption Isotherm: -  

                                                       Freundlich Adsorption Isotherm is given by freundlich in 1909, gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.
The relationship can be expressed by following equation:-
Surface Chemistry
Where ‘x’ is the mass of gas adsorbed on the ‘m’ mass of adsorbent at pressure p, k & n are constant which depend on the nature of the adsorbent and the gas at a particular temperature.

Relationship given by curve:-
Freundlich Adsorption isotherm Surface Chemistry Notes
Freundlich Adsorption isotherm
The curve indicates that at a fixed pressure there is decrease in physical adsorption with increase of temperature.

Taking logarithm of eq..(i)
Surface Chemistry Notes
The validity of freundlich isotherm can be verified by plotting  ‘log x/m’ on Y- axis and ‘log P’ on X – axis it comes to be a straight line.
free download notes of surface chemistry
The adsorption varies directly with pressure.

Adsorption from solution phase: - 

                                        solid can adsorb solutes from solution also.
Example:- litmus solution when shaken with charcoal becomes colourless.

Factors affecting Adsorption from solution phase

a)      The extent of Adsorption decreases with increase of temp.
b)      The extent of adsorption increase with an increase of surface area of the adsorbent.
c)      The extent of the adsorption depends upon the concentration of the solute in the solution.
d)     The extent of Adsorption depends upon the nature of the adsorbent and the adsorbate.

Applications of Adsorption

1)      Production of high vacuum

2)      Gas masks

3)      Control of humidity

4)      Removal of coloring matter from solution

5)      Separation of inert gases

6)      Froth floatation process

7)      Chromatographic analysis


               Berzelius suggested the term catalyst, substance which alter (change) the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalyst and the phenomenon is known as catalysis.

Promoter: -

                substance that enhance (increase) the activity of catalyst.

Poisons: -  

                it decreases the activity of catalyst.

Homogeneous Catalysis: -  

                                      When the reactants and the catalyst are in the same phase (i.e. liquid or gas).
Examples: -
Homogeneous Catalysis

Heterogeneous catalysis: - 

                                      The catalytic process in which the reactant and the catalyst are in different phase is known as heterogeneous catalyst.
Example: -
hetrogeneous catalysis

Shape Selectivity catalysis by Zeolite: -

                                                     The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called as Shape Selectivity catalysis.
“Zeolite” is good shape selective catalysts because of their honey comb like structure. They are micro porous aluminosilicates with three dimensional networks of silicates in which some silicone atoms is replaced by aluminium atoms giving AL-O-Si framework.
An important Zeolite catalyst used in petroleum industry is ZSM-S.
ZSM-S converts directly Alcohols into Gasoline (Petrol) by dehydrating them to give a mixture of hydrocarbons.

Enzyme Catalyst: - 

                       Enzymes are complex nitrogenous organic compound which are produced by living plants and animals work as a catalyst in many life process termed as Biochemical Catalysts (enzyme) and the phenomenon is known as Biochemical Catalysis.
Example: -
Enzyme Catalyst

Characteristics of Enzyme Catalysis:-

a)      Most highly efficient: - One molecule of enzyme may transform one million molecule of reactant per minute.
b)     Highly specific nature: - Each enzyme is specific for a given reaction.
c)      Highly active under optimum temperature: - The rate of an enzyme reaction is maximum at definite temperature called optimum temperature (298K-310K).
d)     Highly active under optimum PH :- Rate of enzyme reaction is maximum at optimum
PH (5-7)
e)      Increasing activity in presence of activators and co-enzymes:- The enzymatic activity in presence of certain substance called co-enzymes (vitamins) and activators are generally Na+, CO2+, Mn2+, Cu2+, etc.
f)       Inhibitors and poisons decrease or stop the rate of enzyme reaction.

Mechanism of Enzyme Catalysis: - 

Enzyme Catalysis
                                                 There are number of cavities present on the surface of colloidal particles of enzymes. the  molecules of the  reactant  (substrate ), which have complementary shape ,fit into the  theses  cavities  just like  a key fits into a lock .on the  account of the presence of actives groups .an Activated  complex is formed which then decompose as to yield the   products .

Colloidal: - 

            A colloidal is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called   dispersion medium.
Colloidal particles are larger than simple molecules but smaller enough to remains suspended .their range of diameter is between 1and 1000nm (10-9 to 10-6m)

Classification of collides: -  

                                     On the basis of (1) Physical state of dispersed phase and dispersion medium
   (2) Nature of interaction between dispersed phase and dispersion medium.
   (3) Types of particles of dispersed phase.
       I.            Classification based on physical state of dispersed phase and  dispersion medium:-

Dispersed phase
Dispersion medium
Types of  colloids
Solid sol
Some coloured glasses
Smoke, dust
Cheese, jellies
Milk, Hair cream
Fog, mist
Solid sol
Pumice, Stone
Froth, Whipped cream

    II.            classification based on nature of interaction between dispersed phase and dispersion medium :- in it  colloidal state  soles  are  divided  into two categories  :- lyophillic (solvent  attractive) ,
Lyophobic (solvent repelling)

a)      Lyophillic colloids: - 

                             The word ‘lyophillic’ means liquid loving. Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber etc. With a suitable liquid (dispersion medium) are called lyophillic sol. These sols are also called reversible sols.

b)      Lyophobic Colloids: -

                                   These words ‘Lyophobic’ means liquid hating substance like metals their sulphides etc. When simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared by only special methods; such sols are called lyophobic sols. These sols are also called irreversible sols.

 III.            Classification based on type of particle of dispersed phase:- In it, colloid are classified as multi-molecular, macro-molecular and associated colloids.

a)      Multimolecular Colloids :- 

                                     on dissolution, a large no of substance aggregate together to form  species having size in the colloidal range (diameter <1nm) the species thus formed are called  Multimolecular colloids

b)      Macromolecular Colloids :-

                                            macromolecules in suitable solvents form solution in which the size of macro molecules may be in colloidal range .such systems are called macromolecule colloids

c)      Associated Colloids (Micelles):- 

                                                     these are some substances which at low concentration behave as a normal strong electrolyte, but at higher concentration exhibit colloidal behaviours due to the formation of aggregates, the aggregates particle thus formed are called micelles or associated colloids .the formation of micelles takes place only above a particular temperature called Kraft temperature, and above a particular concentration called critical concentration (CMC).

Mechanism of Micelles formation :- 

                                                soap is the sodium or potassium salt of higher fatty acid and may be represented as RCOO- Na (e.g. sodium stearate , (CH3(CH2)16COO-Na+])  when dissolve into  RCOO- and  Na+  ions ,the RCOO- ions ,consist of two parts – a long hydrocarbon chain (also called  non – polar tail) which is hydrophobic (water repelling ) and a polar group  COO-   (polar head) which is hydrophilic (water loving ).
Mechanism of Micelles formation
            At  critical micelle concentration the  anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chain pointing towards the center of the sphere with COO-   part remaining outward on the surface of the sphere .
   An aggregate thus formed is known as Ionic micelle
Ionic micelle
Cleansing action of soaps :- the soap molecule in such a way that hydrophobic part of the stearate ions is in oil droplet and hydrophobic part projects out of the grease droplet like the bristles (hairs)
Cleansing action of soaps
Since the polar groups can interact   with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface thus soap help in emulsification and washing away of oils and fats  

Preparation of Colloids:-

a)      Chemical method: - 

                                  colloidal solution can be prepared by chemical reaction leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to form sols.
Preparation of Colloids:-  a) Chemical method

b)      Electrical disintegration or Bredig’s Arc Method: - 

                                                                            this process involves dispersion well as condensation. Colloidal sol of metals such as gold silver etc can be prepared by this method. in this method electric arc is struck between electrodes of metal immersed in the dispersion medium the intense heat product vaporised the metal, which then condenses to form particles of colloidal size.
Electrical disintegration or Bredig’s Arc Method

c)      Peptization: - 

                      it is defined as the “process of converting a precipitate into colloidal sol” by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. During peptization: - the particulate absorbs the one of the ions of the electrolyte on its surface. This cause the development of +ve charge on precipitate, which ultimately break up into small particles of the size of a colloid.

Purification of colloidal Solution: - 

                                               The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution it is carried out by following methods

(I)                Dialysis: - 

                               It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.Since, particles (ions or smaller molecular) in a true solution can pass through an animal membrane (bladder) or parchment paper or colloidal particles. The molecules and ions diffuse through membrane into the outer water and pure colloidal solution to left behind.

(II)             Electro dialysis: - 

                                          The process of dialysis is quite slow it can be made faster by applying an electric field electrodes are fitted in the compartment. The ions present in the colloidal solution migrate out to the oppositely charge to electrodes.
Electro Dialysis

(III)          Ultra filtration: - 

                                    In these methods special filters are used, which are permeable to all substances except the colloidal particles. An ultra filter paper may be prepared by soaking the filter paper in a colloidal solution, hardening by formaldehyde and then finally drying it. Thus, by using ultra filter paper the colloidal particles are separated from rest of the materials. The colloidal particles left on the ultra filter paper are there stirred with fresh dispersion medium (solvent) to get a pure colloidal solution.

Properties of colloidal solutions

(I)                Colligative Properties: - 

                                                  The values of colligative properties (osmotic pressure, lowering in vapor pressure, depression in freezing point, elevation in boiling point) are of small order as compared to values shown by true solution at same concentration.

(II)             Tyndall effect: - 

                                        It is may be defending as the scattering of the light by the colloidal particles present in the colloidal solution.

(III)          Colour: - 

                           The colour of the colloidal solution depends on the wavelength of the light scatter by the dispersed particles. The wavelength of light further depends on the size and nature of the particles.

(IV)          Brownian Movement: -

                                                It may be defined as continuous zigzag movement of the colloidal particles in a colloidal solution is known as Brownian movement.

(V)             Charge as colloidal particles:-

                                                     Charge as colloidal particles always carry is electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either +ve  or –ve.

(a)     +ve charged solution:-

1.      Haemoglobin
2.      Hydrated Metallic oxides
E.g. AS2S3.xH2O

(b)    –ve charged solution:-

1.      Metals
E.g. Cu, Ag, Au
2.      Metallic sulphides
E.g. AS2S3, Sb2S3, CdS solutions.

(VI)          Electrophoresis: -  

                                     The movement of colloidal particles under the influence of an electric field is called electrophoresis. –ve charged particles move towards the cathode and +ve charged particles moves towards anode.

Coagulation: - 

               The process of setting of colloidal particles is called coagulation or precipitation of the sol.
 The coagulation of the lyophobic sols can be carried out in the following ways:-
1.      By electrophoresis: - The colloidal particles move towards oppositely changed electrodes get discharged and precipitate.
2.      By mixing two oppositely charged sols: - Oppositely charged sols when mixed almost equal proportional neutralise then changed and get partially or completely precipitated.
3.      By Boiling: - When a sol is boiled the adsorbed layer is disturbed due to increased collisions with the molecules of the dispersion medium. This reduces the charge on the particles and ultimately led to setting down in the form of a precipitate.
4.      By Persistent dialysis: - On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely collides become unstable and ultimately coagulate.
5.      By addition of electrolyte : -  When excess of electrolyte is added the colloidal particles precipitated, the reason is that colloids interact with ions carrying change opposite to that present on themselves, this causes neutralisation leading to their coagulation.

Coagulation of lyophilic sols: - 

                                       There are two factors which are responsible for stability or the lyophilic sols. These factors are change and salvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated. This is done,
(i)     By addition of an electrolyte
(ii)   By adding a suitable solvent

Protection of colloids:-

Lyophilic sols are more stable than lyophobic sols. Lyophilic colloids have a unique property of protecting lyophobic colloids. When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) covering up the particles of lyophobic sol.


An emulsion is a colloidal dispersion in which both the dispersion medium and dispersed phase are liquids generally; one of the two liquids is water. There are two types of emulsions.
1.      Oil dispersed in water (o/w type) and
2.      Water dispersed in oil (w/o type)

1.      O/W type – water act as a dispersion medium.
                                 Example: - Milk and vanishing cream.
2.      W/O type - oil act as dispersion medium.
                  Example: - Butter and cream.

Colloids around us: - 

                            Most of the substances all we come across in our daily life are colloids, for example meals, clothes, wooden, furniture, houses, newspaper are largely composed of colloids.

Application of colloids: - 

                                      colloids are widely used in the industrial sector.


·         Electro precipitation of smoke: - 

                                                               The smoke, before is comes out from the chimney, is led through a chamber containing plates having a charged opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge get precipitated, the particles thus settle down on the floor of the chamber. The precipitator is called Cottrell precipitator.
Electro precipitation of smoke

·         Purification drinking water: - 

                                                    alum is added to water (that contain impurities) to coagulate the suspended impurities make water fit for colloidal in nature.
Example: - Argyrol is silver sol is used as an eye lotion.

·         Medicines :- 

                             Most of the medicines are colloidal in nature Ex:- Argirol is a silver sol used as an eye lotion.

·         Tanning  :- 

                         when a hide (Animal skin) , which has +ve  charged particles is soaked in tanning (or chromium salt) which contains – ve  charged particles , mutually coagulation take place. This result in the hardening of leather. This process is termed as tanning.

·         Photographic plates and films: - 

                                                          photographic plates and films are prepared by coating an emulsion of the light sensitive silver bromide in gelatine over glass plates or celluloid films.

·         Rubber industry: - 

                                   Latex is a colloidal solution of rubber particles which are negative charged Rubber is obtained by coagulation of latex.

·         Industrial products: - 

                                       paints inks, synthetic plastics, rubber, cement, graphite lubricants, etc. Are all colloids solution.


12 Class Chapter 4- Chemical kinetics

Chemical Kinetics

The branch of chemistry, which deals with the study of reaction rates and their mechanism, called as chemical kinetics.

Rate of a chemical reaction:- 

                                          “ The rate of a reaction can be defined as the change in concentration of a reactant or product in unit time”
Let a reaction whose volume remain constant                  R--->P
One mole of reactant R produces one mole of product P. [R1] & [P1] and [R2] & [P2] are the concentrations of R & P at time t1 & t2 respectively.
Chemical kinetics
Both above expression show average rate of reaction

Units of rate of reaction:-

1.      Concentration                         time-1

2.      Mol                                          L-1s-1

Instantaneous rate of reaction:-

                                                  It is the rate of change of concentration (i.e. change of concentration per unit time) of any one of the reactants or products at that particular instant of time.

Factors influencing Rate of a reaction:-

1.      Concentration:-

                              As concentration of reactant increase, rate of reaction also increases.

2.      Temperature:

                              Rate of reaction increases with increase of temperature mostly reaction rate double with rise of 100 temperature.

3.      Catalyst :-

                    Catalyst generally increase the rate of reaction without undergoing in the reaction, it also help in attaining the equilibrium quickly without disturbing the equilibrium state in reversible reaction.

Rate expression and rate constant:-

Consider a general reaction aA + bB --> cC + dD
Where,      a, b, c, and d are stoichiometric coefficient of reactants and products.
The rate expression for this reaction is-
Rate is directly proportional to [A]x [B]y  ………………………..(iii)
Where,      component x & y may or may not be equal to the stoichiometric coefficient (a & b) of the reactants
Also,   Rate = k [A]x[B]y   ………………………………(iv)
chemical kinetics

This form of equation (v) is known as differential rate equation, where k is proportionality constant called rate constant. And the equation (iii) which relates the rate of a reaction to concentration of reactants is called Rate law or rate expression.


         Rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each terms raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balance chemical reaction.
EX:-  Reaction --> Experimental rate expression
  CHCl3 + Cl2 --> CCl4 + HCl
      Rate = k[CHCl3] [Cl2]1/2
      Rate = k[CH3COOC2H5]1 [H2O]0
2NO + O2 --> 2NO2

Order of a reaction:-  

                                The sum of powers of the concentration of the reactants in the rate of low expression is called as the order of that chemical reaction.
Rate = k [A]x[B]y
Order = x+y
Order of reaction may be 0, 1, 2, 3 or even in fraction, Zero order reaction is independent of concentration. To Download Chemistry Notes in PDF join our Telegram Channel (Search @ChemistryNotesInfo on Telegram App).

Unit of rate constant (k):-

            aA + bB --> cC + dD
  Rate = k [A]x[B]y
Where,                        x+y = n = order of reaction
jitendra singh sandhu

Molecularity of a reaction: -

                                          The no. of reacting species (atoms, ions, 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.

Integral rate equation: - 

1.      Zero order reaction: - 

                                         consider a reaction-
                        R   -->
sandhu saab
                        d[R] = -k dT
Integrating both sides,
                        [R] = -kt + c ……………………….(1)
Where, c is constant of integration at t = 0, the concentration of reactant R=[R]0
Where, [R]0 is initial concentration of reactant.
Substitute in equation (1),
                        [R]0 = -k ´ 0 + c
                        [R]0 =  c   ………………………………(2)
From equation (1) & (2),
                        [R] = -kt + [R]0
                         Kt = [R]0 – [R]

     Rate = k[NH3]0 = k

2.      First order reaction: - 

                                       consider a reaction-
                        R   --> P
study notes

Integrating this equation, we get
            ln[R] = -kt + c   ……………………………….(1)  [where c is constant]
At t=0, R=[R]0 where [R]0 is the initial concentration of the reactant
Put these values in equation (1), we get
                        ln[R]0 = -k x 0 + c
                        ln[R]0 =  c      ……………………..….(2)
From equation (1) & (2)
                        ln[R] = -kt + ln[R]0  ……….…….…(3)
                             kt = ln [R]0 – ln[R]
                        k = 1/t[ln([R]0 /[R]………………………..(4)
At time t1 from equation (3)
            ln[R]1 = -kt1 + ln[R]0    ………………………...(5)
At time t2,
            ln[R]2 = -kt2 + ln[R]0  ……………………………(6)
            [R]1  & [R]2 are the concentration of the reactant at time t1 & t2 respectively
Subtracting equation (5) from equation (6), we get
            ln[R]1 – ln[R]2 = -kt1 – (-kt2)
chemistry notes


·         A plot between ln[R] and t for a first order reaction
·         A plot between log[R]0 / [R] and time for a first order reaction
graph notes

Half life of a reaction: -

                                i.            For zero order reaction: - 

 Rate constant is given by –
zero order reaction

                              ii.            For first order reaction: -
first order reaction

Temperature dependence of the rate of a reaction: -

                                                                                    Most of chemical reactions are accelerated by increase in temperature. It has been found that for a chemical reaction with rise in temperature by 10°, the rate constant is nearly double.
The temperature dependence of a chemical reaction can be accurately explained by Arrhenius equation-
k = A e-Ea/RT   …………………………(1)
Where, A is Arrhenius factor or frequency factor
R is gas constant
Ea is activation energy measured in joules/mole (jmol-1)
Also, in this reaction-
H2 (g) + I2 (g)  -->  2HI (g)

According to Arrhenius, This reaction can take place only when a molecule of hydrogen and molecule of iodine collide to form an unstable intermediate. It exist for a very short time and then break up to form two molecule of hydrogen iodide.
Temperature dependence of the rate of a reaction

The energy required to form this intermediate, called ‘activation complex’ (C), is known as activation energy (Ea).
Also, taking natural logarithm of both side of equation (1), we get-
  The plot of ‘ln k’ vs ‘1/T’ gives a straight line according to equation (2)
physical chemistry
Fig: A plot between ‘ln k’ vs ‘1/T’
In figure, slope = -Ea/R and Intercept = ln A
So, we can calculate Ea and A using these values.
At temperature T1, equation (2) is-
temperature dependance
Since A is constant for a given reaction k1 and k2 are the value of rate constant at temp. T1 and T2 respectively.
Substrate eq. (3) & (4)
We get,
chemical kinetics notes

Effect of catalyst:- 

                            A catalyst which alters the rate a reaction without itself undergoing any permanent chemical change.
notes of chemical kinetics
The action of a catalyst can be explained by intermediate complex theory.
According to this theory, A catalyst participate in a chemical reaction by forming temporary bonds with the reactants resulting in an intermediate complex and decompose to yield products and catalyst.
R + C --> R-C --> P + C
Reactant + catalyst --> intermediate complex --> product + catalyst

Collision theory of chemical reactions:-  

                                                                According to this theory “The reactant molecules are assume to be hard spheres and reaction is postulated to occur when molecule collide with each other”
    “The no. of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z)”
    Another factor which affects the rate of a chemical reaction is activation energy for a bimolecular elementary reaction.
A + B --> Product
Rate of reaction can be expressed as
            Rate = ZAB e-Ea/RT
Where,  ZAB represents the collision frequency of the reactants, A & B and
e-Ea/RT represents the fraction of molecules with energies equal to or greater than Ea.

  All collision do not lead to the formation of product. the collision in which molecule collide with sufficient kinetic energy (Threshold energy) and proper orientation, so as to facilitate breaking of bonds between reacting species and formation of new bonds to form products are called as effective collision.
Note:- Threshold energy = Activation energy + Energy possessed by reacting species.
EX:- Formation of methanol from bromoethane
CH3Br + OH- --> CH3OH + Br-
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