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2-Fsc-I(Chemistry)

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					               Chemistry Notes(Fsc-I) By Rai Faisal
                   2- 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 behaviour 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 collegative 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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