Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

1|Page

Chemistry Notes for class 12 Chapter 4
Chemical Kinetics
The branch of chemistry, which deals with the rate of chemical reactions
...
is called chemical kinetics
...
Fast/instantaneous reactions Chemical reaction which completes in less than Ips (1012
s) time, IS known as fast reaction
...
g
...
organic substitution reactions
...
Slow reactions Chemical re actions which completes in a long time from some minutes
to some years are called slow reactions
...
g
...
transformation of diamond
etc
...
Moderately slow reactions Chemical reactions which are intermediate between slow
and fast reactions are called moderately slow reactions
...
It is expressed in mol L-1 s-1 or Ms-1 or atm time-1 units
...
(rav can be calculated by dividing
the concentration difference by the time interval)
...
ncerthelp
...

For reaction,

Methods for measuring reaction rate (i) pH measurement, (ii) change in optical activity, (iii)
change in pressure, (iv) change in conductance
...

Factors Affecting Rate of Reaction
1
...
Temperature
www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

3|Page

3
...
Radiations and catalyst
5
...
The above expression is known as rate law
...

Rate constant may be defined as the specific rate of reaction when the molar concentrations of
the reactants is taken to be unity, i
...
,
Rate = k, if [A] = [B] = 1
Units of rate constant or specific reaction rate for a nth order reaction is given as
K = (1/Time) x (1/[Conc
...

2
...

4
...

Each reaction has a particular value of rate constant at a particular temperature
...

The value of rate constant for a reaction does’t depend upon the concentration of the
reactants
...
ncerthelp
...
ncerthelp
...


www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

6|Page

For first order chemical reactions,

(Vo, Vt, and ∞ are the volumes of NaOH solution used for the titration of same volume of the
reaction mixture after times 0, t and ∞ respectively
...
In case of pseudo first order reaction, chemical reaction between
two sr” stances takes place and one of the reactant is present in execess
...
g
...


www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

7|Page

[rO rt, and r∞
...
]
Methods to Determine Order of Reaction
(i) Graphical method

(ii) Initial rate method In this method, the order of a reaction is determined by varying the
concentration of one of the
reactants while others are kept constant
...

(iv) Half-life period (t1/2) method In general half-life period (t1/2) of a reaction of nth order is
related to initial concentration of the reactant as

www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

8|Page

This method is employed only when the rate law involved only one concentration term
...

Temperature Dependence of Rate of a Reaction
For every 10°C rise in temperature, the rate of reaction becomes double, but only 16%
collisions increases
...

Temperature coefficient is the ratio of rate constant of a reaction at two temperature differing
by 10
...
It is also called pre-exponential factor

www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

9|Page

R = gas constant
Ea = activation energy
Activated complex (or transition state)
Activated complex is the highest energy unstable intermediate between the reactants and
products and gets decomposed immediately (having very short life), to give the products
...


Threshold energy (ET) The minimum amount of energy which the reactant must possess in
order to convert into products is known as threshold energy
...

⇒ Ea = ET – ER
Lower the activation energy, faster is the reaction
...

Larger the value of Eo, smaller the value of rate constant and greater is the effect of a given
temperature rise on K
Important points about Arrhenius equation
(i) If ℜ2 and ℜ1 are rate constant at temperature T2 and T1; then


...
ncerthelp
...

(iv) Ea does’t depend on temperature, volume, pressure, etc
...

In the Arrhenius equation, when T → ∞ then ℜ = Ae° = A when Ea = 0,k = A and the rate of
reaction becomes independent temperature
...

In the chemical reactions, catalyst provides an alternate pathway or reaction mechanism by
reducing the activation energy between reactants and products and hence
...


In the presence of catalyst, activation energy decreases and hence
...

Theory of Reaction Rates
Collision Theory

www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)

11 | P a g e

According to this theory, the reactant molecules are assumed to be hard spheres and the
reaction is postulated to occur, when molecules collide with each other
...

Here, rate = – (dv/dt) = collision frequency x fraction of effective collision
= ZAB x f = ZAB x e-Ea/RT
where, ZAB represents the collision frequency of reactants, A and B e-Ea/RT represents the
fraction of molecules with energies equal to or greater than Ea
...

So, rate = PZABe-Ea/RT
The Activated Complex Theory or Transition State Theory
Reactants ⇔ Activated complex → Products
This theory is based on the fact that bond cleavage and bond formation, involved in a chemical
reaction, must occur simultaneously
...
There is an energy barrier or activated complex [intermediate product with partially
formed bond] between the reactants and products
...
The height of the barrier determines the threshold energy
...

1
...

2
...

Quantum yield or quantum efficiency of a photochemical reaction,
φ = (number of reactant molecules reacting in a given time / number of photons (quanta) of
light absorbed ill the same time)

www
...
com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)



---