CHEMICAL KINETICS
The
branch of chemistry that deals with the study of rate of reaction, factors on
which rate of reaction depends and mechanism of the reaction is called chemical
kinetics.
RATE OF REACTION:
The
rate of change of concentration of reactants or products per unit time is called
rate of reaction.
Consider
a general reaction
R → P
Rate = -Δ [R] /Δ t =
Δ [P]/ Δ t
Δ[R]/ Δ t
= [final conc.] ─ [initial conc.]/[final time] - [initial
time]
UNITS: - mol ⁄ lit ⁄ sec
TIME
SCALE USED: - Millisecond = 10-3
Microsecond
= 10-6
Nanosecond = 10-9
Pico second = 10-12
Femtosecond = 10-15
INSTANTANEOUS RATE OF REACTION
It is the rate of
change of concentration of reactants or products at a particular instant
of time.
Consider
a general reaction
R → P
Instantaneous
rate of reaction will be given as:
Rate = -d[R]/dt = d[P]/dt
/[mol
L-1]n-1
FEMTOCHEMISTRY
The branch of chemistry that deals with the study of very fast reactions
approximately in the order of 10-15 sec is called femtochemistry.
NUMERICAL:
- Write over all rate of the following reaction.
N2 + 3H2
→ 2NH3
Ans:
- Rate = - Δ [N2]/ Δt = - 1 Δ
[H2]/ 3 Δt = 1 Δ [NH3]/2 Δt
NUMERICAL:
- Express the rate of following reaction.
2N2O5
→ 4NO2 + O2
Rate
= - 1 Δ[N2O2]/ 2Δt = +
1 Δ[NO2]/ 4 Δt = + Δ[O2]/Δt
NUMERICAL:
- Write rate of the following reaction
H2 + Cl2
→ 2HCl
Rate
= - Δ[H2]/ Δt = - Δ [Cl2]/ Δt = +1 Δ [HCl]/2 Δt
NUMERICAL:
- Express the rate of the following reaction.
4NH3 + 5O2 → 4NO + 6H2O
Rate
= -1 Δ [NH3]/4 Δt = -1 Δ [O2]/5 Δt = +1 Δ[H2O]/6 Δt
NUMERICAL: - PCl5 → PCl3 + Cl2
Rate = - Δ[PCl5]/ Δt = +
Δ[PCl3]/ Δt = +Δ[Cl2]/ Δt
FACTORS AFFECTING RATE
OF RACTION
(1)
NATURE OF THE REACTANTS: - Rate
of reaction
depends upon the nature of reactants. The reactions of ionic compounds are very
fast as compared to covalent compounds.
(2)
CONC. OF THE REACTANTS: - Rate of reaction is directly proportional to the
concentration of the reactants.
(3)
TEMPERATURE: - Rate
of the reactions increases with the increase in temperature. Generally it
becomes double for every ten degree rise in temperature
(4)
CATALYST: -
The rate of reaction increases with the use of catalysts.
(5)
SURFACE AREA: -
Greater the surface area of the reactants faster will be the rate of reaction.
(6)
RADIATION: - The
rate of some reactions increases when the reactants are exposed to radiations.
E.g. when a mixture of hydrogen and chlorine is exposed to sunlight, it reacts
violently.
PHOTOCHEMICAL
REACTIONS:
The chemical reactions which occur in the
presence of light are called photochemical reactions. E.g. when a mixture of
hydrogen and chlorine is exposed to sunlight, it reacts violently.
H2 + Cl2
→ 2HCl
(violent reaction)
1Q:
- Rate of reaction becomes double for every 10 degree rise in temperature. Why?
Ans:
- This is due to the reason that for every ten degree rise in temperature the
fraction of molecules undergoing effective collision becomes double.
2Q:
- The rate of reaction increases with the increase in concentration of the
reactants. Why?
Ans:
- With the increase in concentration the number of molecular collisions
increases and hence the reaction rate also increases.
3Q:
- Rate of reaction increases with the use of catalyst. Why?
Ans:
- Catalyst lowers the activation energy barrier by changing the path of the
reaction. As a result of which greater number of reactant molecules are
converted into products by crossing over the energy barrier and rate of
reaction increases.
4Q:
- Why does the rate of reaction keeps on decreasing with time?
Ans:
- We know that rate of reaction is directly proportional to the concentration
of reactants. In the beginning the conc. of reactants is maximum hence the
reaction rate is also maximum .But with the passage of time conc. of reactants
decreases which eventually leads to decrease in rate of reaction.
5Q:
- Why does the rate of reaction not remain constant throughout?
Ans:
The rate of reaction depends upon conc. of the reactants which keeps on
decreasing .As a result of which rate of reaction does not remain constant
throughout.
6Q:
- Instantaneous rate of reaction is preferred over average rate of reaction
.why?
Ans:
- The rate of reaction decreases with time and therefore average rate of
reaction does not have any significance for the reaction.
7Q:
- How is rate of reaction related to the concentration of the reactants?
Ans:
- Rate of reaction is directly proportional to concentration of the reactants.
8Q:
- Why a negative sign is assigned to the rate of reaction when it is expressed
in terms of change in the conc. of reactants?
Ans:
- Due to the fall in the conc. of reactants
LAW OF MASS ACTION
This
law states that the rate of reaction is directly proportional to the product of
molar concentrations of all the reactants. Consider following general reaction
A + B → C
Rate α [A] × [B]
Rate = k × [A] × [B]
Here k is called rate constant
When [A] = 1; [B] = 1
Then Rate = k
Thus
rate constant is defined as rate of reaction when concentration of each of the
reactant is unity.
Q:
- What is temperature coefficient of a reaction?
Ans:
- It is the ratio of rate constant of a reaction at two different temperatures
which differ by 10 degree.
Temp coefficient = Rate constant at
(T+ 10) K/Rate constant at T K
CHARACTERISTICS OF RATE
CONSTANT
(1)
It is the measure of rate of reaction.
(2)
Its value is different for different reactions.
(3)
It changes with the change in temperature.
(4)
It is independent of the concentration of reactants.
(5)
Its units depend upon order of the reaction.
1)Q:
- Define specific rate constant.
Ans:
- It is defined as rate of reaction when concentration of each of the reactant
is unity.
2)Q:
- In what respect specific reaction rate differs from the rate of the reaction?
Ans:
- specific reaction rate is the rate of reaction when concentration of each of
the reactant is unity.
Where as the rate of reaction is the rate of change of
concentration of reactants or products per unit time.
3)Q:
- How is rate constant related to the conc. of reactants?
Ans:
- It does not depend upon the concentration of the reactants.
RATE LAW
The
equation which describes the rate of reaction in terms of molar concentration
of the reactants as determined experimentally is called rate law.Consider
a general reaction
nA + mB → C
According
to rate law:-
Rate
= k[A]α[B]β
Here
α = n or α ≠ n
And β = m or β ≠ m
ORDER
OF REACTION: - It is the sum of powers to which concentration terms are raised
in the rate law equation.
Consider a general reaction
A + 2B → C
Rate
= k[A]1[B]2
ORDER
= 1+2=3
NUMERICAL:
- Write Rate law for the following reactions and find out the order of the
reactions.
2NO2
+ F2 → 2NO2F
Rate
law for above reaction is
Rate
= k [NO2]1 + [F2]1
ORDER
= 1+1 = 2
NUMERICAL:
- CH3CHO → CH4 + CO
According
to Rate law
Rate
= k [CH3CHO]1.5
ORDER=1.5
NUMERICAL:
- 2N2O5 → 4NO2 + O2
According
to Rate law
Rate
= k[N2O5]1
ORDER=1
NUMERICAL:
- H2 + Cl2 → 2HCl
According
to rate law
Rate
= k[H2]0[Cl]0
ORDER=0
NUMERICAL:
- 2NO + O2 → 2NO2
According
to rate law
Rate
= k [NO]2 [O2]1
ORDER
=2+1=3
NUMERICAL:
- 2N2O → 2N2 + O2
According
to Rate law
Rate
=k[N2O]1
ORDER=1
UNITS OF RATE CONSTANT
FOR DIFFERENT ORDER REACTION
FORMULA
1 Sec-1 /[mol
L-1]n-1
UNITS
FOR 0 ORDER RXN
= 1
Sec-1/[mol
L-1]0-1
= [mol L-1]1 Sec-1
= mol L-1 Sec-1
UNITS
FOR 1ST ORDER RXN
= 1
Sec-1/[mol
L-1]1-1
= [mol L-1]0 Sec-1
= Sec-1
UNITS
FOR 2ND ORDER RXN
= 1 Sec-1/[mol
L-1]2-1
= [mol L-1]-1Sec-1
= L mol-1 Sec-1
UNITS
FOR 3RD ORDER RXN
= 1 Sec-1/[mol
L-1]3-1
= [mol L-1]-2 Sec-1
= L2 mol-2 Sec-1
COLLISION THEORY
To
explain the effect of temperature on the rate of reaction Arrhenius put forward
a theory known as collision theory. According to this theory the reacting
molecules should undergo effective collisions to bring about a chemical
reaction. In order to undergo effective collisions molecules must possess
threshold energy and proper orientations. Now with the rise in temperature the
number of effective collisions increases as a result of which rate of reaction
gets enhanced.
THRESHOLD ENERGY
The minimum amount of energy which the
colliding molecules must possess to bring about a chemical reaction is called
threshold energy.
ACTIVATION ENERGY
The
extra amount of energy which the reacting molecules have to absorb so that
their energy becomes equal to threshold energy is called activation energy. It
is denoted by Ea.
Ea = Threshold energy – Actual
energy of the reactants
ACTIVATION ENERGY
BARRIER
The
energy barrier which the reactants have to cross by absorbing activation energy
to form products is called activation energy barrier.
ACTIVATED COMPLEX
The
reactants are converted into products via the formation of high energy intermediate
species called activated complex.
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