Chemical Kinetics 2 Flashcards
Molecularity of a reaction
The number of reacting species
(atoms, ions or 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
examples of different molecularity
i) unimolecular
ii) bimolecular
iii)trimolecular
i) The reaction can be unimolecular when one reacting species is involved,
for example, decomposition of ammonium nitrite.
NH4NO2 → N2+ 2H2O
ii)Bimolecular reactions involve simultaneous collision between two
species, for example, dissociation of hydrogen iodide.
2HI → H2+ I2
iii)Trimolecular or termolecular reactions involve simultaneous collision
between three reacting species, for example,
2NO + O2 —-> 2NO2
The probability that more than three molecules can collide and
react simultaneously is very small. Hence, reactions with the
molecularity three are very rare and slow to proceed.
what is rate determining step
Similarly, the overall rate
of the reaction is controlled by the slowest step in a reaction called the
rate determining step.
Consider the decomposition of hydrogen peroxide:
2H2O2 ——I-/Alkaline medium——> 2H2O + O2’
This reaction is first order with respect to both H2O2 and I–
. Evidences suggest that this reaction takes place in two steps
(1) H2O2+ I– → H2O + IO–
(2) H2O2 + IO– → H2O + I– + O2
Both the steps are bimolecular elementary reactions. Species IO-
is
called as an intermediate since it is formed during the course of the
reaction but not in the overall balanced equation. The first step, being
slow, is the rate determining step. Thus, the rate of formation of
intermediate will determine the rate of this reaction.
There for
Rate = k [H2O2] [I-]
differentiate between order and molecularity
(i) Order of a reaction is an experimental quantity. It can be zero and
even a fraction but molecularity cannot be zero or a non integer.
(ii) Order is applicable to elementary as well as complex reactions
whereas molecularity is applicable only for elementary reactions.
For complex reaction molecularity has no meaning.
(iii) For complex reaction, order is given by the slowest step and
molecularity of the slowest step is same as the order of the overall
reaction.
zero order reactions
Zero order reactions are relatively uncommon but
they occur under special conditions. Some enzyme
catalysed reactions and reactions which occur on
metal surfaces are a few examples of zero order
reactions.
example of zero order reaction. why is it zero order?
The decomposition of gaseous ammonia
on a hot platinum surface is a zero order reaction at high pressure.
2NH3 (g) ——1130k/Platinum catslyst→N2(g) +3H2 (g)
In this reaction, platinum metal acts as a catalyst. At high pressure,
the metal surface gets saturated with gas molecules. So, a further
change in reaction conditions is unable to alter the amount of ammonia
on the surface of the catalyst making rate of the reaction independent
of its concentration.
The thermal decomposition of HI on gold surface
is another example of zero order reaction.
1st order reaction examples
Hydrogenation of ethene is an example of first order reaction.
C2H4(g) + H2(g) → C2H6(g)
Rate = k [C2H4]
All natural and artificial radioactive decay of unstable nuclei take
place by first order kinetics.
what is the half life of a reaction
The half-life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration.
It is represented as t1/2.
For zero order reaction t
1/2 prop. [R]0
. For first order reaction t1/2 is independent of [R]0
.
what are pseudo first order reactions?
A chemical reaction in which value of order of reaction is one but molecularity is more than one are known as pseudo unimolecular/pseudo first order reaction.
explain pseudo first oder reactions with an example
consider the acidic hydrolysis of ethyl acetate
In reality, it is a second order reaction and concentration of both ethyl acetate and
water affect the rate of the reaction. But water is taken in large excess for hydrolysis, therefore, concentration of water is not altered much
during the reaction. Thus, the rate of reaction is affected by concentration of ethyl acetate only.
CH3COOC2H5+ H2O —-H+——>CH3COOH + C2H5OH
Rate=k’[CH3COOC2H5][H2O]
but [h2o] is a const so
rate=k[CH3COOC2H5]
where k=k’[H2O]
pseudo 1st order (inversion)
C12H22O11 + H2O + →H C6H12O6+ C6H12O6
cane sugar glucose fructose
rate=k[C12H22O11]
