Chemical Kinetics
Order
It can be defined as the sum of the exponents of the concentration terms of the rate law
Or
It may be defined as the sum of the powers to which the concentration terms are raised in a rate law expression. This is an experimentally determined factor.
For the reaction
Rate = k [A]x [B]y
The order ‘n’ of the reaction = ‘x’ + ‘y’ where ‘a’ and ‘b’ may or may not be equal to ‘x’ and ‘y’
What are the different orders of reaction?
Zero, First, Second, third etc.
A zero order reaction is independent of the concentration of reactants. The rate of the reaction does not depend on the concentration terms or in other words an increase in the quantity of the reactants does not increase the rate of the reaction.
Photochemical formation of HCl from hydrogen and chlorine is a zero order reaction.
Units of Zero order reaction:
First order reaction:
The rate of a first order reaction depends on one concentration term only. It is possible that there could be more than one reactants involved in the reaction.
All nuclear fission reactions are first order decay or disintegration.
Hydrolysis of esters: are first order reactions
Unit of first order reaction:
Second order reaction:
The rate of a second order reaction is dependent on two concentration terms. This depends on the mechanism of the reaction. Example of a second order reaction is given below
Unit of ‘k’ for second order reaction:
Problem:
When the concentration of a reaction doubles the rate of the reaction increases 4 times what is the order of the reaction
4 = 2n
log 4 = n log 2
n = log 4/log2 = 2
order of the reaction = 2
Molecularity
The number of reacting species (atoms, ions or molecules) that must collide with each other simultaneously so as to result in a chemical reaction is called molecularity.
Pseudo Uimolecular reactions
Reactions in which the ORDER is one but molecularity is two is called pseudounimolecular reactions. E.g. Hydrolysis of Esters.
CH3COOH + CH3CH2OH « CH3COOC2H5
Mechanism of a reaction
A sequence of elementary reactions or steps written to account for the overall reaction is called the mechanism of the reaction.
Example:
| NO2 + F2 => NO2F + |
The rate-determining step is the slowest step. The rate law is written using this step.
Rate = k[NO2] [F2]
Order ‘n’ = 1+1 = 2
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Life Time
The time taken for the reaction to complete is know as life time (usually 98%)
Half Life ‘t1/2’
The time taken for half of the reaction to complete is called half life period of the reaction.
or
The time taken for the concentration of the reactants to reach 50% of its initial value:
Problem:
Find the amount of reactants present after time‘t’ if half life (‘t1/2’) is know
Af = Ao x (½)n ‘n’ = number of half lives, total time/number of half lives
Af = Final concentration, Ao = Initial concentration
Factors affecting the rates of reactions;
- Nature of reactants
- Concentrations of reactants
- Temperature
- Catalyst
- Radiation
Nature of reactants
1. Physical state of reactants
a) Gaseous reactions are faster than liquid reactions which are greater than solid reactions. This is because the chances of interaction of the gaseous molecules are greater than in liquids which are greater than in solids.
2. Particle size of reactants
a) Greater the surface area greater is the chance of interaction of the reactant species. This can be seen from the following reactions.
i) Powdered coal burns faster than apiece of coal
ii) powdered sugar dissolves faster than a lump of sugar
iii) pulverized wood and wood shavings burn faster than wood of the same mass
3. Chemical nature of reactants
a) The rate of a reaction depends on the chemical nature of the reactants.
i) MnO4– + 8H+ + 5Fe2+ => Mn2+ + 5Fe3+ + 4H2O ( fast)
ii) MnO4– + 16H+ + 5C2O42- => Mn2+ + 10CO2 + 8H2O ( slow)
The two reactions occur at different rates due to the difference in the nature of the reducing agents ie Fe2+ and C2O42-
Concentrations of reactants
The rate of the reaction decreases with decrease in concentration. This is obvious from the law of mass action. When the concentration is large the chance of reactants coming in contact with each other is greater. Hence the rate of the reaction will increase.
Effect of Temperature
As a rule generally speaking a rise in temperature increases the rate of the reaction (whether endothermic or exothermic) this is because all reactions occur only if the reactants molecules possess Threshold Energy.
Threshold Energy
It may be defined as the minimum amount of energy which the colliding molecules must posses in order that the collisions may become effective.
Activation Energy
The extra amount of energy which the molecules of the reactants have to absorb so that their energy becomes equal to the threshold energy is called Activation Energy. (Graph)
Activation Energy = Threshold Energy – Energy possessed by the reactant molecules.
Fast reactions have low activation energy
Slow reactions have high activation energy
Orientation Of Molecules
All collisions does not lead to the formation of the products. Only those molecules which possess threshold energy and have a proper orientation form the Transition state or Activated Complex, which can further change into products.
Thermodynamic and Kinetic Stability
Every reaction which is spontaneous on the basis of free energy change ( i.e. having G negative) may not take place under ordinary conditions due to high energy of activation such reactions may be thermodynamically unstable but kinetically stable. This is why fuels do not undergo combustion spontaneously.
Presence of Catalyst
A catalyst is a substance that increases the speed of a reaction without itself undergoing any chemical change.
A catalyst provides an alternate path with lower energy barrier. It decreases the energy of activation of the reaction.
Effect of radiation
Those reactions which take place only in the presence of electromagnetic radiation’s are known as Photochemical reaction’s ( In the absence of light the reactions do not occur)
E.g.
Photography
Photosynthesis
Blue Printing
Reaction between H2 and Cl2 in the formation of HCl
There are three steps involved in the reaction
Chain initiation step:
Chain Propagation step:
Chain Termination step:
Difference between order and molecularity of a reaction
[table id=3 /]
| Order of a reaction | Molecularity of a reaction |
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Based on bond’s present in compounds predict the rate’s of reaction
- Compounds with covalent bonds undergo slow reactions
- Ionic compounds in aqueous solutions undergo fast reactions at room temperature
- Combustion reactions are fast once the reactions are initiated.
- Reactants in the gaseous state undergo reactions faster than reactions in liquid and solid state.
- Reactants in the powdered form undergoes quicker reactions due to increased surface area
Mechanism of Reactions:
A sequence of elementary reactions or steps written to account for the overall reaction is called the mechanism of the reaction.
Example:
| NO2 + F2 → NO2F + |
Slow reaction step IFast reaction step II |
| 2NO2 + F2 → 2NO2F |
In the above example the slowest step is the rate determining step. The rate determining step involves only 2 reactants therefore it is a second order reaction.
Rate = k [NO2][ F2] order n = 2
(The rate-determining step is the slowest step. The rate law is written using this step.)
When will you suggest a mechanism for a reaction?
When the rate law is not equal to the law of mass action it implies that the reaction does not take place in a single step hence a mechanism should be suggested to satisfy the rate law if known.
What are elementary steps in a mechanism?
In a multi step reaction the single steps involved are called the elementary steps.
The different elementary steps should add up to represent the actual reaction.
What are reactive intermediates?
Reactive intermediates are molecules, atoms or ions formed as a result of molecular interaction of the reactants or the cleavage of the covalent bonds in them. (the F atom in the above reaction is a reactive intermediate with high energy and looking for ways to stabilize ASAP)
What happens to the reactive intermediates in a reaction involving a multi step reaction?
The reactive intermediates in the elementary steps have very short existence and high energy and usually they change rapidly into other products and will not be present in the final equation as they will cancel out in the subsequent elementary steps.
Why is the slowest step an important step in determining the rate of the reaction?
The elementary steps that involve cleavage of covalent bonds in aqueous solutions require large amount of energy and hence are slow reactions Energy of activation is high. The quantity of products formed depends on the rate of the slowest step or RDS. The reactive intermediates are formed in the slow steps and hence the rate of reaction of the intermediate does does not alter the rate of the reaction.
Elementary steps which are slow are the rate determining steps- hence rate law is written using this step. (The reactive intermediates are high energy species and hence will change quickly into other products rather fast and such elementary steps will usually be faster than the rate determining steps.)
Videos on Initial rate method problem
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