Chemical Kinetics Flashcards
Intro of chemical kinetics
• Greek word “kinetics” means “moving”
• chemical kinetics is the branch of chemistry which deals with the study of:
1. Rates of chemical reaction
2. Factors affecting rate
3. The mechanism through which the reaction proceeds
4. Optimum conditions for the maximum yield of a product
Imp points (1)
∆ during the course of chemical reaction, the molar concentration of reactants gradually decreases, where as that of product increase
Rate of reaction
It is defined as the quantity of a reactant consumed or quantity of product formed per unit time, i.e:
•Rate= incre in conc of products or decr in conc reactants/ time taken for change
Hence the over all rate of reaction as defined above is actually the average rate of reaction
Velocity of reaction
The rate of reaction at a particular given moment i.e at a specific time
Velocity of reaction = dx/ dt
•dx= change in conc
• dt= small interval of time
Velocity of reaction may also be termed as instantaneous rate of reaction
Factors affecting the rate of reaction
The rate of reaction involving collisions of molecule is influenced by manufacturers enlisted below:
1. Concentration of reactants
2. Nature of reactants
3. Temperature
4. Presence of catalyst
5. Surface area of reactant (heterogeneous reactions)
6. Radiation
Concentration of reactant (factors of rate of reaction)
- number of moles or molecules of substance per unit volume (def)
- rate of reaction warries with the concentration of reactant
- rate of reaction is directly proportional to concentration of reactants
-this is because with the increase in concentration the number of molecules of reactants also increase - so frequency of collisions between them increases resulting in the increase in rate of formation of products
Eg: xA + yB => C
Rate of Rxn. α[A]^x [B]^y
dx/dt = K [A]^x[B]^y
In this rate expression the sum of exponents of concentration i.e (x+y) is called order of reaction.
-in case of reactants in gas their concentration increased by increasing their partial pressures
-sometime concentration of reactant become double and rate of reaction increases 4 time
Characteristics of ‘k’
- It has a fixed value at particular temperature.
- It’s value varies with temperature.
- It’s value at temperature remains unchanged when the concentration of either or of all reactance is changed.
Determination of rate of reaction
There are two methods for the measurement of rate of chemical reaction. They are as follow:
(a) Physical Methods
(b) Chemical Methods
Physical method
(i) Spectrometry:
This method for the measurement of rate of reaction is only applicable if a reactant or a product absorbs ultraviolet, visible or infrared radiations. The rate of reaction is
measured by measuring the amount of radiation absorbed.
Note: In this method, rate of change in amount of radiations absorbed is equal to the rate of reaction.
(ii) Electrical Conductivity method:
The rate of reactions which involve ions is measured by the electrical conductivity method. The conductivity of such a solution depends upon the rate of change of concentration of the reacting ions or ions which may form during the chemical reaction.
The conductivity will be directly proportional to the rate of change in the concentration of such ions.
Note: In this method, rate of change of conductivity of reaction mixture is equal to the rate of reaction.
(iii) Dilatometric Method:
This method is applicable for the measurement of rate of those reactions which involve small volume change in the solutions. The volume change is directly proportional to the
extent of reaction.
Note: In this method, rate of change of volume of reaction mixture is equal to the rate of reaction.
(iv) Refractrometric Method:
The method is applicable to reactions in solutions where there are changes in refractive indices of the substances taking part in the chemical reactions.
Note: In this method, rate of change of refractive indices of reacting substances is equal to the rate of reaction.
(v) Optical Rotation Method:
In this method, the angle through which plane polarized light is rotated by the reaction mixture is measured by a polarimeter. The extent of rotation determines the concentration of optically active substance. If any of the species in the reaction mixture
is optically active, then this method can be followed to find out the rate of reaction.
Note: In this method, rate of change of angle through which plane polarized
light is rotated is equal to the rate of reaction.
Chemical methods
In chemical method the concentration of reactants and products are analysed
chemically.
Energy of Activation:
The minimum amount of energy which is required for an effective collision leading to products is called activation energy.
Explanation:
Collision between reactant molecules is a necessary condition for a reaction to take place.
When physical methods are not available or suitable than a proper chemical method is used
In a chemical method generally samples are drawn from the reacting vessel at regular intervals of time
Reaction is stopped at that particular movement by suddenly chilling the sample adding it to a suitable chemical
The amount of a reactant or product present at that time is generally found by titrating the sample against a proper reagent
Gives the rate of reaction (dx/dt) at that movement
Nature of reactant (factors of rate of reaction)
•more active reactants give fast rate of reaction
•number of electrons present in intermost shell effect the reactivity of reactant
•elements of IA (ns1) are more reactive than IIA (ns2)
•neutralization reaction and decomposition reactions are very fast
•oxidation reduction reactions are slower
•ionic reactions are also fast
•depend upon activation energy “Ea”
•reaction is with high “Ea” are slow Rxn.
Eg: CO+ O2 -> CO2 (slow)
(high Ea)
•Reaction is with low “Ea” are fast Rxn.
Eg: NO + O2 -> NO2 (fast)
(low Ea)
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