CBSE Class 12 Chemistry Chemical Kinetics Notes Set A

CHEMICAL KINETICS

summary 

Chemical kinetics- The branch of physical chemistry which deals with the study of rate of reaction and their mechanism is called chemical kinetics. 

Rate of chemical reaction- The change in concentration of any reactant or product per unit time is called rate of reaction. Its Unit is moll^-1 sec^-1 or atm sec^-1 

TYPES OF RATE OF REACTION- 

1. Average rate of reaction- The rate of reaction measured over the long time interval is called average rate of reaction.   R ——→ P 

Avg rate = -Δ[R]/Δt = +Δ[P]/Δt 

2. Instantaneous rate of reaction- The rate of reaction measured at a particular time is called instantaneous rate of reaction.  

Instantaneous rate = -d[R]/dt=+d[P]/dt 

RATE OF REACTION FOR THE EQUATION WITH STOICHIOMETRIC COEFFCIENTS: 

aA+ bB —→   C + dD 

rate of reaction= -1d[A] = -1d[B] = 1d[C] = 1d [D]                             

                              a dt       bdt        cdt       d dt 

FACTORS AFFECTING RATE OF REACTION- 

1. Concentration of reactant: rate of reaction is directly proportional to concentration of reactants. 

2. Surface area: greater the surface area faster is the reaction. 

3. Temperature: rate of reaction increases with temperature 

4. Nature of reactant: rate of reaction is faster in ionic compounds than covalent compounds. 

5. Presence of catalyst: it increases the rate of reaction by lowering activation energy. 

6. Radiation: rate of reaction increases with radiation in photochemical reactions  

RATE LAW- The rate of reaction is directly proportional to the product of concentration of reactant and each concentration is raised to some power which may or may not be equal to stereochemistry experimentally 

For a reaction 

aA+bB → cC+dD 

Rate law = k[A]^p[B]^q 

Where powers p and q are determined experimentally which may or may not be equal to stoichiometry coefficient. 

RATE CONSTANT (k)- It is equal to the rate of reaction when molecular concentration of reactant is at unity. 

Units of rate constant for nth order : (molL^-1) ^1-n sec^-1 or (atm or bar) 1-n sec-1 for gaseous reaction 

Units of k for zero order is moll^-1 sec^-1 

Units of k for 1st order is sec^-1 

Units of k for 2nd order is mol2l^-2 sec^-1 

MOLECULARITY – The total no. of reactants taking part in elementary chemical reaction is called molecularity. 

ORDER OF REACTION- The sum of powers to which the concentrations terms are raised in a rate law expression is called order of reactions. For above case order = p+q: orders of rn is determined experimentally. 

Zero order reaction: rate of reaction does not depend upon concentration of reactants. For eg decomposition of ammonia 

2NH3 —→ N₂+ 3H₂ 

rate = k[R]⁰ 

First order reaction: decomposition of sulphuryl chloride 

SO₂Cl₂  —→  SO₂+ Cl₂ 

rate = k[R] 

Second order reaction: for eg decomposition of HI 

2HI —→ H₂+ I₂ 

rate = k[R]² 

Third order reaction: for eg 

2NO+ O₂ —→ 2NO₂ 

METHODS TO CALCULATE ORDER OF REACTION: 

1. Graphical method 

2. Initial rate method 

3. Integrated rate method  

4. Half-life method.  

Integrated rate equation:  

Zero order reaction: k=[R₀]-[R] /t

first order reaction: k= 2.303/t log [R0]/ [R] 

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