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Kinetics III
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Law of mass action

Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

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  1. XIl -o4 Kinetics-111


  2. Instantaneous rate: The rate of change of concentration of any one of the reactants or products over a very small interval of time In general if in a reaction: Rate =-1d[A] a dt b dt = +4 c dt 1D d dt


  3. Experimental Determination of instantaneous rate of reaction:- In order to determine changes in concentration of reactants or products, one should take small portions of the reaction mixture at suitable intervals of time and freeze them rapidly to about 0 C as to stop the reaction. The concentration is then measured with the help of suitable method. Time--+ A curve is ploted between concentration and time. A tangent is drawn to the curve at the point corresponding to time interval 't'. the slope of this tangent gives the instantaneous rate of reaction: Instantaneous rate of reaction = slope of curve - Intercept along ordinateAX intercepta along abscissa At


  4. Reaction life time: It is defined as the time taken by the reaction to proceed to 98% of completion The shorter the life time, faster the reaction.


  5. Law of mass action: > At a given temperature, the rate of a reaction at a particular instant is proportional to the product of the active masses of the reactants at that instant raised to the powers which are numerically equal to the numbers of their respective molecules in the stoichiometric equation describing the reaction.


  6. Active mass molar concentration of the substance no. of moles of the substance volume in litres M x V For a reaction aA + bB cC dD According to law of mass action Rate o [A]a[Bjb Rate k[A]a[B]b If [A] 1 [B] 1 then Rate k


  7. The value of rate constant depends on: (i) Nature of the reactant (ii) Temperature (iii) Catalyst.


  8. Units of Rate constant: Unit of rate constant depend upon order of reaction and is therefore different for different reaction Unit of Rate constant - time-1 unit of concentration sec- mol/litre litre1n-1 mol -1 sec


  9. Rate of reaction It is the speed with which reactants are converted into products S.No Reaction rate constant 1. It is proportionality constant. It is measured as the decrease of concentration of reactants or increase in concentration of products It is equal to the rate of reaction when concentration of reactant is equal to unity 2. It is independent of the concentration of reactant It depends upon inital concentration of reactants 3.