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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Rate of chemical reactions

The rate of a reaction tells us how fast the reaction occurs. Let us consider a simple reaction. A + B -- > C + D

Rate of chemical reactions

 

The rate of a reaction tells us how fast the reaction occurs. Let us consider a simple reaction.

 

A + B -- >  C + D

As the reaction proceeds, the concentration of the reactant A and B decreases with time and the concentration of the products C + D increase with time simultaneously. The rate of the reaction is defined as the change in the concentration of any reactant or product in the reaction per unit time.

For the above reaction,

Rate of the reaction

1.    Rate of disappearance of A

2.    Rate of disappearance of B

3.    Rate of appearance of C

4.    Rate of appearance of D

 

During the reaction, changes in the concentration is infinitesimally small even for small changes in time when considered in seconds. Therefore differential form of rate expression is adopted. The negative sign shows the concentration decrease trend and the positive sign shows the concentration increase trend.

Rate = concentration change / time taken = - [A] / ∆t

=  -d[A] / dt =  -d[B] / dt = -d[C] / dt = -d[D] / dt

For a general balanced reaction, written with stoichiometries like x,y, for the reactant and l,m for the product, such as

 

xA + yB -- >  lC + mD. The reaction rate is

Rate = - (1/ x)  d[A] / dt  = - (1/ y)  d[B] / dt  = + (1/ l)  d[C] / dt  =

+ (1/ m)  d[D] / dt 

For example : In the reaction,

H2 + Br2 -- >  2HBr

The overall rate of the reaction is given by

 

Rate = -  d[H2] / dt   = -  d[Br2] / dt    =    d[HBr] / dt  

Consider the reaction, 2NO + 2H2  -- > N2 + 2H2O

Rate = - 1/2 d[NO] / dt   =- 1/2 d[H2] / dt  =  d[N2] / dt  =  1/2 d[H2O] / dt 

Units of Rate

 

Reaction rate has units of concentration divided by time. Since concentration is expressed in mol lit-1 or mol dm-3 the unit of the reaction rate is mol lit-1 s-1 or mol dm-3 s-1.

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