Two similar reactions have the same rate constant at 25°C, but at 35°C one of the reaction has a higher rate constant than the other. Account for these observations.


K = Ae
Ea/RT. It is just accidental that both reactions have same k at 25°C, since K depends upon A, Ea and T. The reaction with the higher rate at 35°C has the larger activation energy, because it was able to use the increase in energy more effectively.
129 Views

A first order reaction has a specific reaction rate of 2.31 x 10–3 sec–1. Calculate the half-life period of the reaction.

For the first order reaction the half life can be shown written as :

t1/2 = 0.693/k

As we have given k= 2.31 x 10-3s-1

putting this value in above equation, we get

t1/2 = 0.693/2.31 x10-3s-1 = 300s

197 Views

What is meant by the ‘rate constant’ K of a reaction? If the concentration be expressed in mol L–1 units and time in seconds, what would be the units for K

(i) for a zero order reaction
(ii) for a first order reaction? 

Rate constant: Rate constant is a proportionality constant in the experimentally determined rate law. For a chemical reaction

AB-dAdt=kA

The rate constant of a reaction is defined as the rate of reaction when concentration of reactant is unity, e.g. 1 mol L–1.

(i) For a zero order reaction: mol L–1 sec–1.
(ii) For a first order reaction: sec–1.
586 Views

Advertisement

Following reaction takes place in one step:

2NO(g) + O2(g) 2NO2(g)

How will the rate of the above reaction change if the volume of the reaction vessel is diminished to one third of its original volume? Will there be any change in the order of the reaction with the reduced volume ?


The below reaction take place in one step, thus rate of reaction is 

2NO(g) + O2(g)  2NO2(g)

       Rate = kNO2 O2

Suppose x moles of NO and Y moles of O2 are taken in the vessle of volume V litre, then

r1 = kxV2 yV

If the volume of vessel is reduced to V/3, then for the same moles of NO2 and O2

           r2 = kxV/32 yV/32    = 27kyV2 yV


         r2r1 = 27   or   r2 = 27 r1


The rate of the reaction becomes 27 times the initial rate. Order of reaction remains unaffected.
1416 Views

Advertisement
A reaction is first order in A and second order in B.
(i) Write differential rate equation.
(ii) How is the rate affected if the concentration of B is tripled?
(iii) How is the rate affected when the concentrations of both A and B are doubled?
What is the significance of rate constant in the rate expression?

A reaction is first order in A and second order in B.

(i)        Rate = kA1 B2

       
 (ii)   r0 = kA1 B2       r1 = kA1 3B2   r1 = 9 × r0


  (iii)        r0 = kA1 B2             r2 = k2A1 2B2        r2 = 8 × r0

Rate constant of a reaction is equal to the rate of reaction when concentration of each of the reactant is unity.

120 Views

Advertisement