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Order of reaction numerical part 5

Order of reaction-

source : www.gardencity.k12.ny.us

Order of reaction-

Q 1)

i) If concentrations  of  A and B are expressed in terms of mole dm-3 and time in min. Calculate the units of  rate constant for the following reaction-

A + B ———>AB

ii) What are the units for a zero order reaction? (concentrations are expressed in mole / litre and time in seconds)

Solution)

Reaction,

A + B ———>AB

Rate       [A][B]

Rate = K[A][B]

mole dm -3/min = K [mole dm-3]2

unit of K = mole dm-3 /min (mole dm-3)2

 unit of K = dm3 mole-1 min-1 Ans.

ii) For zero order reaction,

Rate = K

dx/dt = K

unit of K =( mole / litre)/sec

Unit of K = mole  litre-1sec-1 Ans.

Q2) Write the units of the rate constant for a nth order reaction . Deduce from this the units of rate constant for ,

i) half order reaction

ii) 3/2 th order reaction

iii) third order reaction = K[A]n

Solution)

Rate = K[A]n

mole litre-1/sec. = K [mole litre-1]n

unit of K = [mole litre-1sec-1] / [mole litre-1]n

unit of K =  [mole litre-1](1-n )sec-1

i) If n=1/2, then

unit of K =  [mole litre-1](1-n)sec-1

unit of K =  [mole litre-1](1-1/2) sec-1

unit of K =  [mole litre--1]1/2 sec-1 Ans.

ii) If n=3/2, then

unit of K =  [mole litre-1](1 -n)sec-1

unit of K =  [mole litre-1](1- 3/2) sec-1

unit of K =  [mole litre-1]-1/2sec-1 Ans.

iii) If n=3 , then

unit of K =  [mole litre-1](1 -n)sec-1

unit of K =  [mole litre-1](1 -3) sec-1 =  [mole litre-1]-2 sec-1

Unit of  K = litre2mole-2sec-1 Ans.

Q3 ) State the order  with respect to each reactant and overall order for the following reactions-

i) 2 H2 + 2 NO ——-> 2H2O + N2

Rate = K [H2][NO]2

ii) H2O2 + 3I + 2H+ ——-> 2H2O + I3

Rate = K [H2O2][I]

iii) CH3CHO ——–> CH4 + CO

Rate = K [CH3CHO]3/2

iv) 2NOBr ———> 2 NO + Br2

Rate = K[NOBr]2

v) 3NO ——-> N2O + NO2

Rate = K [NO]2

What are the dimensions of rate constant in each case ?

Solution )

i)  Rate = K [H2][NO]2

order with respect to H2 =1 Ans.

order with respect to NO =2 Ans.

Overall order = 1 +2= 3 Ans.

Rate = K [H2][NO]2

unit of K = rate / [H2][NO]2

unit of K = [mole litre-1sec-1] / [mole litre-1]3

   unit of K = [mole litre-1]2 sec-1 = mole 2 litre -2 sec-1 Ans.

ii)  Rate = K [H2O2][I]

order with respect to H2O2 =1 Ans.

order with respect to I = 1Ans.

Overall order = 1 +1= 2 Ans.

Rate = K [H2O2][I]

unit of K = rate / [H2O2][I]

unit of K = [mole litre-1sec-1] / [mole litre-1]2

   unit of K = [mole litre-1 ]sec-1 = mole -1 litre  sec-1 Ans.

iii)

Rate = K [CH3CHO]3/2

order with respect to CH3CHO =3/2 Ans.

Overall order = 3/ 2 Ans.

Rate = K [CH3CHO]3/2

unit of K = rate / [CH3CHO]3/2

unit of K = [mole litre-1sec-1] / [mole litre-1]3/2

   unit of K = mole-1/2 litre1/2 sec-1 Ans.

iv)

Rate = K [NOBr]2

order with respect to NOBr =2 Ans.

Overall order =  2 Ans.

Rate = K [NOBr]2

unit of K = rate /[NOBr]2

unit of K = [mole litre-1sec-1] / [mole litre-1]2

   unit of K = [mole litre-1 ]sec-1 = mole -1 litre  sec-1 Ans.

v)

Rate = K [NO]2

order with respect to NO =2 Ans.

Overall order =  2 Ans.

Rate = K [NO]2

unit of K = rate /[NO]2

unit of K = [mole litre-1sec-1] / [mole litre-1]2

   unit of K = [mole litre-1 ]sec-1= mole -1 litre  sec-1 Ans.

 

 

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