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Specialty Chemistry Forums => Chemical Engineering Forum => Topic started by: titanium on January 20, 2008, 04:11:23 AM

Title: Chemical Kinetics
Post by: titanium on January 20, 2008, 04:11:23 AM

Hi,

Can anyone help me with the following questions please?

1) What are the rate laws and orders of reactions for the folloing reactions?
(a) A reaction with the general stoichiometry of A<=>R?
(b) Unimolecular reversible reactions of the type A<=>R?

2) Consider the following two possibilities for the reactions involving A, B and C:
A -> B + C
A -> B -> C

Describe qualitatively how the ratio rA / rB changes with time in each case (invariant, increasing or decreasing with time) if only A is present initially.

3) For a gas phase reaction at 400 K the rate is reported to be

-dpA/dt = 3.66pA^2, atm/hr

(a) What are the units of the rate constant?
(b) What is the value of the rate constant for this reaction if the rate equation is expressed as

-rA = -(1/V)(dNA/dt) = kCA^2, mol/(m^3.s)

4) For the reversible reaction A + B <=> 2C, if the forward rate law is known to be rCf = kCf.cA.(cB^2) (note that the forward rate is expressed in terms of the formation of product C), suggest a thermodynamically consistent rate law for the reverse reaction with respect to the step.

I appreciate any help. Thank you.

Title: Re: Chemical Kinetics
Post by: eugenedakin on January 23, 2008, 09:56:24 PM

Hello titanium,

I am glad that you found this forum to help you out.

One of the stipulations of this forum is that you need to attempt to answer the question yourself.  Could you attempt to answer the question and then I can help you from there.

I look forward to your attempted solutions.

Sincerely,

Eugene

Title: Re: Chemical Kinetics
Post by: titanium on January 26, 2008, 12:56:51 AM

Hi Eugene,

Thanks for the reminder. I know how to solve the questions except this one:

2) Consider the following possibility for the reactions involving A, B and C:
A -> B -> C

Describe qualitatively how the ratio rA / rB changes with time in each case (invariant, increasing or decreasing with time) if only A is present initially.

This is my working:

The series reaction can be written as
Reaction (1)  A -> B
Reaction (2)  B -> C

The rate equation for rxn 1 is -rA = rB1 = r1. The rate eqn for rxn (2) is -rB2 = rc = r2.

rB = rB,net = rB1 - rB2

Hence rA/rB = -rB1/(rB1 - rB2). But I am not sure whether this ratio (rA/rB) is invariant, increases or decreases with time.

Please try to help me. Thank you so much.

Title: Re: Chemical Kinetics
Post by: eugenedakin on January 26, 2008, 08:32:36 AM

Hello titanium,

No problem at all.  Thanks for looking into the question.

Lets consider all three possibilities and determine a trend for each reaction.  Lets say the starting material (just for arguements sake) 'A' is given the value of 5 (I just picked the number randomly).

Lets determine the reaction rate value for 'B' (given the formula you have already calculated) if the value of 'A' decreases, to lets-say '2'. Then calculate out the value for the ratio rA/rB.  Did the ratio change?

The question mentions that 'A' is present initially, so my guess is that the concentration of 'A' is decreasing with time. The above number estimation should help you answer your question.

I hope this works out well for you.   :)

Sincerely,

Eugene