[newbie]

The decomposition of iodoethane in the gas phase proceeds according to the following equation:
C2H5I(g) → C2H4(g) + HI(g)
At 660.K, k = 7.2 x 10–4s–1; at 720.K, k = 1.7 x 10–2s–1. 
a.   What is the rate constant for this first-order decomposition at 325°C

b.   If the initial pressure of iodoethane is 894 torr at 245°C, what is the pressure of iodoethane after three half-lives?



So I found the activation energy. I am going to use Arrenhius equation to solve for k but I'm missing variable A, the concentration. How do I solve for that?

[Alpha-Omega]

I am sending you a link to a site that has an actual calculator that will help you solve for A.

www.shodor.org/UNChem/advanced/kin/arrhenius.html

The relationships are all laid out for you....

This calculator calculates the effect of temperature on reaction rates using the Arrhenius equation.

The first option in RED is what you need....Let me know if you need more *delete me*!!!!!!!!!!!!!!!!!!!!!

[Alpha-Omega]

1)   If you have Ea and T...then you can solve for A

2)  You can solve for k if you have Ea, T, and A

Hope this helps...

[trinhn812]

Wait I only know Ea and T.
The equation is ln(k) = -(Ea/R*(1/T)) +ln(A)
I know R, Ea, and T. So how do I find A?
The caculator assumed I already know A which I'm unsure how to find.

[Padfoot]

I don't think you even need A for this problem.  Just see how the exponential term in the Arrenhnius eqn changes and apply that (i.e. if exponential term doubles, rate constant doubles etc).  You have already calculated Ea which is needed for this. 

[Alpha-Omega]

A is a constant

[Alpha-Omega]

I am not really sure if I understood your question.  if I reread what you say and let me pull the sentence:

The decomposition of iodoethane in the gas phase proceeds according to the following equation:
C2H5I(g) → C2H4(g) + HI(g)
At 660.K, k = 7.2 x 10–4s–1; at 720.K, k = 1.7 x 10–2s–1. 

a.   What is the rate constant for this first-order decomposition at 325°C

b.   If the initial pressure of iodoethane is 894 torr at 245°C, what is the pressure of iodoethane after three half-lives?



So I found the activation energy. I am going to use Arrenhius equation to solve for k but I'm missing variable A, the concentration. How do I solve for that?

Here you indicate you are looking for the Variable A...the Concentration of A...that is from the rate eqn...not the Arrhenius eqn...see attched gifs for both equations with labeled variables...Both equations do have "A" in them....but they have entirely different meanings.

In the rate eqn  the A is a concentration for a species in the reation...it is a VARIABLE....

In the Arrhenius Eqn "A" is NOT A VARIABLE....it is a FREQUENCY FACTOR...

I have attached two gifs showing the 2 equations and their labeled variables.

You do not need that A...k is given

You for the rate constant at different temperatures, you can use these to work out the activation energy of the reaction.  You say you have done that.....that you have Ea.

So now we are down to the actual question in the problem:


What is the rate constant for this first-order decomposition at 325°C

So you have Ea, You have T, You have R,   all you need to do is plug thos in and solve for k....

I had to really re-read what you wrote.....in this case A is NOT a concentration variable....is is a constant...a frequenct factor...