[jjhixon]

Ok so heres the question

Cl(g) + Hz(g) ----> HCl(g) + H(g)

Use an Arrhenius plot to determine the activation barrier and frequency factor for the reaction

Temp(K)                                                        Rate Constant(L/mol*s)
 
90                                                                             0.00357
100                                                                           0.0773
110                                                                           0.956
120                                                                           7.781 

Ok, so i understand the basic concept and equation involved, which is k=Ae^(-Ea/RT)

First the slope was easy to find Y/X = ln(7.781)-ln(0.00357)
                                                   ----------------------  = -2767.2 K
                                                         (1/120  -  1/90)
Ea= -(slope)R = (-2767.2) x (8.314) x (1/1000)= 23.0kj/mol

That part i get BUT what i Dont get is the frequency which is

lnA= e^(y intercept), and THAT is something i just cannot solve and do not know how to get the Y intercept.

Its supposed to be y= -2767.2x + 25.112 on the graph, but i have no damn clue how to get that 25.112? where did it come from?

Can someone please show me the steps to figure out the Y Intercept, cuz theirs a lotta problems that ask for frequency factor and i cant do them, cuz i cant solve for the intercept. Also I apologize if this is too easy but my calculus is clearly inept.

Thanks

[Astrokel]

What about extrapolating your graph to cut the y-axis? Well alternatively, you could substitute all the known values into your equation to solve for frequency factor.

[jotakabe]

First, if you actually do the plot, you will get the slope and y intercept no problem.  If you don't do the plot, you can find the activation energy using the following equation:

ln(k1/k2) = (E/R)*((1/T2)-(1/T1))

Essentially, this is what you did by using the first and last points in your calculation.  However, you should know that the value that you calculated could be slightly off because you are only picking two points for your slope.

To find A without the plot, the only thing that you can do is plug in your activation energy into the Arrhenius equation and solve for A.  You have a bunch of different points, so you might need to solve for A with all of them and take an average (as someone else suggested).

Hope this helps!