[CHM 112] Temperature Kinetics Graphs

[Andrew J. Pounds]

Hey -- let's all make sure that we do this right...  review the lab
description and then read the following...

The Arrhenius equation is


k=Ae−EaRTk = A e^{-\frac{E_a}{RT}}

In this lab the concentrations are all the same, so the rate is directly
proportional to the rate; we are essentially replacing the rate constant
in the Arrhenius equation with the rate.

We take the natural log of both sides to get ln(rate)=ln(A)−EaR1  
Tln(k) = ln(A) - \frac{E_a}{R} \frac{1}{T}

Notice how this looks like the equation of a line:  y = b + mx

So you plot the natural log of rate on the y axis and 1/T on the
x-axis.  Make sure T is in kelvin.   Since your rates will all be less
than 1 (but not negative) the natural log of your rates will be
negative.   You should get a graph with a negative slope.   That is
okay, however, because that negative slope is equal to −Ea/R-E_a/R.  The
negatives cancel and you should be left with a positive activation energies.

If I see negative activation energies reported bad things will happen...

 

-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University,  Macon, GA 31207   (478) 301-5627

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