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Name: Christian
Status: student
Grade: 9-12
Location: Denmark
Country: N/A
Date: October 2006

Question:
Hi, I have done an experiment looking at the reaction between sodium thiosulphate and hydrochloric acid; now by varying the temperature I have registered some data and made an Arrhenius plot. From this I determined the activation energy for the reaction to be 52 kJmol^-1. Now I have been looking ALL around for an accepted value, but have been unsuccessful. Could you please help me by indicating either a source that contains the accepted Ea or could you provide me with the value?



Replies:
I found one source from which you might be able to compute their value of the activation energy:

http://physics.slss.ie/downloads/ch_me_6.2teacher.doc

However, remember that the concept of activation energy is empirical and a gross oversimplification of the temperature behavior of most chemical reactions. There are several problems: 1. The mechanism may involve several paths, and activation energy assumes a single "simple" path. 2. There is a constant in the equation: k = A*(exp^-Ea/RT) the so called "pre-exponential factor, A" which is also a function (usually not known) of the temperature. 3. When you plot the ln(k) vs. 1/T(kelvins) to determine Ea, the activation energy you frequently find that the activation energy, Ea, appears to be a function of temperature!! That is the plot isn't linear. And even if it is linear it is not clear what it is you are measuring because very complicated reaction mechanisms often produce simple appearing concentration and temperature behavior.

Too frequently, all of the limitations of the concept of activation energies are not spelled out for students. It's empirically good for estimating the rate of a reaction at a different temperature, but don't read a lot into its fundamental importance.

Vince Calder



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