Energy of Activation

Objective

To determine the activation energy of a chemical reaction.

Discussion

The activation energy of a reaction is the amount of energy needed to start the reaction. It represents the minimum energy needed to form an activated complex during a collision between reactants. In slow reactions the fraction of molecules in the system moving fast enough to form an activated complex when a collision occurs is low so that most collisions do not produce a reaction. However, in a fast reaction the fraction is high so that most collisions produce a reaction. For a given reaction the rate constant, k, is related to the temperature of the system by what is known as the Arrhenius equation:

k=A*10^(-Ea/2.303*R*T)=A*e^(-Ea/R*T)

where R is the ideal gas constant [8.314 J/(mol-° K)], T is the temperature in degrees Kelvin, Ea is the activation energy in joules per mol, and A is a constant called the frequency factor; which is related to the fraction of collisions between reactants having the proper orientation to form an activated complex.
In this experiment you will measure the rate constant for a chemical reaction, the same one investigated in " Determination of a Rate Law" , at several temperatures, plot log k Vs. 1/T, and determine the activation energy of the reaction from the slope. (It is left to the reader to determine how the slope is related to the activation energy.) This lab will probably be done after the lab entitled " Determination of a Rate Law" . Review the data collected in that lab to determine the best test solution to used for this investigation and create your own procedure. At least four data points should be collected for a good plot: room temperature, one below room temperature, and two above room temperature.

Procedure

The class will be responsible for mixing up all solutions. Divide this responsibility among yourselves and have your procedure approved by your instructor prior to starting the experiment.

Questions

1. Plot log(k) Vs. 1/T and determine the activation energy from the slope. Place your activation energy on the chalkboard.

2. How would the recorded time for the blue color to appear be affected if the solutions being mixed at the temperatures above room temperature cooled after mixing? Explain

3. How would the error addressed in question 2 above affect the calculated activation energy? Explain.

4. How would the recorded time for the blue color to appear be affected if the solutions being mixed at the temperatures below room temperature warmed after mixing? Explain.

5. How would the error addressed in question 4 above affect the calculated activation energy? Explain.

4. From the class values listed on the chalkboard for the energy of activation determine which ones would be considered outliers at the 96% confidence level and rejected. After rejecting these values calculate the class average for the energy of activation and the standard deviation.

5. Summarize your findings for both this lab and the lab entitled " Determination of a Rate Law " in a nice neat table.


Teacher Notes

1. This document was formatted to display properly with Netscape v1.1. If your browser does not support the Netscape extensions you may not see this document as intended, that is, you may have weird looking characters on the screen.

2. In my Chem II class this lab follows the lab entitled " Determination of a Rate Law " . I feel that students rely to heavily on cook-book type experiments. Therefore, I do several labs with my Chem II students where they have to devise there own procedure in an attempt to get them to do some thinking on their own. Students are to look at the data from the Rate Law lab and determine which test solution would be the best to use for both cold and warm solutions.

3. In order to determine the Energy of Activation p and q will have to be used from the Rate Law lab.


Results from the Chemistry II class of Lapeer East High School.


 
Questions? Comments??
Revised on: 09/15/2003 at 15:11:17