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Name: Nicholas
Status: student
Grade: 9-12
Country: Hong Kong
Date: Fall 2009


Question:
I am a Grade 12 Student currently doing the IB Diploma. I am in the midst of writing an Extended Essay in Chemistry. My research question is: "How do different Acid Catalysts effect the activation energy of the reaction between Propanone and Iodine." This is a very common experiment that uses colorimetric method to find the end-point of the reaction. I have been having lots of trouble finding information on the mechanism of the Acid Catalyst and how it acts as a catalyst to reduce activation energy. All my research has been able to tell me is that it provides an alternate pathway for the reaction to occur. Further questions that if answered could help me a great deal are: -Why/how do chemicals have different colors? -The three acids I am using are H2SO4, HCl and H3PO5; this is in order of effectiveness as a catalyst in reduce rate of reaction. How can we calculate how much the H+ ion disassociates and its relationship to its effectiveness as a catalyst? -Do you have any theoretical values or any values obtained from a more sophisticated lab?



Replies:
Nicholas,

The reaction you mention has, indeed, been studied in quite some detail. How the acid helps catalyze the reaction can be answered by studying the mechanism of the reaction. The catalyst is involved in the rate determining step of the mechanism, that is forming the enol from the ketone. This step actually consists of two parts:

-protonating the ketone oxygen (this is where the H+ comes in) to form a carbocation

-losing an H+ from the alpha carbon (regenerating the catalyst) to form the enol

I would suggest you research the mechanism fully before beginning your investigation. Any organic text book will document the mechanism under: halogenation of aldehydes or ketones, you may also want to look up: ketone-enol tautomerisation. The best text for a comprehensive overview is "March's Advanced Organic Chemistry" by Jerry March (this is the Organic bible) if you can find one, Chapter 12.

Here is also a web page to get you started:

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch18/ch18-3-1.html

You should design an investigation using exactly the same conditions, but varying the nature of the acid catalyst. Your question about the degree of ionization of these acids is an interesting one, you should look up the pKa's for the acids to answer this:

http://www.chem.tamu.edu/xray/pdf/notes/salttable.pdf

But you must also bear in mind other factors such as:

-sulfuric and phosphoric acid are polyprotic (can provide more than one H+ per molecule)

-pKa depends upon the solvent being used and pKa's may not vary consistently between the acids with different solvents

-are there any anion effects from the anions produced along with the H+ from the acids

Finally: colour in chemistry is one of the hardest things to predict and explain. Colour generally comes from electronic transitions arising from absorption of photons of visible light. The gap between electronic energy levels must be quite small to absorb photons in the visible region, most molecules absorb in the UV. Classic examples of this are transition metals with split d-subshells (hence coloured complexes of transition metals). The halogens are interesting in their coloured natured and I suggest you research why this could be. Start with a web search and then try consulting some inorganic texts, "Chemistry of the Elements" by Greenwood and Earnshaw is dated but EXCELLENT for this sort of question.

Sorry I have not given you a more direct answer, but this is YOUR research project after all!!! Good luck with it!

Best wishes.

Tom Collins


You have a "barrel" of questions here. There is no way I can provide a single coherent answer here all the issues you range. You need to focus your project objectives. Just for a start, check with your language teacher. Language: "Affect" is a "verb". Example: We affect the result. "Effect" is a noun. The "effect" of this experimental method resulted in the following results.

It is difficult to reliably measure the pH of an actively reacting system. You need to establish that a change in "color" (presumably the change being from the amount of residual iodine remaining.)

The color of the solution compounds the problem. Have you measured the analytical method using known but similar, controlled substances? Yes, that is a "boring" experiment, but if the results do not calibrate your method, you are wasting your time and effort.

You need to ensure that the analytical methods you are going to use do in fact measure the experimental results you hope to observe. While "boring", these control experiments are the important base upon which to plant your subsequent experimental results.

Your statement: "Why/how do chemicals have different colors? -The three acids I am using are H2SO4, HCl and H3PO5; this is in order of effectiveness as a catalyst in reduce rate of reaction. How can we calculate how much the H+ ion disassociates and its relationship to its effectiveness as a catalyst?" defines whole research projects itself. Given any one of the three acids you selected, what are the endpoint changes you observed, or expect to observe? HCl has a single hydrogen ion, H2SO4 has two hydrogen ions, and H3PO5 has three hydrogen ions available. How are you going to unravel these differences?

Be aware that you have not yet gotten to your objective, but that is OK. More often than not the important part of a research program is "nailing down" the possible. Welcome to the REAL world of research.

Vince Calder



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