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Topic: Writing a conclusion for the "Clock Reaction" Lab  (Read 2657 times)

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Offline docnet

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Writing a conclusion for the "Clock Reaction" Lab
« on: September 10, 2016, 11:21:33 PM »
Hello, I'm writing a lab report for a Chem 2 "clock reaction" lab. and have hit a bit of a dillemma.

A quick summary of the lab:

The rate law for the following reaction was determined by doubling the initial concentrations of Hydroxide, iodide, and bromate ions in seperate trials.

6I-(aq) + BrO3-(aq) + 6H+(aq)  :rarrow: 3I2(aq) + Br-(aq) + 3H2O(l)



the reactions contained a the same moles of thiosulfate ions which consumed the Iodine molecules, and added starch (which makes the solution turn black in the precence of Iodine molecules, so we could visually measure the time it took to reach the "indication point" with a stopwatch. The "indication point" was our method of comparison between the different trials containing different initial concentrations of the reactants.

After this part of the lab, we did more trials with the solution at different temperatures to determine the activation energy of the reaction and the frequency factor. Then we did a trial with a catalyst, which leads to my question.

1. Based on your results for the trial containing the catalyst, how does the catalyst effect the value of the activation energy of this reaction?

(I feel like the wording of this question is misleading).

In the presence of the catalyst, the reaction reached the "indication point" more quickly than the reaction without the catalyst. Therefore, the catalyst must either increase the number of effective collisions or decrease the activation energy. One way a catalyst could speed up a reaction is providing an alternate reaction path that has a lower activation energy, which increases the number of effective collisions of the reactant molecules without affecting the K value of the original, un-catalyzed reaction.

2. What additional experiment could you perform to determine the value of the activation energy for the catalyzed reaction?

a person could run multiple trials of the reaction including the catalyst at various temperatures. Plotting the rate constants in the form of ln (k) as a function of 1/T should allow us to calculate the value of the activation energy from the slope of the resulting line.

3. What is one possible difficulty you would face in performing the experiment that you described in your answer to the previous question? How Beowulf you overcome this difficulty?

A possible difficulty in performing the experiment described could be that the catalyzed reaction happens too quickly, making time measurements inaccurate. a way to relay the "indication point" would be to decrease the molarities of the reactants while keeping the concentration of the thiosulfate the same. Or, one could run the experiment only at low temperatures so that the reaction proceeds more slowly.


Is the answer to my last question acceptable? I can't ignore the feeling that I'm missing an important part of the conclusion. Why would the lab manual ask a question like this? (this is a community college science course btw).
« Last Edit: September 12, 2016, 05:31:55 AM by Arkcon »

Offline Vidya

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Re: Writing a conclusion for the "Clock Reaction" Lab
« Reply #1 on: September 11, 2016, 11:33:23 PM »
Your experiment is temperature dependent ,time dependent and concentration dependent ...all these are involved in calculations also ...for the last question you need to check if any possible errors you did which can add error in your results.

Offline docnet

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Re: Writing a conclusion for the "Clock Reaction" Lab
« Reply #2 on: September 12, 2016, 01:45:15 PM »
OK... I did some thinking and came up with two possible issues with my answer to #3.

1. If the concentrations of the initial reactants are dilute enough that BrO3- ions are completely consumed before [S2O3-] reaches zero, then there won't be an "indication point" because there won't be any I2 to react with the starch.

2. By diluting the concentrations of the initial reactants, the calculated initial rates from -Δ[BrO3-]/Δt won't accurately portray the actual initial rates. The change in concentration of the bromate ion has to be neglegible compared to the final concentration of the bromate ion. The change in concentration of the bromate ion should be taken from the very early stages of the reaction, and not when the reaction has reached halfway point or after. This is because in a reaction that is to the fourth order, the initial rate probably decreases quickly initially mixing the reactants. Diluting the reactants while keeping the thiosulfate ions the same concentration would make the "indication point" occur later in the reaction progress, so our assumption wouldn't be ok.


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