The question: In the lab described below, why did the decolorization reaction described, which should be first order in each of the two reactants, go most slowly when the reactants were most concentrated?
As part of the introduction to our kinetics unit, I asked my students for factors that they thought would influence the amount of time it takes a reaction to occur. Not surprisingly, they came up with a variety of things, including temperature and the concentration of reactants. I set up a simple lab to compare temperature and concentration conditions based on the reaction of crystal violet with sodium hydroxide. Later in the unit we will come back to the reaction and analyze its kinetics more carefully, but for the moment, I just wanted my students to do a more basic analysis.
The students did trials for 4 conditions, and each involved the same quantity of reactants: 25 ml of 0.10 mM crystal violet and 25 ml of 1.0M sodium hydroxide. The reaction was performed by adding the crystal violet to a flask, adding water to dilute, adding the sodium hydroxide to the flask, stirring, and timing the reaction until it no longer showed visible purple. (Yes, that's a sloppy endpoint, but it was sufficient for this activity's purposes.)
Trial 1: 25 ml crystal violet, 150 ml room temp distilled water, 25 ml sodium hydroxide (a 1:8 dilution of stock reactants)
Trial 2: 25 ml crystal violet, 150 ml hot distilled water, 25 ml sodium hydroxide
Trial 3: 25 ml crystal violet, 50 ml room temp distilled water, 25 ml sodium hydroxide (a 1:4 dilution of stock reactants)
Trial 4: 25 ml crystal violet, 0 ml distilled water, 25 ml sodium hydroxide (a 1:2 dilution of stock reactants)
- The hot reaction went much faster than the cold.
- The 1:4 dilution went faster than the 1:8 dilution. (~6 minutes vs 9 minutes, respectively)
- The 1:2 dilution went much, much more slowly than any other. (~16 minutes)
Result #3 is the anomaly. It involved the most concentrated solutions and so should have been the fastest of the 3 room temperature reactions, but it was far and away the slowest.
Things I've considered:
The reaction isn't significantly endo- or exothermic, so the solution didn't have a different temperature due to its minimal mass. (All of the reactions except for the one with hot water added were within a couple of degrees of each other.)
The students didn't make a mistake in execution - this was observed by multiple groups over multiple trials.
The difference was not due to the sloppy endpoint of the reaction. Though the reduced volume meant that a given amount of remaining crystal violet would be most purple in this trial, the difference in color was enormous and obvious.
The reverse reaction never becomes significant with such a massive excess of sodium hydroxide.
New idea: The unusual result is the only trial without distilled water - maybe the water is contaminated or something like that, but in a way that is promoting the reaction? I was excited when I thought of this (I actually popped up out of a dead sleep with it in my head,) but it seems unlikely, I used the exact same container of water an hour earlier to make each of the stock solutions...
I've wracked my brain, but I can't figure out a reasonable explanation for what we observed. Any suggestions would be welcome.