Steam
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How to Keep Mg ribbon from breaking up.
I am doing a lab on rate laws for my school and I have to measure the times it takes for a quantity of Mg ribbon to react with various concentrations
of HCl.
What keeps happening is the Mg ribbon breaks up and floats to the surface and then only have of it actually reacts with the HCl solution.
We tried making a cage of copper wire but again the pieces broke up and floated to the surface.
We have copper mesh but I heard that other lab groups were having trouble with it because the bubbles of H2 were getting caught in it and thereby
protecting the Mg from reacting. Any thoughts?
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DraconicAcid
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Glue the ribbon to a stopper so that you have a constant half of it exposed to the solution.
Also, only measure the rate of the reaction at the beginning of the process, so that the surface area of the magnesium doesn't change significantly.
[Edited on 1-4-2014 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Steam
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Got it, thanks!
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Nicodem
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Quote: Originally posted by Steam  | | I am doing a lab on rate laws for my school and I have to measure the times it takes for a quantity of Mg ribbon to react with various concentrations
of HCl. |
The school is making you doing rate laws for a heterogeneous reaction? Even one where the rate depends on the interaction between three different
phases?
The quality of the schools sure is going down since the old times. I hope you are not in of those countries where you even have to pay for such lousy
schooling.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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annaandherdad
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I agree with Nicodem. The experiment is ill conceived. Even if there were no gas bubbles to get in the way, the reaction of a liquid containing
reactants with a surface is complicated. As the reaction proceeds, the concentration of reactants is diminished near the surface, which slows the
reaction down until new reactants can arrive from elsewhere, either by diffusion or by bulk fluid motions. The latter can be assisted by thermal
effects if there is any heat of reaction (positive or negative), or density variations etc. Fluid mechanics is complicated, and depends on geometry,
boundary conditions and other things. Chaotic motion, which is intrinsically unpredictable, is common in fluid motions. Under these circumstances
you have to be careful to get any result of any general validity.
In your case the bubbles make things that much more complicated. Your observations are unlikely to reveal anything useful about the theory of
reaction rates, or even anything useful in a strictly empirical sense.
Any other SF Bay chemists?
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Chemosynthesis
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I agree with the above two posts. To me, it sounds like they took material from a P-chem or instrumental lab on diffusion coefficients and tried
peddling it to other courses to save even more money.
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DraconicAcid
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I think it's a teacher with an imperfect understanding of kinetics trying to come up with a simple lab using a simple reaction, not realizing that
having two phases will make it more complicated instead of simpler.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Steam
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I completely agree, I am glad my parents tax dollars pay him! *sarcastic tone*
Unfortunately I have to work with what I got, I am just trying to get half decent results. I feel especially bad for my classmates who don't have a
previous understanding of rate laws. Definitely not the way one should learn it.
I did glue the mg strips on a stopper and only had the mg in solution for a set amount of time as to not disturb the surface area too much.
I think I got fairly good data, and I will plot it tonight to see if it is first or second order with respect to the acid.
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Chemosynthesis
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| Quote: Originally posted by DraconicAcid | | I think it's a teacher with an imperfect understanding of kinetics trying to come up with a simple lab using a simple reaction, not realizing that
having two phases will make it more complicated instead of simpler. |
I don't want to believe. That is so depressing.
| Quote: Originally posted by Steam | | I think I got fairly good data, and I will plot it tonight to see if it is first or second order with respect to the acid. |
Upload your plot! At least an image.
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vmelkon
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I have done this experiment in high school but the purpose was to determine how much hydrogen gas would be produced. Measure it accurately. Use PV =
nRT to calculate n. Use n to calculate the mols of Mg and then convert that to grams.
I think the prof had weighted the Mg with an analytical balance.
The idea was to encourage the student to be precise and careful in his measurements.
Signature ==== Is this my youtube page? https://www.youtube.com/watch?v=tA5PYtul5aU
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Steam
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yeah, but teacher made his own lab for the rate unit...
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