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?DraconicAcid - 1-4-2014 at 09:02
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]Steam - 1-4-2014 at 09:10
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.annaandherdad - 1-4-2014 at 09:44
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. Chemosynthesis - 1-4-2014 at 10:05
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.DraconicAcid - 1-4-2014 at 10:13
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.Steam - 1-4-2014 at 10:41
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. Chemosynthesis - 2-4-2014 at 21:23
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 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.vmelkon - 4-4-2014 at 12:27
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.Steam - 4-4-2014 at 19:45
yeah, but teacher made his own lab for the rate unit...
