Reaction Rate, Surface Area, and Geometry
Date: Winter 2010-11
Hello, I recently did a chemistry experiment where i
measured the effect of surface area on the rate of the reaction, the
reaction between magnesium and hydrochloric acid. 15ml of 3.0mol/L
HCl was used and a 0.4*1.0cm strip of magnesium was used for each
trial. to change the surface area, i folded the magnesium (in half,
in thirds, in the shape of a V,and cylindrical). The flat piece of
magnesium and the cylindrical one did have the same surface area
though. The reaction was measured by the collection of hydrogen gas
(downward displacement of water).three trials were done, all trials
showed similar data. i know that as surface area increases the rate
of reaction should also but there were 2 outliers to my data: flat
and cylindrical piece both of which had the highest surface area.
The flat piece of magnesium was supposed to have the fastest rate,
but it had the lowest, by 10ml/s. The cylindrical piece had the 2nd
highest rate but it had the same surface area as the flat piece. I
have no clue why that happened, how to account for it or how to
Did you perform repeat measurements on each way of folding the Mg? A
single piece of data has little significance in science, we must
verify any result by repeating it.
I would not expect that folding the Mg would really change the
surface area, as acid will get in between the fold, but it still
could if you sealed it well.
I would advise you to repeat and repeat!!
Nidhi - I am wondering if your flat piece was lying flat on the bottom
of the vessel while reacting.
If true, then the bottom face was getting much less access to acid,
and its contribution to reaction rate may have been nearly suppressed.
On the other hand, if the bubbles were furious and the strip was tumbling
in the acid,
then the bottom face is unlikely to have been suppressed,
and then I have no explanation to offer you.
Measuring rates is always difficult - so do not get frustrated. The
reason rates measurements are tough is because many things can
control rates and it is often difficult to keep those things the
same throughout your experiments. Things like fluctuations in
temperature can drastically affect rates. Changes in air pressure,
even the height of the collecting tube over the pan of water, can
affect how fast the water is displaced. Since Mg is easily oxidized,
the amount of oxidation coating the strip can also have some effect.
Here are some suggestions: (1) Be sure to clean the strips with a
fine sand-paper. (2) Try to get a strong difference in rates first
by really changing the surface area between two samples. For example
you could hammer one strip into a flat strip measuring approximately
0.4*0.25cm (folding the strip twice and flattening it). (3) Make
sure that the height of the water-filled collection tube relative to
the pan and to the surface of the floor is the same for all trials.
(4) See how much of a difference a compact strip will have versus an
open one. This way, you can tell whether a small variation, such as
a V- or cylindrical shape, is statistically significant.
Greg (Roberto Gregorius)
Is it possible that there was something preventing contact between
the Mg and the acid solution?
How had that piece of Mg been handled compared with how the other
pieces had been handled?
3 replicates of each treatment may not be the most robust for
establishing the significance of differences among your test cases,
but it is better than nothing. What you will probably find is that
the differences among treatments are all insignificant, i.e. the
differences between trials within one type of treatment are as big
as differences between treatments.
Now as for why that would be, my first suspicion is that your
treatments really did not appreciably change the surface area
available for reaction- your folded pieces have to be folded so
tightly that fluid/gas is incapable of (or at least restricted from)
reaching the interior surface. Something to coat part of the Mg
piece that can withstand the HCl - paint/epoxy/nail polish/wax -
would be a more foolproof way of reducing the surface area.
Another factor that cannot be established from your description is
the orientation of your Mg pieces in a tube/beaker/flask. Some
orientations could enhance/reduce advective currents which would
increase/decrease apparent reaction rates, and orientations would
need to be consistent within a treatment to get good reproducibility.
Between these variables, there might be things you could improve to
reduce variability within treatments, and give you a better shot at
finding consistent and significant differences among treatments. If
you cannot redo the experiments, think about what the most likely
causes of variability are, and note that in your write-up.
There are not enough of details to re-analyze your results.
You do not
mention any shape "controls" for each geometry. If you double the surface
area of a particular shape, does the rate double? How were you able to
calculate the surface area and volume of the strip without knowing its
thickness? Were the origins of the strip the same (assuming the 0.4 x 1 cm
strip is the length and width, and not the thickness of the ribbon)? Was the
cylinder a rolled up tube of the strip of Mg, or a solid piece of Mg from
the same sample of metal? Was the surface area of the cylinder (including /
not including) the ends of the cylinder? Did the rate of reaction of
various flat pieces (depend / not depend) on whether the surface area was
changed by folding (as you did) the same as cutting pieces of the foil?
You have learned an important lesson, although giving you an answer
you did not expect. Science is all about digging out the explanation of
unexpected results. If we knew the answer before we began, there would not be
any reason to do the experiment in the first place! Welcome to the world of
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Update: June 2012