

Rate of Reaction Mg and HCl
Name: Trang
Status: student
Grade: Key Stage 4
Location: Great Britain
Date: August 2008
Question:
Hi, I am currently working on a case study about rate of
reaction. I did an experiment with Mg and HCl. according to the
collisions theory, every time I double the concentration of HCl, the
rate of reaction will double as it is inversely proportional to each
other however when I calculated my proportion I found out that my
rate of reaction is actually in proportion of 1 to 4. I am not able
to explain why. Can you please help me?
Replies:
Trang,
The rate of a reaction (R) is a function of the rate constant (k), the
concentration of reactant A ([A]) raised to some power (m) which is either
0, 1, or 2. If there is another reactant B ([B]) than this will also be
included. Mathematically, this is written as:
R = k [A]^m [B]^n
Assuming that you hold [B] constant (do not change the concentration (or if B
is a pure solid), and if we divide the equation 1 for a reaction with a
concentration of A1:
R1 = k [A1]^m
by the equation describing another instance of this reaction using a different
concentration A2:
R2 = k [A2]^m
then what we have is:
R1/R2 = ([A1]/[A2])^m
If we were smart enough to make sure that A1 is twice that of A2, then the above
reaction reduces to:
R1/R2 = 2^m
Now you can see that if m = 0 then R1/R2 = 1, if m=1 then R1/R2 = 2, and if m=2
then R1/R2 = 4.
I think from this you can figure out the explanation for your observations.
Greg (Roberto Gregorius)
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Update: June 2012

