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Name: Trang
Status: student
Grade: Key Stage 4
Location: Great Britain
Date: August 2008

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?


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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