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Change in concentration vs. reaction rate

by Maylis
Tags: concentration, rate, reaction
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Maylis
#1
Sep1-14, 05:46 AM
PF Gold
P: 579
Hello,

I am wondering, why is it that

##\frac {d[C]}{dt} \ne k[C]## in general, where ##C## is a chemical species, and the product ##k[C]## is the reaction rate, ##r##. ##r_{c} = k[C]##

My thoughts is that because the units aren't necessarily the same, therefore they can't be the same. But I was wondering about a more physical explanation.
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Einstein Mcfly
#2
Sep1-14, 10:10 AM
P: 163
What you're describing is a unimolecular reaction. If you have a reaction that is more than just one thing changing on its own, you have at least a bimolecular reaction and you can't talk about it just in terms of a single concentration.

The units of the rate constant are whatever they have to be for whatever type of reaction you have. You'll learn all about this in P-chem.
DrDu
#3
Sep2-14, 01:48 AM
Sci Advisor
P: 3,633
What do the square brackets stand for, exactly?

Maylis
#4
Sep2-14, 02:29 AM
PF Gold
P: 579
Change in concentration vs. reaction rate

Concentration of the species
DrDu
#5
Sep2-14, 04:50 AM
Sci Advisor
P: 3,633
You have to be careful, here. The velocity of the reaction may in deed be formulated as the change of the concentration of a species with time. However, even in unimolecular reactions, the expression on the right hand side depends rather on the chemical activity than on concentration.


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