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When there is a reversible
reaction, A + B
↔
C,
The reaction rate of the forward reaction is:
V1 = k1 [A][B] (1)
And the reaction rate of the backward reaction is:
V2 = k2 [C] (2)
On the other hand, there is another equation which shows the
reaction rate. In the case of A, the equation is:
V3 = - Δ[A] / Δt (3)
I can understand the meaning of the reaction rate in equation
(3) – because this equation shows the change of [A] per unit
of time. However, I can not understand what equations (1) and
(2) stand for. Please tell me about the meanings of equations
(1) and (2).
Gattu Ippatsu (Aug 2001)
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Answer:
In summary, V1, V2, and
V3 all show the reaction rate. So what is the difference?
V3 is the definition of the reaction rate, as you
mentioned.
V1 and V2 are “equations showing reaction rate
as a function of concentration”. (This is called “rate law”.) The rate
law is not the definition of the reaction but is obtained
experimentally. This is the difference.
The rate law of A + B
↔
C is V = k [A][B] in some cases. However, there
are also many cases where this is not
true.
For example, there is a reaction; “Hydrogen reacts with bromine and
becomes hydrogen bromide.” This type of reaction is A + B
↔
C, but we can not express the velocity as V = k [A][B]
in this case. The equation would be more complex. Why is the equation
different? The reason is that the reaction mechanism is different.
A + B
↔
C reaction might be A + B
↔
D
↔
C. There may be an interaction between A and B,
which affects the reaction. Thus, there are many kinds of reaction
mechanisms on which the rate law depends.
We can estimate a reaction mechanism if we can find a rate law by
experiment. This is one of the advantages of the rate law. Another
advantage of the rate law is that we can estimate a rate of reaction
at an unknown concentration once we know the rate law and rate
constant. Therefore, it is important to obtain a rate law experimentally.
Acknowledgement
We thank
Dr. Toshikuni Yonemoto for his suggestions in this report.
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp. Please let us know if you find any errors.
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