Why are coefficients not accounted for in derivations of concentration-time equations

by sodium.dioxid
Tags: accounted, coefficients, concentrationtime, derivations, equations
sodium.dioxid is offline
May8-12, 02:14 PM
P: 51
Every book I look at, they state

rate = -Δ[A]/Δt = k[A] , for A → B

From there, they go on to derive the concentration-time equation.

Well, my concern is what if we have: aA → B

Shouldn't "a" be accounted for in the derivation.

In other words, why don't we derive a more general equation using rate = -Δ[A]/aΔt = k[A]?

It seems like the book wants me to use ln[A]_t = -kt + ln[A]_0 even when I have aA → B

For some reason, it's always the chemistry books horrible at explaining things (unlike Biology and Physics).
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Borek is offline
May8-12, 03:57 PM
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Quote Quote by sodium.dioxid View Post
In other words, why don't we derive a more general equation using rate = -Δ[A]/aΔt = k[A]?
I don't see how it is more general. Speed is a change of concentration per time unit, period.
epenguin is offline
May11-12, 07:42 PM
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Maybe he means bimolecular reactions sUch as maybe 2NO2 → N2O4 ?

The answer is that treatments are found in the texbooks for a=2. For a>2 they assentially do not happen becuase of the high improbablilty of a productive trimolecular collision. Of course soichiometries a>2 exist, but the fundamental kinetic laws for elementary i.e. essentially single-step reactions which you are styuding now are only ever straight first order or second order.
There are then more complicated mechanisms which you will come across with all sorts of kinetic laws, but they are all made of several elementary reactions which each follow first or second order kinetic law.

MrSid is offline
May16-12, 07:45 AM
P: 81

Why are coefficients not accounted for in derivations of concentration-time equations


Have you checked the wiki page for derivations of rate equations? the general problem is set up as aA + bB --> cC + dD and the equations are then resultant of several assumptions that simplify solving the differential equations and integrating the results.

It is these simplifying assumptions that always have to be met by the experiments if one can attempt to use the simple 0, 1st, and 2nd order integrated equations under steady state approximations. Experimental data fits to the equations are then used to model the system. Bad fits usually mean new experiments with different simplifying criteria, and then a reanalysis.

see wikipedia Rate_equation_(chemistry) and in the english article use the link for Steady_state_approximation

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