Ask A Scientist© Chemistry Archive

name         Mireille C.
status       student
age          17

Question -   What is the effect of a monobasic or a dibasic acid on
the order of a reaction between an acid and a metal?
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It is not possible to say. The rate of a chemical reactions is complicated,
usually involving a number of steps that are often confounded, i.e. it is
difficult to isolate one of the reactions from the rest.

The "order" of a reaction derives from the EMPIRICAL observation that the
(overall) rate of many reactions can be expressed (in the simplest form):
d(C)/dt = (+/-)k (C)^n where 'n' is the order of the reaction. Of course the
rate of many reactions can be EMPIRICALLY fitted by more complicated
functions of the concentration (pressure) of the reactants and products. It
would be more general (and) correct to start with: d(C)/dt = f(C) where f(C)
is some empirical function to the concentrations (to be determined). When
all is said and done the selection of f(C) = (+/-)k(C)^n often fits the
experimental data with adequate accuracy. One motivation for selecting this
functional form is that the differential equations for  functions of this
form can be solved. One could just as well start from the premise that the
rate of reaction could be expressed as a Taylor-MacLauren expansion:
d(C)/dt = SUM (C^n/n!), but the differential equations become more difficult
to solve explicitly.

The point is that the form for the reaction rate is an EMPIRICAL choice, and
says virtually nothing about the reaction rate except for processes like
radioactive decay where the first order process results from the random
decay of radioactive nuclei. The functional expression of the rate of a
reaction, d(C)/dt, can change under different conditions, e.g. temperature,
reactant/product concentration, reaction pressure etc.

The bottom line is: You cannot tell much about what is going on at the
atomic/molecular scale from a reaction rate.

Vince Calder
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