Page 57 - Basic physical chemistry for the atmospheric sciences

3

C h e mical kinetics

Our previous discussion of chemical equilibria and chemical thermo­
dynamics allows us to assess whether or not a chemical reaction will
proceed in a certain direction, and what the concentrations of the
reactants and products will be when a system is in chemical equilib­
rium. In this chapter we are concerned with how f a st reactants are
t
converted into produc s , some of the factors upon which the rate of
conversion depends , and the sequence of steps by which the conver­
sion occurs. These subjects are the province of chemical kinetics.

3. 1 Reaction rates
The rate of a chemical reaction could be measured by the rate at which
the concentration of one of the reactants decreases or one of the
products increases with time. In this case, however, as the following
exercise demonstrate , the rate would depend on which reagent was
s
considered.
Exercise . 1 . Compare the rate of disappearance of N:P5(g) and the
3
rates of formation of N02(g) and Oz(g) in the reaction

(A single arrow from left to right indicates that we need only be
concerned with the forward reaction. )
Solution . For every 2 moles of N205(g) that disappear, 4 moles of
N02(g) and I mole of Oz(g) are formed. Hence,

_ !(d[N205(g)]) !(d[N0 (g)]) = (d[Oz(g)])
2
=
2 dt 4 dt dt

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