PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 292

9.3.6 Ostwald’s isolation method.

The methods presented above are not very suitable for determining the partial orders of re-
actions. Ostwald’s isolation method is an experimental procedure which transfers the task of
finding the partial order of a reaction to that of finding the overall order of a reaction. We will
show the principle on the example of the reaction

A + B + C→products (9.88)

whose kinetic equation is [compare with equation (9.79)]

−

dcA

dτ

=kcαAcβBcγC =⇒

dx

dτ

=k(cA0−x)α(cB0−x)β(cC0−x)γ.

If we choose for a kinetic experiment such initial concentrations thatcA0cB0, cA0cC0,
thencB0−x≈cB0andcC0−x≈cC0(compare with the section on pseudofirst-order reactions
in9.2.3). The kinetic equation rearranges to

dx

dτ

=k(cA0−x)αcB0cC0=k′(cA0−x)α

and the partial orderαwith respect to substance A is determined using one of the preceding
methods. Then we perform a kinetic experiment with the excess of substances A and C and
determineβfrom the data. Finally we take substances A and B in excess and determineγ.