PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 272

Obr. 9.1: Time dependence of the reactants concentration for a zero-order reaction of the type
A→B.The initial concentration of the reactant iscA0, the initial concentration of the product is
zero.

Figure9.1shows the time dependence of the concentration of the reactant A and product
B.

Example

The decomposition of ethanol in the human body is a complex sequence of biochemical reactions.

Formally, however, we can describe them using a single zero-order reaction. The rate constant

is 0.1 g/hour per 1 kg of the person’s mass. How long will it take for a person with a mass of

60 kg to degrade all alcohol left in his or her body after drinking a jar (1/2 litre) of light beer?

Assume that a half-litre jar of beer contains 12 g of ethanol. Instead of the usual definition of

concentration, consider the number of grams of ethanol per 1 kg of the person’s mass to be the

“concentration” in this case.

Solution

The initial “concentration” iscA0= 12/60 = 0.2 g/kg. We use relation (9.21) to obtain

τend=

0. 2

0. 1

= 2 hours.