CHAP. 9: CHEMICAL KINETICS [CONTENTS] 301
whereci,eqare the concentrations of the reactants in the state of chemical equilibrium. From
this it follows
csS,eqctT,eq···
caA,eqcbB,eq···
=
k 1
k 2=K , [equilibrium], (9.128)whereK is the equilibrium constant on choosing the standard state of unit concentration,
cst= 1 mol dm−^3 , provided that the equilibrium mixture forms an ideal solution.
Chemical equilibrium is thus the resulting state of reversible reactions. One-way reactions,
as yet the primary focus of our attention, represent special cases when the equilibrium is shifted
entirely in favor of the products.
Note: Stoichiometric coefficients are equal to the partial orders of reaction only in ele-
mentary reactions [see9.5.1]. In addition, equation (9.128) is not thermodynamically
consistent because it contains concentrations and not activities [see equations (8.15)]. Ac-
tivities are equal to concentrations only in ideal solutions.9.4.9 First-order consecutive reactions.
9.4.9.1 Type of reaction
A →k^1 B,
B
k 2
→ C.We will consider a reaction with the initial concentration of substances B and C being zero.9.4.9.2 Kinetic equations.
−
dcA
dτ= k 1 cA, (9.129)
dcB
dτ= k 1 cA−k 2 cB. (9.130)