The Foundations of Chemistry

The SO 2 –O 2 –SO 3 System

Consider the reversible reaction of sulfur dioxide with oxygen to form sulfur trioxide at

1500 K.

2SO 2 (g)O 2 (g) 34 2SO 3 (g)

Suppose 0.400 mole of SO 2 and 0.200 mole of O 2 are injected into a closed 1.00-liter

container. When equilibrium is established (at time te, Figure 17-2a), we find that 0.056

mole of SO 3 has formed and that 0.344 mole of SO 2 and 0.172 mole of O 2 remain unre-

acted. The reaction does not go to completion. These changes are summarized in the

following reaction summary, using molarity units rather than moles. (They are numeri-

cally identical here because the volume of the reaction vessel is 1.00 liter.) The net reaction

is represented by the changesin concentrations.

2SO 2 (g)  O 2 (g) 34 2SO 3 (g)

initial conc’n 0.400 M 0.200 M  0

change due to rxn 0.056 M 0.028 M 0.056 M

equilibrium conc’n 0.344 M 0.172 M 0.056 M

In another experiment, 0.500 mole of SO 3 is introduced alone into a closed 1.00-liter

container. When equilibrium is established (at time te, Figure 17-2b), 0.076 mole of SO 3 ,

0.212 mole of O 2 , and 0.424 mole of SO 2 are present. These equilibrium amounts differ

from those in the previous case, but they are related in an important way, as we will see

in the next section. This time the reaction proceeds from right to leftas the equation is

written. The changes in concentration are in the same 2
 1 
2 ratio as in the previous case,

as required by the coefficients of the balanced equation. The time required to reach equi-

librium may be longer or shorter.

2SO 2 (g)  O 2 (g) 34 2SO 3 (g)

initial conc’n 0 0 0.500 M

change due to rxn 0.424 M 0.212 M 0.424 M

equilibrium conc’n 0.424 M 0.212 M 0.076 M

The numbers in this discussion were
determined experimentally.

A setup such as this is called a
“reaction summary.” The ratio in the
“change due to rxn” line is determined
by the coefficients in the balanced
equation.

710 CHAPTER 17: Chemical Equilibrium

te

Concentrations

Time

(a)Starting with SO 2 and O 2.

0.400 M

0.100 M

te

0.056 M

0.172 M

0.344 M

[SO 2 ]

[O 2 ]

[SO 3 ]

Time

Concentrations

0.400 M

[SO 2 ]

0.424 M

0.212 M

0.100 M 0.076 M

[O 2 ]

[SO 3 ]

te

(b) Starting with SO 3.

0.500 M

Figure 17-2 Establishment of equilibrium in the 2SO 2 O 234 2SO 3 system.

(a) Beginning with stoichiometric amounts of SO 2 and O 2 and no SO 3. (b) Beginning

with only SO 3 and no SO 2 or O 2. Greater changes in concentrations occur to establish

equilibrium when starting with SO 3 than when starting with SO 2 and O 2. The equilibrium

favors SO 2 and O 2.