The Foundations of Chemistry

(c) Sulfur dioxide is oxidized to sulfur trioxide, one step in the production of sulfuric acid.

2SO 2 (g)O 2 (g)88n2SO 3 (g)

Plan

For a process at constant pressure, wP
V(
n)RT.For each reaction, we evaluate 
n,

the change in the number of moles of gaseoussubstances in the reaction.

n(no. of moles of gaseous products)(no. of moles of gaseous reactants)

Because both Rand T(on the Kelvin scale) are positive quantities, the sign of wis opposite

from that of 
n;it tells us whether the work is done on(w) or by(w) the system.

Solution

(a) 
n[2 mol N 2 (g)4 mol H 2 O(g)1 mol O 2 (g)]0 mol

7 mol0 mol7 mol

nis positive, so wis negative. This tells us that work is done bythe system. The large

amount of gas formed by the reaction pushes against the surroundings (as happened with devas-

tating effect in the Texas City disaster).

(b) 
n[2 mol HCl(g)][1 mol H 2 (g)1 mol Cl 2 (g)]

2 mol2 mol0 mol

Thus, w0, and no work is done as the reaction proceeds. We can see from the balanced

equation that for every two moles (total) of gas that react, two moles of gas are formed, so the

volume neither expands nor contracts as the reaction occurs.

(c) 
n[2 mol SO 3 (g)][2 mol SO 2 (g)1 mol O 2 (g)]

2 mol3 mol1 mol

nis negative, sowis positive. This tells us that work is done onthe system as the reaction

proceeds. The surroundings push against the diminishing volume of gas.

You should now work Exercises 75 and 76.

A bomb calorimeter is a device that measures the amount of heat evolved or absorbed

by a reaction occurring at constant volume (Figure 15-9). A strong steel vessel (the bomb)

is immersed in a large volume of water. As heat is produced or absorbed by a reaction

inside the steel vessel, the heat is transferred to or from the large volume of water. Thus,

only rather small temperature changes occur. For all practical purposes, the energy changes

associated with the reactions are measured at constant volume and constant temperature.

nrefers to the balanced equation.

616 CHAPTER 15: Chemical Thermodynamics

Here there are no gaseous reactants.

The decomposition of NH 4 NO 3

produces large amounts of gas,

which expands rapidly as the very

fast reaction occurs. This explosive

reaction was the main cause of the

damage to New York’s World

Trade Center in 1993 and the

destruction of the Federal Building

in Oklahoma City (shown here) in

1995.

The “calorie content” of a food can
be determined by burning it in excess
oxygen inside a bomb calorimeter
and determining the heat released.
1 “nutritional Calorie”1 kcal
4.184 kJ.