Page 154 - Physical chemistry understanding our chemical world

INDIRECT MEASUREMENT OF ENTHALPY 121

of the worse side effects of incomplete combustion during smoking is the formation
of carbon monoxide (CO) in relatively large quantities. Gaseous CO is highly toxic,
and forms an irreversible complex with haemoglobin in the blood. This complex helps
explain why people who smoke are often breathless.
A simple way of overcoming the toxic effects of CO is to oxidize it before the
smoker inhales the tobacco smoke. This is where the ‘anti-smoking’ pipe works. (In
fact, the name is a misnomer: it does not stop someone smoking, but merely makes
the smoke less toxic.) The cigarette is inserted into one end of a long, hollow tube
(see Figure 3.7) and the smoker inhales from the other. Along the tube’s length are
a series of small holes. As the smoker inhales, oxygen enters the holes, mixes with
the CO and combines chemically with it according to

1
CO (g) + O 2 −−→ CO 2(g) (3.34)
2
The CO 2(g) produced is considerably less toxic than CO (g) , thereby averting at least
one aspect of tobacco poisoning.
We might wonder: What is the enthalpy change of forming the CO in Equation
(3.34)? It is relatively easy to make CO in the laboratory (for example by dehydrating
formic acid with concentrated sulphuric acid), so the enthalpy of oxidizing CO to CO 2
is readily determined. Similarly, it is easy to determine the enthalpy of formation
of CO 2 , by burning elemental carbon; but it is almost impossible to determined
1
H c for the reaction C + O 2 → CO, because the pressure of oxygen in a bomb
2
calorimeter is so high that all the carbon is oxidized directly to CO 2 rather than CO.
Therefore, we will employ Hess’s law once more, but this time employing enthalpies
of combustion H c .
The enthalpies of combustion of carbon and CO are obtained
readily from books of data. We can readily find out the following The enthalpy H c(1) is
from such data books or Table 3.2: huge, and helps explain
whyweemploycoke
H c(1) [C (s) + O 2(g) −−→ CO 2(g) ] =−393.5kJ mol −1 and coal to warm a
house; this reaction
1
H c(2) [CO (g) + O 2(g) −−→ CO 2(g) ] =−283.0kJ mol −1 occurs when a coal fire
2
burns.
Again, we have numbered the enthalpies, to save time.
Once more, we start by drawing a Hess-law cycle with the elements at the bottom
of the page. This time, it is not convenient to write the reaction of interest along the

Mouth Cigarette

Small holes to allow in oxygen
Figure 3.7 An anti-smoking device: the cigarette is inserted into the wider end. Partially oxidized
carbon monoxide combines chemically with oxygen inside the device after leaving the end of the
cigarette but before entering the smoker’s mouth; the oxygen necessary to effect this oxidation
enters the device through the small circular holes positioned along its length