I
exemplified
ROBERT C. PLUMB
Steaks or Algae?
Smokestack Plumes; Informative about Pollution?
Illustrating elementary opplicofions o f the first and second lows o f fhermodynomics to the problem o f food production
lllustroting principles o f sfoichiomefry
Topic Suggested by F. T . Bodurtha, Engineering Department, E. I . du Pont de Nemours & Co., I n c
Contribution by Professor Henry A. Bent, North Carolina State University Is ir ~ P T I P OII T :in nvcrpopulntt.d pl:u~rtto r:&e pl.~nts or :iuimnls f*nrfood? Thr lnnr of thcrmorlvniu~~irs II~O< vide an interesting insight into this question. Plants alone fix photons. Using the sun's energy, plants produce metabolic products-chiefly (but not solely) molecular oxygen and carbohydrates (approximate formula C .nHzO)-whose Gibbs energy is greater than that of the chemical reactants-chiefly (if not solely) COz and HzO. That is to say, the items on the left-hand side of the equation ~
~~~
~
+
+
~~
.~ ~
+
Carbohydrate, C.nH*O 0 2 = CO nHzO heat (in the biochemical system's thermal surroundings)
have a lower entropy (and, hence, lower thermodynamic stability) than their chemical and energetic equivalent on the equation's right-hand side. Animals, of course, do not fix photons, directly or indirectly. Nor do animals "fix" heat. No agency in nature, plant or animal, animate or inanimate, .can do that. According to the empirical laws of nature summarized in the second law of thermodynamics, it is not possible, by any means whatsoever, to produce a net increase in useful, available energy solely at the expense of t,he thermal energy of an object. All living animals, however, do produce heat. Thus as one moves along the food chain, from plants to herbivores to carnivores, both the enthalpy and the Gibbs energy of the biochemical system (02, carbohydrates, meat, fat, C02, H20, and other metabolic products) steadily decrease. So if man desires to utilize the sun's energy most eflectively, the degradation of Gibbs energy by the intervening steps of milk, poultry, and beef production should be bypassed. The photons reaching the earth from the sun could support more algae-eating than steak-eating humans. The exempla are designed to show fundamental chemical principles in operation. They deal with phenomena in which stndonts have intrinsic interest; they apply abstract ideas to easily visualized situations. All of us have our pet anecdotes and illustrations which we know will attract the students' interest. Your contributions and suggestions are invited. They may be sent to bhe author.
692
/
Journol o f Chemical Education
Suppose that a well-meaning amateur conservationist shows you two photographs of smokestack emission, as below, which he had taken on a cold January morning. He urges legal action against Factory A. Questioning reveals that he knows that one factory burns natural gas and the other fuel oil, but he is not sure which is which. Reflection on, the stoichiometry of the combustion reaction will help leaven the impulse t o prosecute. Natural gas, a mixture of hydrocarbons but principally methane, is rich in hydrogen while other fossil fuels have a lower hydrogen-to-carbon ratio. Consequently the yield of water from their combustion varies over wide limits. Fuel
%C
."
%H ,"
Natural gas Fuel oil Coal Coke
75 88 74 85-90
25 11 5 -0
Combination CH,
C
+ 202
=
+ 2H10
COX
+ On = C 0 2
On cold winter days, the condensation of water vapor will occur more often and to a greater extent from power plants firing natural gas than from those using other fuels. The public, sometimes unfortunately, thinks everything they perceive is pollution. I n this case natural gas could appear to the public as a greater pollutant than either oil or coal. I n fact, the sulfur content of these fuels, and their release of sulfur dioxide t o the atmosphere, varies in just the opposite direction. The sulfur contents as delivered to the consumers are: natural gas, 4 X 10-4y0 (as added mercaptans); number 6 fuel oil, up to 2.G%; coal, from0.5 to 5%.
Factory A
Fostory B
