Page 1 of 5 Environmental chemistry Almost every pollution problem that we face has a chemical basis. Even the qualitative descriptions of such problems as the greenhouse effect, ozone depletion, toxic wastes. Groundwater contamination, air pollution, and acid rain, to mention a few, require at least an elementary understanding of some basic chemical concepts. And, of course, an environmental engineer who must design an emission control system or a waste treatment plant must be well grounded in chemical principles and the techniques of chemical engineering. STOICHIOMETRY (Chemical reactions and Chemical equilibria) When a chemical reaction is written down, it provides both qualitative and quantitative information. Qualitatively, we can see what chemicals are interacting to produce what products. Quantitatively, the principle of conservation of mass can be applied to give information about how much of each compound is involved to produce the Results shown. The balancing of equations so that the same number of each kind of atom appears on each side of the equation and the subsequent calculations, which can be used to determine amounts of each compound involved, is known as stoichiometry. The first step is to balance the equation. For example, suppose we want to investigate the combustion of m ethane (CH 4 ) the principal component of natural gas and a major greenhouse gas. Methane combines with oxygen to produce carbon dioxide and water, as the following reaction suggests: CH4 + O2 CO2 + H2O The equation is not balanced. One atom of carbon appears on each side, which is fine. However, there are four atoms of hydrogen on the left and only two on the right. In addition, there are on l y two atoms of oxygen on the left while there are three on the right. CH4 + 2O2 CO2 + 2H2O The molecular weight of a molecule is simply the sum of the atomic weights of all of the constituent atoms. If we divide the mass of a substance by its molecular weight. The result is the mass expressed in moles (mol). Usually the mass is expressed in grams, in which case the moles are g-moles; in like fashion. If the mass is expressed in pounds. The result would be lb-moles. In this text, all moles will be assumed to beg-moles. One g-mole contains 6.02 X10 23 one lb-mole is made up of 2.7 X I 0 26 molecules. Moles = Mass Molecular Weight molecules (Avogadro's number), while
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Page 3 of 5 To express the preceding methane reaction in grams, we need first to find the Number of grams per mole for each substance. Using Table above, we find that the atomic weight of C is 12, H is 1, and O is 16. Notice that these values have been rounded slightly, which is common engineering practice. Thus, the molecular weights, and hence the number of grams per mole, are Example1: Combustion of Butane What mass of carbon dioxide would be produced if 100 g of butane (C 4 H 10 ) is completely oxidized to carbon dioxide and water? Solution: First, write down the reaction; C4H10 + O2 CO2 + H 2 O
Page 4 of 5 Example2: Theoretical Oxygen Demand Consider a 1.67 X 10-3 M glucose solution (C 6 H 12 O 6 ) that is completely oxidized to CO2 and H2O Find the amount of oxygen required to complete the reaction. Solution To find the oxygen required to oxidize this glucose completely, we first write a balanced equation, determine molecular weights, and find the mass of each constituent in the reaction: Example3: Carbon Emissions from Natural Gas Worldwide combustion of methane CH 4 ( natural gas) provides about 8.2 x 10 16 kj of energy per year. If methane has an energy content of 39 X 10 3 kj/m 3 (at STP) what mass of CO2 is emitted into the atmosphere each year? Also, express t h a t emission rate as metric tons of
Page 5 of 5 carbon (not C02) per year. A metric ton, which is 1000 kg, is usually written as tonnes to distinguish it from the 2000-lb American, or short, tons. Solution We first need to express that consumption rate in moles. Converting kilojoules of energy into moles of methane is straightforward: CH4 + 2O2 CO2 + 2H2O