Absolutely Small - Michael D. Fayer [115]
In Chapter 14, we discussed carbon-carbon single and double bonds. The types of hybrid atomic orbitals used to form molecular orbitals were explicated. Quantum theory allows us to understand the details of bonding and the effects of the nature of bonding on the shapes of molecules and the strengths of the bonds that hold atoms together. In this chapter, fats have been used to illustrate how the seemingly small details of the molecular bonding, such as single bonds versus double bonds, the number of double bonds, and cis versus trans structure about a double bond, play fundamentally important roles in biology. The geometry of double bonds may literally be a life-and-death issue.
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Greenhouse Gases
IN THIS CHAPTER, we will look at what happens when we burn coal, oil, or natural gas in power plants to create energy. A major point is why coal produces so much more of the greenhouse gas, carbon dioxide, than does oil, which, in turn, produces more than natural gas per unit of energy. Furthermore, the reason that carbon dioxide is such an important greenhouse gas is caused by the fundamental quantum mechanical phenomena of black body radiation and quantized energy levels.
CARBON DIOXIDE FROM BURNING FOSSIL FUELS
Chapter 15 discussed turning wine (ethanol) into vinegar (acetic acid) by adding oxygen to ethanol. When this happens, we say that ethanol has been oxidized to acetic acid. Oxidation is a chemical process that can take many forms, but in the case of turning ethanol into acetic acid, it literally involves the addition of oxygen. The process is facilitated by biological enzymes. Hydrocarbons, such as methane or heating oil, can also be oxidized. However, hydrocarbons are very stable molecules. They will only oxidize at high temperatures. Burning fossil fuels is the process of oxidization. It takes heat to get the process going, but once the oxidation starts, the breaking of chemical bonds and the formation of new molecules liberates additional heat (thermal energy) that keeps the process going.
BURNING METHANE: NATURAL GAS
First, consider what happens when we burn methane (natural gas). A model of methane is shown in Figure 14.1. Methane (CH4) reacts with oxygen to give water (H2O) and carbon dioxide (CO2). The reaction can be written as follows.
CH4 + 2O2 → 2H2O + CO2
This chemical equation shows that one molecule of methane will react with two molecules of oxygen to give two molecules of water and one molecule of carbon dioxide. The arrow points from the reactants to the products. This reaction is said to be balanced because the number of carbon, hydrogen, and oxygen atoms is the same on both the left and right sides. In a chemical reaction, the combination of atoms that make up molecules changes, but the number of each type of atom never changes. Actually, another reaction product is heat. It takes energy to break the C—H bonds of methane. However, energy is released when the O—H and C—O bonds are formed to make the products. More usable energy (called free energy) is released in making the bonds to form water and carbon dioxide than is used to break the methane bonds. The net result is that burning methane releases energy that can be used to do things such as boil water to cook spaghetti or drive a steam turbine to make electricity.
WHAT IS A GREENHOUSE GAS?
Methane is a very good fuel, but it produces the greenhouse gas, CO2. What is a greenhouse gas? A real greenhouse, in which flowers or tomatoes are grown, is a building that lets in a lot of sunlight. Today, such buildings may be constructed with large expanses of plastic that are transparent to sunlight. So the sunlight pours in. When the sunlight lands on the materials inside the greenhouse, much of it is absorbed and converted to heat. You probably have experienced this effect if you have gotten into a car with black or dark color seats that have been illuminated by the sun through the windshield. The seats will be very hot. A black steering wheel may, in fact, even be too hot to touch. As discussed in connection