Fossil Fuels and the Energy Crisis

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 15:10


A chemical fuel is any substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. which will react exothermically with atmospheric oxygen, is available at reasonable cost and quantity, and produces environmentally acceptable reaction products. During the past century the most important sources of heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. energy in the United States and other industrialized countries have been the fossil fuels: coal, petroleum, and natural gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container.. In 1971, for example, the United States obtained 43.5 percent of its energy from oil, 34.7 percent from natural gas, and 19.7 percent from coal. Only 1.5 percent was obtained from hydroelectric power and 0.6 percent from nuclear power. Other industrialized countries also obtain 95 percent or more of their energy from fossil fuels.

Coal, petroleum, and natural gas consist primarily of carbon and hydrogen, and so it is not hard to see why they make excellent fuels. When they burn in air, the principal products are water and carbon dioxide, compounds which contain the strongest double bondAttraction between two atoms (nuclei and core electrons) that results from sharing two pairs of electrons between the atoms; a bond with bond order = 2. (DC=O = 804 kJ mol–1) and the third-strongest single bondAttraction between two atoms (nuclei and core electrons) that results from sharing a single pair of electrons between the atoms; a bond with bond order = 1. (DO―H = 463 kJ mol–1)in Table 1 from Bond Enthalpies. Thus more energy is liberated by bond formation than is needed to break the weaker C―C and C―H bonds in the fuel.



EXAMPLE 1 Use the bond enthalpies given in Table 1 from Bond Enthalpies to estimate the enthalpy change when 1 mol heptane, C7H16, is burned completely in oxygen:


C7H16(g) + 11O2 → 7CO2 + 8H2O

SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. Remembering that the projection formula for heptane is

Image:Heptane.jpg


we can make up the following list of bonds broken and formed:


Bonds Broken             Bonds Formed
6 C―C      
2 088 kJ mol–1
            14 Image:C-Odouble bond.jpg      
–11 256 kJ mol–1
16 C―H      
6 608 kJ mol–1
            16 H―O      
– 7 408 kJ mol–1
11 Image:O-Odouble bond.jpg      
5 478 kJ mol–1
_______________
Total      
14 174 kJ mol–1
            Total      
–18 664 kJ mol–1



Thus      ΔH = 14 174 kJ mol–1 – 18 664 kJ mol–1 = –4490 kJ mol–1



Apart from the hydrocarbon compounds in fossil fuels, there are few substances which fulfill the criteria for a good fuel. One example is hydrogen gas:


2H2(g) + O2(g) → 2H2O(l)      ΔH°(298 K) = –571.7 kJ mol–1


Hydrogen does not occur as the elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. at the surface of the earth, however, so it must be manufactured. Right now much of it is made as a by-productA product of a chemical reaction other than the intended or desired one(s). of petroleum refining, and so hydrogen will certainly not be an immediate panacea for our current petroleum shortage. Eventually, though, it may be possible to generate hydrogen economically by electrolysisA process in which a reaction that would otherwise be non-spontaneous (reactant-favored) is forced to occur by passage of electric current. of water with current provided by nuclear power plants, and so this fuel does merit consideration.