The adjective fossil describes the fossil fuels very aptly, because all of them are derived from the remains of plants or animals which lived on earth millions of years ago. Coal, for example, began as plant matterAnything that occupies space and has mass; contrasted with energy. in prehistoric swamps, where it was able to decompose in the absence of air. Present-day peat bogs are examples of this first stage in coal formation, and in countries such as Ireland dried peat is an important fuel. Over long periods of time, at high temperatures and pressures under the earth’s surface, peat can be transformed into lignite, a brown, soft form of coal. Continued action of geological forces converts lignite into bituminous, or soft coal, and eventually into anthracite, or hard coal. When burned, these latter two types of coal release considerably more heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. than do lignite or peat.
A crucial point to realize about fossil fuels is that the energy we release by burning them came originally from the sun. The plants from which the fuels were derived grew as a result of photosynthesis, the combination of carbon dioxide and water under the influence of sunlight to form organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. compounds whose empirical formulaThe chemical formula of a substance written using the smallest possible integer subscripts that reflect the elemental composition. is approximately
CO2(g) + H2O(l) → [CH2O](s) + O2(g) ΔH ≈ 469 kJ mol–1 (1)
Since a number of different substances are formed by photosynthesis, the empirical formula [CH2O] and the ΔH are only approximate.
Photosynthesis is endothermicIn chemical thermodynamics, describes a process in which energy is transferred from the surroundings to the system as a result of a temperature difference., and the necessary energy is supplied by the absorptionPermeation of a solid by a gas or liquid, or permeation of a liquid by a gas. Absorption differs from adsorption in that the substance absorbed is found throughout the absorbent. of solar radiant energy. This energy can be released by carrying out the reverse of Eq. (1), a process which is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference.. When we burn paper, wood, or dried leaves, the heat given off is really a stored form of sunlight. Plants and animals obtain the energy they need to grow or move about from the oxidation of substances produced by photosynthesis. This oxidation process is called respiration.
After millions of years of geological change, the fossil fuels are significantly different in chemical structure from newly photosynthesized plant or animal material. The changes which occur can be approximated by the equation for formation of methane (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.):
2[CH2O] → CH4(g) + CO2(g) ΔH ≈ –47 kJ mol–1
This reaction is only slightly exothermic, and so very little of the energy captured from sunlight is lost. However, about half the carbon and all the oxygen are lost as carbon dioxide gas, and so a fossil fuel like methane can release more heat per carbon atom (and per gramOne thousandth of a kilogram.) than can wood or other organic materials. This is why anthracite and bituminous coals are better fuels than the peat from which they are formed. The enthalpyA thermodynamic state function, symbol H, that equals internal energy plus pressure x volume; the change in enthalpy corresponds to the energy transferred as a result of a temperature difference (heat transfer) when a reaction occurs at constant pressure. changes which occur during photosynthesis, respiration, and formation and combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. of fossil fuels are summarized in Fig. 1.