# You may already have studied types of reactions

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You may already have studied types of reactions, including combustion, like the example below using octane as the fuel: 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O + energy Recall from earlier in this Teacher’s Guide that gasoline is actually a mixture of hydrocarbons so the reaction shown for octane is only an example of ICE chemistry. In general the stoichiometry of the ICE combustion requires an ideal air to fuel ratio of 14:1. At that ratio the engine gives maximum performance with minimum reaction by-products. It is most important to note that the reaction is exothermic. The energy produced is the energy needed to move the car, as the article notes. The heat produced in the explosion, or rapid combustion, causes the gases in the cylinder to expand and drive the piston downward, turning the driveshaft that is attached. You might want to note that during two of the four cycles the behavior of the gas mixture in the cylinder can be described by one or more of the gas laws. In only two of the four strokes —compression and power—can the piston-cylinder setup be considered a closed system, therefore containing a fixed amount of gas. Since it is a mixture of gases, Dalton’s Law of partial pressures applies. During the compression stroke the volume of the gases is greatly decreased. Boyle’s Law applies here. The pressure of the gas molecules on the piston is now greatly increased. As the spark is produced at the beginning of the power stroke, the gas mixture 118
() undergoes rapid combustion (that is, it explodes) and the heat produced raises the temperature of the gases. This serves to increase the pressure they exert according to Amontons’ (Gay- Lussac’s) Law, and this pressure does work on the piston, driving it toward the crankshaft, delivering power eventually to the wheels of the car. There are several energy conversions in an ICE. The chemical potential energy of the fuel is converted to thermal energy and then to the desired mechanical energy to move the car. However, ICEs are very inefficient. Only about 20 per cent of the energy released by combustion is actually converted to useful motion. The other 80 per cent is lost as heat, friction or drag. The chart below shows more detail about energy losses in an ICE . More on energy in the body, including mitochondria Energy for the human body is produced primarily by aerobic respiration. As the article suggests, the chemistry of this process using the carbohydrate glucose as the food nutrient occurs in multiple steps but in summary looks like this: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + energy Glucose oxygen carbon water dioxide Note first that glucose is already the product of a catabolic chemical process in which a more complex carbohydrate like sucrose (C 12 H 22 O 11 ), a disaccharide composed of glucose and fructose, has been broken down by enzyme action in the stomach. The article emphasizes the