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Enthalpy of Formation and Enthalpy of Combustion

ME372 Instructor: Jesse Adams May 1, 2001 By: Chun Lee


Introduction Have you ever wonder how people on National Geographic can walk across a bed of red-hot coals, like figure 1? The answer to this question is not magic and it’s not because the walkers have supernatural powers. The answer is in simple

concept in thermodynamics called enthalpy. Enthalpy is equal to the quantity of U + PV, where U is equal to internal energy of a system, P is the pressure of the system, and V is the volume of the system. Human tissue is mainly composed of water, which has a relatively high specific heat capacity, which means that a relatively large amount of energy must be transferred from the hot coals to significantly change the temperature of the feet. But, during the brief contact between the walker’s feet and the coals, there is relatively little time for energy to flow, so the feet do not reach a high enough temperature to cause damage. Secondly, although the surface of the coals has a very high temperature, the redhot layer is very thin, resulting in a very small volume in the system. Therefore, the quantity of energy available to heat the feet is smaller than might be expected. These two factors result in a very small enthalpy change between the coals the firewalker’s feet. This factor points out the difference between temperature and heat. Temperature reflects the intensity of the random kinetic energy in a given sample of matter. The amount of energy available for heat flow, on the other hand, depends on the quantity of matter at a given temperature. For example, the tiny spark from a sparkler does not hurt when it hits your hands. The spark has a very high temperature but has so little mass that no
Figure 1: Picture of a person walking on hot coals8


significant energy transfer occurs to your hand. This same argument applies to the very thin layer on the coals. First Law of Thermodynamics The first...