Chapter 6 - Chapter 6: Thermochemistry Understanding Heats...

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Chapter 6 06 1 of 10 Chapter 6: Thermochemistry Understanding Heats of Reaction The first part of this chapter lays the groundwork for understanding what we mean by heats of reaction. Thermodynamics is the science of the relationship between heat and other forms of energy. Thermochemistry is one area of thermodynamics. It involves the study of heat absorbed or evolved during chemical reactions. 1. Energy and Its Units Energy is the ability to move matter. a. Kinetic Energy, E K ; Units of energy The energy an object has due to its motion is kinetic energy. 2 K 2 1 mv E = In this formula, m is in kilograms, kg, and v is in m/s. E K is then in kg m 2 /s 2 . This unit is defined as a joule (pronounced “jewl”), J. Another unit of energy is the calorie, cal. It is defined at 4.184 J. (Originally it was defined as the amount of energy required to raise the temperature of one gram of water by one degree C.) To calculate kinetic energy first convert to the required units, then substitute into the equation and solve. b. Potential Energy, E P The energy an object has due to its position in a field of force is potential energy. The most common example of this is gravitational potential energy, the energy an object has by virtue of its altitude. E P = mgh In this formula m is in kilograms, kg, g is the acceleration due to gravity, a constant equal to 9.8 m/s 2 , and h is the altitude in meters, m. E P is then in kg m 2 /s 2 which is equal to a joule, J. c. Internal Energy, U Internal energy is the energy of the molecules and atoms that make up the object. The total energy of an object is therefore, E total = E K + E P + U d. Law of Conservation of Energy Energy may be converted from one form to another, but the total quantity of energy remains constant.
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Chapter 6 06 2 of 10 For example, imagine an object that is held stationery at 25 m above ground. At this point E K = 0. Once dropped, the potential energy is converted to kinetic energy so that by the time that it reaches the ground all the potential energy has become kinetic energy. We see this illustrated in Figure 6.3 where water crosses a dam converting its potential energy into kinetic energy. 2. Heat of reaction We first need to learn to analyze the situation being studied. We divide it into the system (the substance or mixture being studied) and the surroundings (everything else in the vicinity of the system). a. Definition of Heat Heat is the energy that flows into or out of a system because of a difference in temperature between the thermodynamic system and its surroundings. Heat flows until thermal equilibrium is established, that is, until they are the same temperature. The flow of heat can be explained by the kinetic- molecular theory. In Figure 6.5 we see two containers that share a wall. The molecules in the vessel on the left are at a higher temperature than those on the right. As the molecules at a higher temperature collide with the shared wall, they lose energy, slowing down. Conversely, as the molecules at lower temperature collide with the wall, they gain energy, speeding up. This continues until both have
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This note was uploaded on 04/02/2008 for the course CHM 1220 taught by Professor Barber during the Fall '07 term at Wayne State University.

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Chapter 6 - Chapter 6: Thermochemistry Understanding Heats...

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