Chapter6 - 1 Thermochemistry Chapter Six 2 Energy is the...

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Unformatted text preview: 1 Thermochemistry Chapter Six 2 Energy is the capacity to do work (to displace or move matter). Energy literally means work within; however, an object does not contain work. Potential energy is energy of position or composition. Kinetic energy is the energy of motion. E k = mv 2 Energy has the units of joules (J or kg . m 2 /s 2 ) Energy 3 Thermochemistry is the study of energy changes that occur during chemical reactions. System : the part of the universe being studied. Surroundings : the rest of the universe. Thermochemistry: Basic Terms 4 Open : energy and matter can be exchanged with the surroundings. Closed : energy can be exchanged with the surroundings, matter cannot. Isolated : neither energy nor matter can be exchanged with the surroundings. Types of Systems A closed system; energy (not matter) can be exchanged. After the lid of the jar is unscrewed, which kind of system is it? 5 Internal energy ( U ) is the total energy contained within a system Part of U is kinetic energy (from molecular motion) Translational motion, rotational motion, vibrational motion. Collectively, these are sometimes called thermal energy Internal Energy ( U ) Part of U is potential energy Intermolecular and intramolecular forces of attraction, locations of atoms and of bonds. Collectively these are sometimes called chemical energy 6 Technically speaking, heat is not energy. Heat is energy transfer between a system and its surroundings, caused by a temperature difference. More energetic molecules transfer energy to less energetic molecules. Heat ( q ) How do the root-mean-square speeds of the Ar atoms and the N 2 molecules compare at the point of thermal equilibrium? Thermal equilibrium occurs when the system and surroundings reach the same temperature and heat transfer stops. 7 Like heat, work is an energy transfer between a system and its surroundings. Unlike heat, work is caused by a force moving through a distance (heat is caused by a temperature difference ). A negative quantity of work signifies that the system loses energy. A positive quantity of work signifies that the system gains energy. There is no such thing as negative energy nor positive energy; the sign of work (or heat) signifies the direction of energy flow. Work ( w ) 8 For now we will consider only pressure-volume work. work ( w ) = P V Pressure-Volume Work How would the magnitude of V compare to the original gas volume if the two weights (initial and final) were identical? 9 The state of a system: its exact condition at a fixed instant. State is determined by the kinds and amounts of matter present, the structure of this matter at the molecular level, and the prevailing pressure and temperature....
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Chapter6 - 1 Thermochemistry Chapter Six 2 Energy is the...

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