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Unformatted text preview: Thermochemistry Chapter 5 Thermochemistry Thermochemistry Energy • The ability to do work or transfer heat. ¾ Work: Energy used to cause an object that has mass to move. ¾ Heat: Energy used to cause the temperature of an object to rise. Thermochemistry Potential Energy Energy an object possesses by virtue of its position or chemical composition. Thermochemistry Kinetic Energy Energy an object possesses by virtue of its motion. 1 2 KE = ⎯ mv 2 Thermochemistry Units of Energy • The SI unit of energy is the joule (J) . • An older, nonSI unit is still in widespread use: The calorie (cal) . 1 cal = 4.184 J 1 J = 1 ⎯⎯ kg m 2 s 2 James Joule James Joule 1818 18181889 1889 Thermochemistry System and Surroundings • The system includes the molecules we want to study (here, the hydrogen and oxygen molecules). • The surroundings are everything else (here, the cylinder and piston). Thermochemistry Work • Energy used to move an object over some distance. • w = F x d , where w is work, F is the force, and d is the distance over which the force is exerted. Thermochemistry Heat • Energy can also be transferred as heat. • Heat flows from warmer objects to cooler objects. Thermochemistry First Law of Thermodynamics • Energy is neither created nor destroyed. • In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa. ∆ ∆ E = q + w E = q + w heat energy transferred heat energy transferred energy energy change change work done work done by the by the system system Thermochemistry Internal Energy The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E. • • PE + KE = Internal energy (E or U) PE + KE = Internal energy (E or U) By definition, the change in internal energy, ∆ E , is the final energy of the system minus the initial energy of the system: ∆ E = E final − E initial Thermochemistry Changes in Internal Energy • If ∆ E > 0, E final > E initial ¾ Therefore, the system absorbed energy from the surroundings. ¾ This energy change is called endergonic . Thermochemistry Changes in Internal Energy • If ∆ E < 0, E final < E initial ¾ Therefore, the system released energy to the surroundings. ¾ This energy change is called exergonic . Thermochemistry Changes in Internal Energy • When energy is exchanged between the system and the surroundings, it is exchanged as either heat ( q ) or work ( w ). • That is, ∆ E = q + w. Thermochemistry ∆ E , q , w , and Their Signs Thermochemistry Exchange of Heat between System and Surroundings • When heat is absorbed by the system from the surroundings, the process is endothermic . Thermochemistry Exchange of Heat between System and Surroundings • When heat is absorbed by the system from the surroundings, the process is endothermic ....
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 Spring '06
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 Thermodynamics, Enthalpy, Thermochemistry, Energy, Heat

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