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lecture6 - CHAPTER 6 Thermochemistry 6.1 The Nature of...

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6 - 1 CHAPTER 6: Thermochemistry 6.1 The Nature of Energy energy: the capacity to do work or produce heat Law of Conservation of Energy: energy can be neither created nor destroyed. energy can be converted from one form to another Potential Energy: energy due to position or composition Kinetic Energy: energy due to motion KE = 1/2mv 2 Work is force acting over a distance State Function (state property): a property of a system that depends only on its present state. Chemical Energy system: part of the universe under study surroundings: everything else Exothermic: a process which releases heat to the surroundings. Endothermic: a process which absorbs heat from the surroundings. In an exothermic reaction, chemical potential energy is converted into thermal energy which is released to the surroundings as a positive heat flow. In an endothermic reaction, thermal energy absorbed from the surroundings is converted into chemical potential energy which result is in a negative heat flow. Thermodynamics: study of energy and its interconversions First Law of Thermodynamics: the energy of the universe is constant E = q + w where q = heat flow (into system) and w = work done on system) Calculate E for a system if 12.5 kJ of heat flows into the system and 4.1 kJ of work is done by the system on the surroundings. E = q + w = (+12.5 kJ) + (-4.1 kJ) = +8.4 kJ
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Chapter 6 Thermochemistry 6 - 2 PV Work work = force x distance w = F h P = F/A so F = PA so W = P x A x h but V = V final – V initial = A x h, therefore w = P x A x h = P V since V is positive if the system does work on the surroundings, we must change the sign so w is positive when done on the system w = -P V P = F/A P = F/A Δ h Calculate w for a gas compressed from 5.0 L to 1.0 L at a constant external pressure of 5.0 atm.
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