chem 9 - Chemistry 9 The sign of E indicates the direction...

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Chemistry 9 The sign of E indicates the direction of energy flow. The energy change of a system is defined as the system's final energy minus its initial energy; that is, E = E final - E initial . When: E > 0 means that the energy of the final state of the system is greater than that of the initial state. There is a net flow of energy into the system to increase the system energy. E < 0 means that the energy of the final state of the system is less than that of the initial state. There is a net flow of energy out of the system to decrease its energy. E = 0 means that the energy of the final and initial states of the system are the same. There can be no net flow of energy. First Law of Thermodynamics Energy can be moved from one place to another, but it cannot be created or destroyed The system in a thermodynamic problem is simply that portion of the universe that is being studied. It exchanges energy with its surroundings . The system and its surroundings constitute the thermodynamic universe , i.e., E univ = E + E sur Note that both the universe and surroundings quantities are subscripted, but the system quantity is not. The first law of thermodynamics, states that energy is neither created nor destroyed, i.e., E univ = 0 for all processes. * E univ = 0 implies that E = - E sur We conclude that the first law of thermodynamics implies that the energy of a system can be changed only by exchanging energy with its surroundings *It is really mass-energy that is conserved. However, the mass changes in all but nuclear processes are so small that they can be neglected.
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Heat and Work Heat (q) and work (w) are the two most common ways to move energy between a system and its surroundings. The energy of a system increases when it absorbs heat or when work is done on it, and its energy decreases when it gives off heat or does work. Thus, the following is another expression for the first law of thermodynamics: E = q + w q is the heat absorbed by the system and w is the work done on the system When: -q > 0 The system absorbs heat, which increases its energy. Processes in which heat enters the system are said to be endothermic -q < 0 The system gives off heat, which lowers its energy. Processes in which heat leaves (exits) the system are said to be exothermic -w > 0 Work is done on the system, so the energy of the system increases - w < 0 Work is done by the system, so the energy of the system decreases Enthalpy of Reaction The enthalpy of reaction, H, is the heat absorbed by a reaction when it is carried out at constant temperature and pressure. Its sign indicates the direction of heat flow. H defined as the heat absorbed, so it is positive if the reaction absorbs heat from the surroundings and negative if the reaction gives off heat to the surroundings. The sign of the heat flow is often designated by indicating that the heat was absorbed (positive) or released (negative). Thus, +10 J is read 'ten joules is absorbed', while -10 J is read 'ten joules is released.'
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Thermodynamic properties, such as E and H, depend upon the states of the substances. Consequently, tabulated values of thermodynamic properties are given for
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chem 9 - Chemistry 9 The sign of E indicates the direction...

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