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SGCh18

SGCh18 - Chapter Eighteen ENTROPY FREE ENERGY AND...

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366 Chapter Eighteen ENTROPY, FREE ENERGY, AND EQUILIBRIUM The Second Law of Thermodynamics Gibbs Free Energy Free Energy and Equilibrium THE SECOND LAW OF THERMODYNAMICS STUDY OBJECTIVES 1. Explain the meaning of the term spontaneous process. 2. Predict, for a given process, whether entropy of the system increases or decreases. 3. State the second law of thermodynamics. 4. Calculate the standard entropy change for a given reaction using a table of standard absolute entropies. Spontaneous Processes. A large and important part of experimental chemistry deals with spontaneous reactions, that is, reactions that take place "without outside influence." One goal of thermodynamics is to gain the ability to predict whether a reaction will take place when a set of given reactants are brought together. Here we want to know what property of a system can be used as a criterion for predicting spontaneous processes. The textbook points out that the sign of H by itself is not an adequate guide to spontaneity because while some spontaneous reactions are known to be exothermic ( H is –), many endothermic reactions ( H is +) are known to be spontaneous as well. It is also important to remember that the term spontaneous doesn't necessarily mean a fast reaction rate. The term describes reactions that occur without outside influence such as the continually supplying energy, but it tells us nothing about how fast or slow the reaction rate is. Entropy. In addition to the heat absorbed or evolved in a spontaneous process, another factor called entropy must be considered. Entropy is a measure of the disorder or randomness of a system. The entropy (S) is a state function that increases in value as the disorder or randomness of the system increases. Entropy has the units J/K·mol. Intuitively we consider a system to be "ordered" if it is arranged according to some plan or method. The system is "disordered" when its parts are helter-skelter within the system and their arrangement is random. Order and disorder in chemical systems are discernible at the molecular level. Crystalline solids are highly ordered, with molecules or ions occupying fixed lattice sites, and with the unit cell repeated identically over and over again. Liquids are less ordered than solids because the solid lattice has broken down, and molecules or ions have kinetic energy of translation. The molecular motion in liquids increases the disorder compared to that of solids. Gases are more random than liquids. On vaporization, the molar volume increases about 1000-fold, and it is much more difficult to pin down the position of any one molecule. For a given substance, the molecular order increases: gas < liquid < solid whereas the entropy (disorder) increases in the opposite direction: S solid < S liquid < S gas

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Entropy, Free Energy, and Equilibrium / 367 Entropy Changes.
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SGCh18 - Chapter Eighteen ENTROPY FREE ENERGY AND...

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