Chapter18 - Chapter 18 Entropy Free Energy and Equilibrium...

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Chapter 18. Entropy, Free Energy, and Equilibrium 18.1 The Three Laws of Thermodynamics Thermodynamics - the scientific discipline that deals with the interconversion of heat and other forms of energy First Law of Thermodynamics - (Chapter 6) energy may be converted from one form to another, but cannot be created or destroyed. (one way to measure: H ) Second Law of Thermodynamics - explains why chemical processes tend to favor one direction Third Law - extension of the second law and only briefly examined in this course 18.2 Spontaneous Processes and Entropy Spontaneous processes water runs downhill salt dissolves in water heat moves to colder object iron rusts methane burns (exothermic, H = -value) acids neutralize bases (exothermic, H = -value) ice melts at room temperature (endothermic, H = +value) ammonium nitrate (NH 4 NO 3 ) dissolves and solution gets cold (endothermic, H = +value) WHY? Two factors effect spontaneity of any change or reaction 1. Enthalpy (H) exothermic processes (negative H) tend to be spontaneous, but not always 2. Entropy (S) - degree of disorder or randomness
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2 18.3 Entropy An ordered state has a low probability of occurring and a small entropy , while a dis ordered state has a high probability of occurring and a high entropy . change in entropy: S = S final - S initial analogies: deck of cards, socks in drawer, molecular motion for a reaction: S = S products - S reactants positive S means: an increase in disorder as reaction proceeds products more disordered (random) than reactants S gas >> S liquid > S solid Figure 18.3, 4 - examples of increasing entropy Soln >> solvent or solute temperature increase – more molecular motion for a reaction, positive S favors spontaneity 18.4 Second Law of Thermodynamics
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