Is a thermodynamic state function that measures

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 is a thermodynamic state function that measures disorder or  randomness of a system  Δ S = + the system is less ordered Δ S = - the system is more ordered Entropy  describes the number of arrangements (positions and energy  levels) that are available to a system in a given state. Nature spontaneously  proceeds toward the states that have the highest probability of existing. Entropy increases are expected to accompany processes in which: 1. Pure liquids or liquid solutions are formed from solids. 2. Gases are formed from either solids or liquids. 3. The number of gas molecules increases in the course of a chemical reaction. 4. The temperature of a substance is increased. Example Problem 1 THIRD LAW OF THERMODYNAMICS:  The entropy of a pure solid at absolute zero (0 K) is  zero.  Means that all motion stops. Most ordered structure possible with only one arrangement of  atoms. translational motion  - movement in which an entire molecule moves in a  definite direction vibrational motion  - movement of atoms within a molecule in which they  move periodically toward and away from one another rotational motion  - movement of a molecule as though it is spinning like  a top Entropy, Randomness, and Ludwig Boltzman: Board discussion Molar enthalpies –  In general the more complex the molecule, the greater the molar  entropy (Figure 19.15 page 817) J. Hilderbrandt, Chem. 12                  2
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CALCULATING ENTROPY CHANGES:  Procedure for calculating  S o  is quite similar to that for  calculated  H o .   Board discussion.   S o  =  Σ n S o products  -  Σ m S o reactants    Example problem 2 GIBBS FREE ENERGY (G) or FREE ENERGY  - a thermodynamic state function that combines  enthalpy (H) and entropy (S).  The sign of  G  is used to predict whether a reaction will be  spontaneous or not.  For a change occurring at constant temperature and pressure, the change  in free energy is   G =  H - T S    Board discussion Δ G = +  the reaction is not spontaneous as written Δ G = -  the reaction is spontaneous as written Δ G = 0 the reaction is at equilibrium  Free energy, spontaneity, and temperature:  Table 19.4 page 825   Example problem 3 STANDARD FREE ENERGY CHANGES Standard free energy change ( G o )  - the change in free energy that will  occur if the reactants in their standard states are converted to products in  their standard states. 
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