is a thermodynamic state function that measures disorder or randomness of a system ΔS = + the system is less orderedΔS = - the system is more orderedEntropy 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 1THIRD 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 directionvibrational motion - movement of atoms within a molecule in which they move periodically toward and away from one anotherrotational motion - movement of a molecule as though it is spinning like a topEntropy, Randomness, and Ludwig Boltzman: Board discussionMolar 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 ∆So is quite similar to that for calculated ∆Ho. Board discussion.∆So = ΣnSoproducts - ΣmSoreactantsExample problem 2GIBBS 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. ΔG = + the reaction is not spontaneous as writtenΔG = - the reaction is spontaneous as writtenΔG = 0 the reaction is at equilibrium Free energy, spontaneity, and temperature: Table 19.4 page 825Example problem 3STANDARD FREE ENERGY CHANGESStandard free energy change (∆Go) - 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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