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THERMODYNAMICS

THERMODYNAMICS. Enthalpy, entropy, free energy and equilibrium. WHAT DOES EACH TERM TELL US?. Enthalpy - H Energy content + endo - exo Entropy - S Chaos of system + disorder - order Free energy - G Spontaneity + non-spont. - spont. Equilibrium - K

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THERMODYNAMICS

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  1. THERMODYNAMICS Enthalpy, entropy, free energy and equilibrium

  2. WHAT DOES EACH TERM TELL US? • Enthalpy - H • Energy content + endo - exo • Entropy - S • Chaos of system + disorder - order • Free energy - G • Spontaneity + non-spont. - spont. • Equilibrium - K • Extent of reaction >1 favors products

  3. CALCULATIONS • H, S and G may all be calculated from tables of standard values, from Hess’ Law or from the Gibb’s equation G = H - TS Caution: units on H and S often differ!

  4. Other important relationships: • G = - RT lnK • G = Gº + RT log Q • G º = - nFEº

  5. Students should be able to: • Predict entropy given a chemical reaction • Use thermodynamic quantities to calculate each of the terms • Identify what each answer means • Use Gibb’s equation to predict thermo quantities not given • Find thermodynamic boiling points • Predict and calculate K values

  6. Conditions of G H S Result negative positive spont at all temp positive positive spont at high temp negative negative spont at low temp positive negative not spontaneous, EVER

  7. Relationship to K GK E 0 at equilibrium 0 negative >1, products favored + positive <1, reactants favored -

  8. 1980 D (a) State the physical significance of entropy. (b) From each of the following pairs of substances, choose the one expected to have the greater absolute entropy. Explain your choice in each case. Assume 1 mole of each substance. (1)Pb(s) or C(graphite) at the same temperature and pressure. (2)He(g) at 1 atmosphere or He(g) at 0.05 atmosphere, both at the same temperature. (3)H2O(l) or CH3CH2OH(l) at the same temperature and pressure. (4)Mg(s) at 0C or Mg(s) at 150C both at the same pressure.

  9. Answer: (a)Entropy is a measure of randomness, disorder, etc. in a system. (b) (1) Pb has greater molar entropy, Pb, with metallic bonding, forms soft crystals with high amplitudes of vibration; graphite has stronger (covalent) bonds, is more rigid, and thus is more ordered. (2)He(g) at 0.05 atmosphere has greater molar entropy. At lower pressure (greater volume) He atoms have more space in which to move are so are more random. (3)CH3CH2OH has greater molar entropy. Ethanol molecules have more atoms and thus more vibrations; water exhibits stronger hydrogen bonding. (4)Mg(s) at 150C has greater molar entropy. At the higher temperature the atoms have more kinetic energy and vibrate faster and, thus, show greater randomness.

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