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# hw_5 - MS&E 303 Fall 2004 Homework#5(v.2 Due Friday October...

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MS&E 303 – Fall 2004 Homework #5 (v.2) – Due Friday October 1 Some random notes on units from real tables: Tables of thermodynamic properties use a wide array of units. For specific entropy (entropy per mole), one common unit is the e.u. – energy units. As s has the same units as the gas constant, this is just the entropy normalized by R . s (e.u.) = s/R . S o 298 (Ba) = 62 . 4 J/K-mole = 7 . 505 e.u.. The enthalpy of compounds is variously referred to as Δ H o 298 or Δ H f 298 . Strictly the latter is better representing the standard state enthalpy of formation rather than just the standard state enthalpy . The convention of lower case letters for specific (per mole) thermodynamic quantities is generally followed. The exception is in the standard state quantities where they refer to a specific reference state. For such standard states, the reference specifies one mole as well as the temperature and pressure. So S o 298 = 62 . 4 independent of the number of moles in the system. As a consequence, the units of Δ H o 298 is validly either Joules or Joules/mole. This ambiguity only extends to standard state values where quantities are strictly defined. Standard state is a nebulous specification. Most commonly temperature is not part of “standard” but is included in the value. So we have S o 298 and Δ H f 298 with 298 K. It is the specification of material conditions that really identify “standard”. All materials are in the equilibrium state (unless other specified) at the temperature of interest and a one atmosphere pressure. 1. A two-level energy model can be used to describe many phenomena, such as the cis-trans configuration in an organic molecules. In this model, a site is considered to be either occupied with an energy of - ² or unoccupied with an energy of 0. The sites do not, to first order, interact.

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