notes06-07-page324 - Also recall that the mathematical...

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Unformatted text preview: Also recall that the mathematical definition of activity ai of some species i is implicitly stated as ai lim =1 (46.9) ζ →ζ ª g (ζ ) where g(ζ ) is any reference function (e.g., pressure, mole fraction, concentration etc.), and ζ ª is the value of ζ at the reference state. This implicit definition is awkward so for convenience one defines the activity coefficient as the argument of the above limit, γi ≡ ai g(ζ ) (46.10) which we can rearrange as ai = γ i g (ζ ). (46.11) The definition of activity implies that γ i = 1 at g (ζ ª ) (the reference state) That is γ i → 1 as the real system approaches the reference state. Connecting with the chemical potential we saw last semester that the deviation of the chemical potential at the state of interest versus at the reference state is determined by the activity at the current state (the activity at the reference state is unity by definition). μi − μª = RT ln ai . (46.12) i 46.3.2. Raoult’s Law In discussing both Raoult’s law and Henry’s law, we are describing the behavior of a liquid solution by measuring the vapor (partial) pressures of the components 312 ...
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