Standard change in entropy and change in entropy

  1. In the Gibbs free energy equation, does the standard change in entropy equal q(sys)/T(system)?

    Or in math terms:
    T(surr) * q(sys)/T(sys) = T(surr) * dS(standard)
    dS(standard) = q(sys)/T(sys)

    (surr) = surroundings
    (sys) = systems
    (standard) = at standard conditions
  2. jcsd
  3. DrDu

    DrDu 4,639
    Science Advisor

    Not usually. E.g. in any spontaneous adiabatic chemical reaction ##\Delta S>0## but q=0.
  4. Do you know what the difference is between the change in entropy and the standard change in entropy? Is it just the reaction conditions?
  5. DrDu

    DrDu 4,639
    Science Advisor

    The standard change refers to the change of entropy when all reactants are in their standard state (i.e. e.g. activity=1 mol/L, p=1000 hPa, etc).
  6. To elaborate on what Dr. Du said, you start out with the pure reactants in the reference state, and you end up with the pure products in the reference state. To get the standard change in entropy, you would need to identify a reversible process for bringing about this change. For that process, the standard change in entropy is equal to the integral of dQ/T. The reversible process you identify might involve gas expansions, gas compressions, a reactor with the components in equilibrium, and semipermeable membranes to introduce and remove reactants and products from the reactor reversibly. Fortunately for you, you do not meed to make these measurements yourself. The standard free energies of the reactants and products have been tabulated for you.

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