Standard change in entropy and change in entropy

AI Thread Summary
The discussion centers on the relationship between standard change in entropy and the Gibbs free energy equation. It clarifies that the standard change in entropy, denoted as dS(standard), is not simply equal to q(sys)/T(sys) under all conditions, particularly in spontaneous adiabatic reactions where q equals zero despite ΔS being greater than zero. The standard change in entropy specifically refers to the entropy change when all reactants are in their standard states, which are defined by specific conditions such as concentration and pressure. To determine this change, a reversible process must be identified, and the standard change in entropy can be calculated as the integral of dQ/T. The discussion highlights that standard free energies for reactants and products are already tabulated, alleviating the need for direct measurements.
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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)
Thus
dS(standard) = q(sys)/T(sys)

(surr) = surroundings
(sys) = systems
(standard) = at standard conditions
 
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Not usually. E.g. in any spontaneous adiabatic chemical reaction ##\Delta S>0## but q=0.
 
Do you know what the difference is between the change in entropy and the standard change in entropy? Is it just the reaction conditions?
 
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).
 
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.

Chet
 
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