I’d like some help with a question from physical chemistry
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For a certain amount of ideal gas, what are the changes in △U, △S, and △G during a throttling expansion process? The answer is △U=0, △S>0, △G
Reply 1# emmanual: There are detailed explanations in the physical chemistry book; go and check it out.
You need to remember that throttling expansion is a constant-enthalpy process. This is the key to solving problems related to throttled expansion. For an ideal gas, both U and H are functions only of temperature T; therefore, a constant H value implies a constant U value, and the rest can be determined from that.
The throttling process is not an isenthalpic process. The enthalpy value before throttling = the enthalpy value after throttling. The intermediate process is relatively complex and not isenthalpic. Actually, there’s something wrong with this question
The correct question should be about the changes in ΔU, ΔS, and ΔG before and after the throttling expansion process
Reply to 4# penryn: The throttling process is an isenthalpic process, not a constant-enthalpy process.
Reply to 6# adog8207: The throttling process is not an isenthalpic process; this has been asked in numerous multiple-choice questions in thermodynamics. Only the enthalpy value before adiabatic throttling is equal to the enthalpy value after adiabatic throttling. Because the enthalpy decreases near the shrinkage cavity due to the increased flow velocity; as the fluid passes through this cavity, its kinetic energy increases and the pressure drops, resulting in intense turbulence and friction. Given the chaotic flow pattern near the shrinkage cavity, the parameters are only average estimates. The process line also only indicates the trend of change. On the throttling diagram, they are all represented by dashed lines. The occurrence of such disturbances and friction is due to the conversion of increased kinetic energy into thermal energy that is absorbed by the fluid, as the enthalpy remains constant before and after the adiabatic throttling process. Pressure drops. Entropy increases. Decline in functional capacity
Reply to 7# penryn: You really understand it thoroughly. Impressive.