Date: December 17, 2020

Topic: Thermodynamic constraints

Recall

What do the following terms mean: isothermal, isobaric, isochoric and adiabatic?

What characterises adiabatic and diathermal walls?

What can be transferred through these walls?

What is an isolated system?

What is a closed system?

What is an open system?

Notes

Changes of state with constraints

We have many words to describe what constraints have been put in place during a (generally reversible) transition

• Isothermal - temperature is kept fixed during the transition
• Isobaric - pressure is kept fixed during the transition
• Isochoric - volume is kept fixed during the transition
• Isoentropic or Adiabatic - entropy is kept fixed during the transition

Walls

In thermodynamics, it is critical to define how the system under study interacts with the rest of the universe. In actuality, all systems will exchange heat and matter with the external world to a certain degree. In physics, however, we consider a simplified version of reality when we construct our models and theories. We thus construct barriers (walls) between the system and the rest of the universe if you will of the following types:

• adiabatic - a wall that does not allow heat or matter to cross it The system can exchange mechanical work with the outside world through the wall. A system with adiabatic walls has an entropy that is fixed.
• diabatic/diathermal - a wall that does not allow the system to do work on the universe and that does not permit exchange of matter with the universe. However, the system can exchange heat with the universe through the wall. A system with diabatic walls has a fixed volume.

Closed, Open and Isolated Systems

A last important piece of nomenclature concerns the type of system. In particular, we have the following three types:

• Isolated - systems that cannot do work on the rest of the universe or exchange heat with the rest of the universe. These systems also cannot exchange matter with the rest of the universe - the number of atoms (mols) contained within them is fixed.
• Closed - systems that can do work on the rest of the universe and that can exchange heat with the rest of the universe. However, these systems cannot exchange matter with the rest of the universe - the number of atoms (mols) contained within them is fixed. Most of the closed systems you will encounter in this course are one phase systems with one chemical component. Consequently, the Gibbs phase rule tells us that their thermodynamic state can be specified using only 2 thermodynamic variables.
• Open - systems that can do work on the rest of the universe and that can exchange heat with the rest of the universe. These systems are also allowed to exchange matter with the rest of the universe - they do not contain a fixed number of atoms.

<aside> 📌 SUMMARY: It is important to know whether heat and work can be done on the system. If heat cannot be transferred the entropy of the system is fixed. If work cannot be transferred then the volume is often fixed.

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