More on Walls and their Properties
in Thermodynamics
(©2002, François G. Amar, All rights reserved)

In order to be quantitative about energy changes in chemical systems, we need to be clear about the boundaries between the SYSTEM we are studying and the SURROUNDINGS in which we, as experimenters, situate ourselves. Thus we can write:

    SYSTEM + SURROUNDINGS = UNIVERSE

We call the boundary between the SYSTEM and the SURROUNDINGS, a WALL.

The WALL can have different properties or attributes:

 

TYPE of WALL	Property Governed
Insulating or "adiabatic"	Heat transfer is prevented, q=0
Rigid	Work is prevented (-PDV work), w=0
Impermeable	Mass transfer is prevented, closed system

Some examples of chemical systems:

1. A beaker of salt water on a table. The WALL is the glass beaker and is:

Permeable since the beaker doesn't have a tight cover: open system

Non-insulating since heat can be transferred through the glass, q not = 0

 

2. Some He gas in rubber balloon. The WALL is the balloon and is:

Closed since the He cannot escape the balloon.

Non-insulating since if we aim a hair dryer at the balloon we can heat the gas inside and increase the size and volume of the balloon (V~T), q not = 0

Nonrigid since the system can change volume in response to external pressure; work (w= -PDV) may be non-zero, w not = 0.

 

3. The contents of an insulated steel sphere used for underwater exploration (bathyscaphe). The WALL is the sphere and is:

Closed since that gases inside cannot escape the sphere.

Adiabatic since the insulation keeps heat from being absorbed or given off by the interior of the sphere, q=0

Rigid since the system cannot change volume in response to external pressure; work (w= -PDV) is zero, w=0.

 

Let us remember that the categories of WALLS are ideal categories and that in the world, sometimes called the "real world", no WALLS are perfectly insulating or perfectly rigid etc. For very precise work, we need to estimate the non-ideality of our walls and include this in our calculations.

Let us also remember that in order to measure properties inside a closed system we actually have to get inside in some way or get information out through some mechanical, electrical or optical channel. We thus may not have a truly closed system.

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