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Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.
A contact equilibrium may be regarded also as an exchange equilibrium. There is a zero balance of rate of transfer of some quantity between the two systems in contact equilibrium. For example, for a wall permeable only to heat, the rates of diffusion of internal energy as heat between the two systems are equal and opposite.
The word equilibrium implies a state of balance. Equilibrium thermodynamics, in origins, derives from analysis of the Carnot cycle. Here, typically a system, as cylinder of gas, initially in its own state of internal thermodynamic equilibrium, is set out of balance via heat input from a combustion reaction. Then, through a series of steps, as ...
Thermodynamic equilibrium is characterized not only by the absence of any flow of mass or energy, but by “the absence of any tendency toward change on a macroscopic scale.” [2] Equilibrium thermodynamics, as a subject in physics, considers macroscopic bodies of matter and energy in states of internal thermodynamic equilibrium.
This can be illustrated by the equilibrium of carbon monoxide and hydrogen gas, reacting to form methanol. C O + 2 H 2 ⇌ CH 3 OH. Suppose we were to increase the concentration of CO in the system. Using Le Chatelier's principle, we can predict that the concentration of methanol will increase, decreasing the total change in CO.
He allows that two systems might be allowed to exchange heat but be constrained from exchanging work; they will naturally exchange heat till they have equal temperatures, and reach thermal equilibrium, but in general, will not be in thermodynamic equilibrium. They can reach thermodynamic equilibrium when they are allowed also to exchange work. [23]
As the name suggests, this process is a not a true equilibrium since the system is still evolving. Non-equilibrium fluid systems can be successfully modeled with Landau-Lifshitz fluctuating hydrodynamics. In this theoretical framework, diffusion is due to fluctuations whose dimensions range from the molecular scale to the macroscopic scale. [3]
For example, if two systems of ideal gases are in joint thermodynamic equilibrium across an immovable diathermal wall, then P 1 V 1 / N 1 = P 2 V 2 / N 2 where P i is the pressure in the ith system, V i is the volume, and N i is the amount (in moles, or simply the number of atoms) of gas.