The Steady State Simply Explained


Add two solutions together in a test tube, substance a and substance b and they react to give a-b. By the Principle of Microscopic Reversibility, which is said to be independent of the Second Law of Thermodynamics (very mysterious to Sporus - the independence) at equilibrium, when the contents of the test tube stop changing, the rate of all forward reactions is the same as the rate of the corresponding back reactions. The rate of a + b = a-b is the same as the rate of the decomposition a-b = a + b.

Steady State

Steady state is different: Imagine the test tube has a hole in it, and continuously pour in a and b. The inch of fluid in the bottom of the test tube - the stuff waiting to spurt out of the hole - is stirred continuously. A steady state is reached where the composition of this fluid does not change. The composition is unchanging but it is different from the equilibrium condition. The rate of back reactions is not, in general, the same as the rate of forward reactions.

Counter intuitive things may happen in the steady state. For example the inch of fluid may go suddenly black, stay that way, then go suddenly clear, and carry on doing this - the famous 'clock reaction'.

When the inventor of the clock reaction tried to publish he was refused on the ground it was against the Second Law of Thermodynamics - not so: it is not at equilibrium, so free energy is available to drive the effect. The inventor killed himself.

ref: The Thermodynamics of the Steady State; Denbigh K.G. (London, Methuen Chemical Monologs, 1951)

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