## Spontaneity and the performance of work

Posted: April 1, 2015 in physical chemistry, thermodynamics
Tags: , , , , , , ,

Imagine a perfect gas contained by a rigid-walled cylinder equipped with a frictionless piston held in position by a removable external agency such as a magnet. There are finite differences in the pressure (P1>P2) and volume (V2>V1) of the gas in the two compartments, while the temperature can be regarded as constant.

If the constraint on the piston is removed, will the piston move? And if so, in which direction?

Common sense, otherwise known as dimensional analysis, tells us that differences in volume (dimensions L3) cannot give rise to a force. But differences in pressure (dimensions ML-1T-2) certainly can. There will be a net force of P1–P2 per unit area of piston, driving it to the right.

– – – –

The driving force

In thermodynamics, there exists a set of variables which act as “generalized forces” driving a system from one state to another. Pressure is one such variable, temperature is another and chemical potential is yet a third. What they all have in common is that they are intensive variables – those which are independent of the quantity of a phase.

Each intensive variable has a conjugate extensive variable – those which are dependent on the quantity of a phase – and together they form a generalized force-displacement pair which has the dimensions of work (= energy ML2T-2). Examples include pressure × volume, temperature × entropy, and voltage × charge.

But back to those intensive variables. Experience confirms that it is these generalized forces which are the agents of change. And spontaneous change results when there are finite differences in intensive variables between one system and another – the direction of change being determined by their relative values.

– – – –

The result is work

In the illustrated example above, the freed piston moves spontaneously to the right because of finite differences in pressure (P1>P2). As a result, the system consisting of the left hand compartment does PV work on the system consisting of the right hand compartment. Likewise, finite differences in other intensive variables such as temperature and chemical potential will act through their respective force-displacement pairs to perform work.

We can thus conclude that finite differences in intensive variables drive spontaneous change, and that due to the dimensionality of their respective conjugate extensive variables, spontaneous change results in the performance of work. It should however be noted that this work is not always useful work. The spontaneous diffusion of two gases into each other is a classic case, where it is difficult to imagine how the work could be usefully obtained.

– – – –

P Mander March 2015

Tucked away at the back of Volume One of The Scientific Papers of J. Willard Gibbs, is a brief chapter headed ‘Unpublished Fragments’. It contains a list of nine subject headings for a supplement that Professor Gibbs was planning to write to his famous paper “On the Equilibrium of Heterogeneous Substances”. Sadly, he completed his notes for only two subjects before his death in April 1903, so we will never know what he had in mind to write about the sixth subject in the list: On entropy as mixed-up-ness.

Mixed-up-ness. It’s a catchy phrase, with an easy-to-grasp quality that brings entropy within the compass of minds less given to abstraction. That’s no bad thing, but without Gibbs’ guidance as to exactly what he meant by it, mixed-up-ness has taken on a life of its own and has led to entropy acquiring the derivative associations of chaos and disorder – again, easy-to-grasp ideas since they are fairly familiar occurrences in the lives of just about all of us.

Freed from connexion with more esoteric notions such as spontaneity, entropy has become very easy to recognise in the world around us as a purportedly scientific explanation of all sorts of mixed-up-ness, from unmade beds and untidy piles of paperwork to dysfunctional personal relationships, horse meat in the food chain and the ultimate breakdown of civilization as we know it.

This freely-associated understanding of entropy is now well-entrenched in popular culture and is unlikely to be modified. But in the parallel universe occupied by students of classical thermodynamics, chaotic bed linen and disordered documentation are not seen as entropy-driven manifestations. Sure, how these things come about may defy rational explanation, but they do not happen by themselves. Some external agency, human or otherwise, is always involved.

To physical chemists of the old school like myself, entropy has always been seen as the driver of spontaneously occurring thermodynamic processes, in which the combined entropy of system and surroundings increases to a maximum at equilibrium. This view of entropy partly explains why many of us had difficulty in absorbing the notion of entropy as chaos, since equilibrium always seemed to us a very calm and peaceful thing, quite the opposite of chaos.

Furthermore, we were quite sure that entropy was an extensive property, i.e. one that is dependent on the amount of substance or substances present in a system. But disorder didn’t at all have the feeling of an extensive property. If one (theoretically) divided a thermodynamically disordered system into two equal parts, would each part be half as disordered as the whole? We didn’t think so. To us, there were serious conceptual obstacles to accepting the notion of entropy as disorder.

But while our fundamental understanding of entropy was grounded in the thermal theories of Rankine and Clausius, we did give a statistical nod in the direction of Boltzmann when seeking to explain spontaneous isothermal phenomena. We accepted the notion of aggregation and dispersal as arbiters of entropy change, which we viewed (rightly or wrongly) as separate and distinct from changes in thermal entropy. We even had a name for it – configurational entropy.

Having not one but two different kinds of entropy to play with turned out to be quite useful at times. For example, it helped to explain counter-intuitive spontaneous happenings such as the following:

This is an experiment I remember well from my college days. The diagram shows a sealed Dewar flask containing a supercooled, saturated solution of sodium thiosulphate (aka thiosulfate). A tiny seeding crystal is dropped through a hole in the lid. Crystallization immediately occurs, with an apparent increase in organisation as piles of highly regular crystals form in the solution. It’s an awesome sight to behold.

The experiment provides an unequivocal demonstration that visually-assessed disorganisation and entropy cannot be regarded as synonymous, for while the former unquestionably decreases, the latter must surely increase because the process is spontaneous.

And in overall terms, indeed it does. Although the configurational entropy of the system decreases due to the aggregation of Na+ and S2O32- ions into crystals, the other kind of entropy – thermal entropy – more than compensates as the heat of crystallization causes the temperature of the system to rise. For the whole process ΔSsystem > 0, and therefore ΔSuniverse >0 since the system is isolated from its surroundings.

As I said, having two kinds of entropy to play with can be useful in explaining things that are otherwise counter-intuitive. The above experiment also serves to show that the fashion in popular culture to interpret entropy simply as mixed-up-ness can end up being more than mildly misleading.