Page 169 - Physical chemistry understanding our chemical world

136 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

Before we address this reaction, we need to emphasize how all these are equilibrium
reactions: at completion, the reaction vessel contains product as well as unconsumed
reactants. In consequence, there is a mixture at the completion of the reaction.
This change in S arises from the mixing of the elements between the two reacting
species: before reaction, all atoms of chlorine were bonded only to other chlorine atoms
in elemental Cl 2 . By contrast, after the reaction has commenced, a choice arises with
some chlorine atoms bonded to other chlorine atoms (unreacted Cl 2 ) and others attached
to fluorine in the product, FCl.
In fact, the experimental value of S is very small and positive.

Aside

Entropy as ‘the arrow of time’

The idea that entropy is continually increasing led many philosophers to call entropy
‘the arrow of time’. The argument goes something like this. From the Clausius equality
(see p. 142), entropy is the ratio of a body’s energy to its temperature. Entropy is
generally understood to signify an inherent tendency towards disorganization.
It has been claimed that the second law means that the universe as a whole must
tend inexorably towards a state of maximum entropy. By an analogy with a closed
system, the entire universe must eventually end up in a state of equilibrium, with the
same temperature everywhere. The stars will run out of fuel. All life will cease. The
universe will slowly peter out in a featureless expanse of nothingness. It will suffer a
‘heat death’.
The idea was hugely influential. For example, it inspired the poet T. S. Eliot to write
his poem The Hollow Men with perhaps his most famous lines

This the way the world ends
not with a bang but a whimper.

He wrote this in 1925. Eliot’s poem, in turn, inspired others. In 1927, the astronomer
Sir Arthur Eddington said that if entropy was always increasing, then we can know the
direction in which time moves by looking at the direction in which it increases. The
phrase ‘entropy is the arrow of time’ gripped the popular imagination, although it is
rather meaningless.
In 1928, the English scientist and idealist Sir James Jean revived the old ‘heat death’
argument, augmented with elements from Einstein’s relativity theory: since matter and
energy are equivalents, he claimed, the universe must finally end up in the complete
conversion of matter into energy:

The second law of thermodynamics compels materials in the universe to move
ever in the same direction along the same road which ends only in death and
annihilation.