Page 163 - Physical chemistry understanding our chemical world

130 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

until the composition of the solution in the saucer is homogeneous,
Mixing occurs spon- with the mixing complete. We never see the reverse process, with
taneously, but we dye suddenly concentrating into a coloured blob surrounded by
never see the reverse
process, with dye sud- clear, uncoloured water.
denly concentrating In previous chapters we looked at the way heat travels from hot
into a coloured blob to cold, as described by the so called ‘minus-oneth’ law of ther-
surrounded by clear, modynamics, and the way net movements of heat cease at thermal
uncoloured water. equilibrium (as described by the zeroth law). Although this transfer
of heat energy was quantified within the context of the first law, we
have not so far been able to describe why such chemical systems
A reaction is ‘spon-
taneous’ if it occurs occur. Thermodynamic changes only ever proceed spontaneously
without any additional in one direction, but not the other. Why the difference?
energy input. In everyday life, we say the diffusion of a dye ‘just happens’
but, as scientists, we say the process is spontaneous. In years past,
it was thought that all spontaneous reactions were exothermic, with
It used to be thought non-spontaneous reactions being endothermic. There are now many
reactions were spon-
taneous if H was exceptions to this overly simplistic rule; thus, we can confidently
negative. This sim- say that the sign of H does not dictate whether the reaction is
plistic idea is incorrect. spontaneous or not, so we need a more sophisticated way of looking
at the problem of spontaneity.

When we spill a bowl of sugar, why do the grains go
everywhere and cause such a mess?

Changes in the extent of disorder

Surely everyone has dropped a bowl of sugar, flour or salt, and caused a mess! The
powder from the container spreads everywhere, and seems to cover the maximum
area possible. Spatial distribution of the sugar granules ensures a
The granules of spilt range of energies; so, for example, some particles reside on higher
sugar have a range of surfaces than others, thereby creating a range of potential energies.
energies. And some granules travel faster than others, ensuring a spread of
kinetic energies.
The mess caused by dropping sugar reflects the way nature always seeks to max-
imize disorder. Both examples so far, of dye diffusing in water and sugar causing
a mess, demonstrate the achievement of greater disorder. But if we are specific, we
should note how it is the energetic disorder that is maximized spontaneously.
It is easy to create disorder; it is difficult to create order. It requires effort to clean up
the sugar when re-establishing order, showing in effect how reversing a spontaneous
process requires an input of energy. This is why the converse situation – dropping
a mess of sugar grains and creating a neat package of sugar – does not happen
spontaneously in nature.