understand that ΔH, whilst important, is not sufficient to explain
spontaneous change (e.g. spontaneous endothermic reactions)
understand that the concept of increasing disorder (entropy change
ΔS) accounts for the above deficiency, illustrated by physical
change (e.g. melting, evaporation) and chemical change (e.g. dissolution,
evolution of CO2 from hydrogencarbonates with acid)
be able to calculate entropy changes from absolute entropy values
Entropy is the term given to the natural disorder of the universe. If left
to itself the universe tends towards disorder. It can be thought of as an inevitable
driving force. Entropy has been called 'nature's arrow'. It is the natural tendency
of systems to become disordered.
we look at a solid, we can see that all of the particles are carefully arranged
in specific locations. This is a highly organised and ordered system. Its entropy
is said to be very low. However, in a gas the particles are free to move randomly
and with a range of speeds. The entropy of gases is high.
A liquid has more entropy than a crystalline solid, but much less than a gas.
A solution contains a mixture of solute and solvent particles and has more entropy
than a simple liquid but once again, far less than a gas.
Disorder can be increased by increasing the number of particles that have freedom
Gas particles have three degrees of freedom:
Translation means moving in a specific direction. Rotation is motion
about an axis, and vibration means movement of the atoms with respect
to one another within molecules, with respect to one another (stretching
and bending of bonds).
Gases then, have a large amount of entropy.
The amount of entropy depends on the number of possible energy levels that
the individual particles can have. This is a function of the temperature. As
the temperature increases the number of energy states available to the particles
also increases. The number of possible arrangements of energy over all of the
particles increases. This gives the system a greater choice of arrangements,
i.e. more entropy.
Hence, entropy is a function of the number of particles and the total energy
available to those particles.
Entropy is defined as the degree of disorder inherent in a system. Unlike the
chemical potential energy of a substance, entropy can be measured from an absolute
When a system has no disorder, i.e. it is perfectly arranged, and it has no
energy, i.e. it is at absolute zero kelvin, it can have no entropy.
These conditions are met in a perfect crystallline substance at absolute zero.
This allows us to measure, or calculate, absolute entropy values, i.e. the
entropy when compared to this absolute zero entropy baseline. There are various
calculations that make this possible, based on thermodynamic data as well as
statistics. Explainations of these go beyond the scope of this book.
The difference in entropy in any process, chemical or physical, is the entropy
of the final situation minus the entropy of the initial situation. For a chemical
reaction this is the difference between the products entropy and the reactants
entropy, called the entropy change. IThe entropy change is symbolised by ΔS,
delta S. When the entropy increases, ΔS is positive.
ΔS = S(final) - S(initial)
When the entropy is determined under standard conditions it is called the standard
the entropy of the system increase or decrease when a kettle boils?
Before boiling the water is a liquid and has low entropy. After boiling
the water becomes a gas and has much higher entropy. Therefore the entropy
has increased and ΔS is positive.
the entropy of the following reaction increase or decrease ?
N2(g) + 3H2(g)
On the left hand side of the equation there are four moles of gas and
on the right hand side of the equation there are two moles of gas. The
total amount of gas particles is decreasing and therefore the entropy
is decreasing, ΔS is negative.
One point to note is that the effect of energy input (temperature increase)
on entropy is not the same at all temperatures, but rather depends on the absolute
temperature. There is a greater increase in entropy at lower temperatures for
a given energy input.
Hence, the entropy change is dependent on the temperature at which the change
takes place. This can be expressed by the equation:
The very fact that endothermic reactions proceed spontaneously tells us that
enthalpy is not a definitive explanation for why reactions happen. However,
it can be shown that all endothermic reactions have an increase in disorder,
i.e. an increase in entropy.
It is clear then that entropy is also involved in some way in the driving force
behind reactions. The actual relationship is explained in the next section.