Generally a reaction is said to occur spontaneously

Generally, a reaction is said to occur spontaneously if the entropy increases.
Entropy (S) is a measure of the disorder in a system.
Spontaneous reactions or processes happen “automatically”, and do not
require an input of energy to occur. Spontaneous processes have a great tendency to
proceed in one direction. In a closed system, entropy always increases over time. In
an open system, energy can be added to a system to cause a decrease in entropy,
but this is not necessarily a spontaneous reaction.
You know already that entropy may be defined as a measure of how much the energy
of the particles become more spread out in a process. Now, how is entropy related to
the spontaneity of a reaction?

Determination of Spontaneity based on Entropy:
There are three factors that combine to predict spontaneity. They are:
1. Energy Change
2. Temperature
3. Entropy Change
The driving force for a spontaneous process is an increase in the entropy of
the universe . The best indicator of spontaneity in a reaction is the change in
Entropy. (S or ∆ S).
Remember: Spontaneity of a process can be determined from the change in entropy
and change in free energy of a system.
Application of Spontaneity
The application of spontaneity with respect to equilibrium is as follows:
A reaction will never spontaneously move away from equilibrium and will
always move spontaneously towards equilibrium.
Although spontaneous reactions are often exothermic, this is not a criteria for
spontaneity.
Entropy change measures the dispersal of energy: how much energy is spread
out in a particular process, or how widely spread out it becomes (at a specific
temperature). This is reflected in the idea that a spontaneous change always
results in the dispersal (spreading out) of energy (and also matter). You see now

how hot pans cooling and chemical reactions belong to the ‘how much' category
where energy is being transferred. Coffee in cream and gas expansion and perfume
in air are ‘how widely' processes whe re the initial energy of the molecules stay the
same but the volume occupied by the molecules increases. You have probably heard
the phrase “the ever expanding Universe”, although this phrase is often taken to
mean our ever increasing knowledge of our Universe, it also is quite true that our
Universe is expanding and this effect is entropic in nature.
ACTIVITIES