Why crystallization is a non-spontaneous slow process

Liquid ---- > Crystals

Is it against 2nd law of thermodynamics? Have you ever questioned this process? Entropy goes from more disorder [liquid] to more order [solid] . Entropy looks like decreases in the process of crystallization which is against 2nd law of thermodynamics.

Have you ever questioned yourself how this process takes place in the direction of reduced entropy?

When a liquid crystallizes into a solid, it gives off heat to its surroundings (the latent heat of fusion). This heat causes an increase in the amount of disorder in the surroundings. So even though the crystal has low entropy, its formation increases the entropy of the surroundings enough so that the process has a net positive entropy change for the universe, Total entropy S = S [system] + S [surrounding] , goes up. 

Gibbs free energy

Coming to Gibbs free energy, it tells us about state of equilibrium in a process. Any process in which free energy change is +ve or near zero is a slow non spontaneous process near a state of equilibrium

Let's take a typical case crystallization of NaCl

Gibbs free energy change ? G= ?H - T x ?S

H is enthalpy, S entropy, T is kelvin

For Nacl enthalpy change of fusion = 27.95 kj/mol, at standard condition T = 298 deg C

Entropy change of fusion, delta S = 26.02 j/mol-k

If these values are inserted in Gibbs free energy equation

?G = 7.7 kj /mol

The crystallization process has very small free energy to do any work.This explains why crystallization is a slow process and stays near state of equilibrium. This explains why seed crystals are added in crystallization. Seeding accelerates nucleation.