During cooling, the kinetic energy of the molecules falls. why does this happen?

Kinetic energy is the energy if an energy in motion. Molecules in a substance has varying kinetic energies because they move at different speeds. During cooling heat is released to the environment.  Thus, when heat is being released, the kinetic energy of the molecules decreases.

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.