Because there is a large amount of empty space between gas molecules, the gas has the property

These gas molecules are much smaller than the distance between them.
So, most of the volume of a gas is therefore empty space.
There is no force of attraction between gas particles or between the particles and the walls of the container.
Because there is a large amount of empty space between gas molecules,the gas has property of low density.

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.