Spontaneous and non spontaneous reactions.

You can consider the process "kitchen salt dissolves in water" as a changing system.
First you have salt(s) and water(l), and afterwards a salt solution appears:

NaCl(s) NaCl(aq)       ΔH > 0
This is not really a chemical reaction, but something happens with the particles:
  1. The ionic lattice is broken
  2. The ions are being hydrated (surrounded by water molecules)
  3. The total process is endothermic (costs energy), and still the total process is spontaneous. The reason for that is dhe enormous increase of entropy during this process.

This yes or no occurrence of a chemical reaction has everthing to do with the thermodynamical data.
there is a so called second main law of thermodynamics, saying:

Processes are spontaneous when there is increase of entropy.

You can also say: if the amount of disorder increases during a chemical process, than this process is spontaneously, even if the process is endothermic.

If ions in an ionic lattice are removed from each other, and can freely move throughout the solution, than the amount of disorder has increased as also the entropy.
Dissolving a salt in water is clearly a spontaneous process.

Now imagine an explosion: a solid, with lots of order (little entropy) changes completely into gases (lots of disorder, lot of entropy); Explosion must be very spontaneous.