[chay722]

Would the dissolving of NH4Cl in water be a spontaneous reaction? I mean it is not reversible so I don't think it is spontaneous. Thanks for any help.

[Yggdrasil]

Any irreversible reaction is spontaneous.  A spontaneous reaction is one in which ΔG < 0, meaning that it is thermodynamically favorable for the reaction to proceed forward.  This also means that the ΔG for the reverse process is positive, so the reverse process is thermodynamically unfavorable.  Because the backward reaction is thermodynamically unfavorable, your reaction will only proceed forward and cannot proceed backward, making an irreversible change.

[Kryolith]

Not every exergonic reaction is spontaneous, but every spontaneous reaction is exergonic.

[Yggdrasil]

By spontaneous, I mean the process can proceed without any outside work being performed on the system (the chemical definition of spontaneous), without regards to the kinetics of the process.  But, you are correct that not every exergonic process will occur at a fast rate (the more colloquial definition of spontaneous).