What are spontaneous reactions? What are the conditions for the spontaneity of a process?

Explanation

(i)A reaction that does occur under the given set of conditions is called a spontaneous reaction.
(ii) The expansion of a gas into a evacuated bulb is a spontaneous process, the reverse process that is gathering of all molecules into one bulb is not spontaneous. This example shows that processes that occur spontaneously in one direction cannot take place in opposite direction spontaneously.
(iii)Increase in randomness favours a spontaneous change.
(iv ) If enthalpy change of a process is negative, then the process is exothermic and occurs spontaneously . Therefore `DeltaH` should be negative.
(v)If entropy change of a process is positive, then the process occurs spontaneously, therefore `DeltaS` should be positive .
(vi) If free energy of a process is negative, then the process occurs spontaneously, therefore `DeltaG` should be negative.
(vii)For a spontaneous, irreversible process, `DeltaH lt 0 , DeltaS lt 0 , DeltaG lt 0` i.e., `DeltaH =-ve , DeltaS=+ve` and `DeltaG=-ve`.