An reaction is possible if the RHS and LHS satisfy conservation of mass-energy.
Example :
A = B + C (But mass of A is larger than B+C) For this reaction to occur , the energy released account for the difference in mass .
If the mass of A is smaller than B+C , than the reaction will not occur. To initiate the reaction would require A to possess some energy equivalent to the difference in the mass. This energy may be in the form of KE or absorption of some high energy photon.
