What is basically Gibb''s free energy change?Why it is required and How is related to spontaneity of a chemical reaction ? 

Ans) The energy associated with a chemical reaction that can be used to do work.  The free energy of a system is the sum of its enthalpy (H) plus the product of the temperature (Kelvin) and the entropy (S) of the system...

               DeltaG=Delta H - T Delta S

then a negative Delta G associated with a reaction shows that it can happen spontaneously. This is consistent with the usual chemistry convention of treating work completed by the system as negative work. Most common reactions can be assessed for spontaneity under standard conditions by looking up the associated thermodynamic quantities for every of the reactants and products. For non-standard conditions one can make use of the the expression for ΔG in terms of the other THERMODNAMIC POTENTIAL...