Question

The reaction Ni(H2O)62+(aq) + 6 NH3(aq) Ý Ni(NH3)62+(aq) + 6 H2O(l) is spontaneous.

Part a. Is the spontaneity due principally to a negative ΔH or to a positive ΔS? Explain your answer.

Part b. Why is the reaction of the products in the above equation with bidentate ligand ethylenediamine more spontaneous than the first reaction.

Ni(NH3)62+(aq) + 3 en(aq) → Ni(en)32+(aq) + 6 NH3(aq)

(Hint: They have similar magnitudes for ΔH)

Answer 1

The spontaneity of the chemical reaction can be judged by the gibbs free energy change of the reaction.

And this free energy change is determined by the enthalpy and entropy change as well as temperature of the system.

It is given as

A negative free energy change confers to spontaneous reaction whereas a positive free energy change refers to non spontaneous reaction.

Part A :

Here since there are lesser moles of aqueous species in the products side , the entropy of the system decreases and becomes negative. So the spontaneity is due to the negative enthalpy change.

Part B :

The reaction has aqueous species on both the sides of the reaction. The entropy of the system increases , and the change in entropy is positive. So this reaction is more spontaneous than the earlier reaction , even though they have similar magnitudes for the enthalpy change.