The standard free energy of formation, δg∘f, of a substance is the free energy change for the formation of one mole of the substance from the component elements in their standard states. these values are applicable at 25 ∘c and are found in thermodynamic tables.

the value of δg∘f for a substance gives a measure of the thermodynamic stability with respect to the component elements. negative values for a formation reaction indicate thermodynamic stability of the product. in other words, the compound formed does not spontaneously decompose back into the component elements. positive values for a formation reaction indicate thermodynamic instability of the product. thus, the compound will spontaneously decompose, though the rate may be slow.

the sign of δg∘f can be used to predict the feasibility of synthesizing a substance from its component elements. the standard free energy change for a reaction, δg∘, is a state function and can be calculated from the standard free energies of formation as follows:

δg∘rxn=∑npδg∘f(products)−∑nrδg∘f(re actants)

where np and nr represent the stoichiometric coefficients in the balanced chemical equation for the reactants and products respectively.

based on the standard free energies of formation, which of the following reactions represent a feasible way to synthesize the product?

a. n2(g)+h2(g)→n2h4(g); δg∘f=159.3 kj/mol
b. 2na(s)+o2(g)→na2o2(s); δg∘f=−451.0 kj/mol
c. 2c(s)+2h2(g)→c2h4(g); δg∘f=68.20 kj/mol
d. 2so(g)+o2(g)→2so2(g); δg∘f=−600.4 kj/mol