[Rock_lee]

What must be true of the free energy change, ?G, for a reaction to be spontaneous

A. It must be negative
 B. It must be greater than the change in entropy.
 C. It is dominated by the enthalpy change, ?H.

This is for homework that is due.  I think the answer is A, but just wanted to double check before i submit.  Can anyone tell me why The other choices are wrong or right? lmk thanks.

[Mitch]

Its A.

[Donaldson Tan]

Part A
A chemical reaction occurs if its final state is lower in energy content than its initial state. This implies that the final value of its free energy (G) must be lower than its original value, thus dG < 0

Part B
Let us consider an chemical system at equilibrium,
dG = dH - TdS
Assuming Part B is true, then dH - TdS > dS
=> dH > (T+1)dS => dS < dH/(T+1)
Since dS = dQ/T = dU/T then
dU/T < dH/(T+1)
H = U + PV => dH = dU + d(PV)
Since system is at equilibrium, its pressure and volume remains constant, ie. d(PV) = 0
dU/T < dU/(T+1)
Since 1/T > 1/(T+1) for T > 0, then dU must be negative, ie. dU < 0 => dH < 0
This means that dG > dS is not valid for all cases of a chemical reaction.