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What condition describes a reaction at equilibrium?
A)    G = 0
B)    G° = 0
C)    G° =  G°
D)   ln K = 0

I know that  G = 0 describes a reaction at equilibrium, but since  G = -RTln(K) and  G=0, then wouldn’t that also imply that ln K = 0 since R and T aren’t 0? Or are we assuming that T could be 0, so A is the answer as opposed to D?

[onenameless]

T is never assumed to be zero since that would imply you are working at absolute 0 which is never the case. A is the answer because G is the free energy and at equilibrium, the forward reaction is equal to the reverse reaction and thus G is zero.

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I'm still confused though as to why then D can't be the answer since if R and T are not 0, then ln K has to be 0 from G=-RTln(K), doesn't it?

[onenameless]

Yes, lnK = 0 would mean that your reaction is at equilibrium in a sense but your equation is not G = -RTln(k), it's G = G^o -RTlnK. So if lnK = 0, then G = G^o in which case, G^o may not be zero depending on what the reaction is. If G = 0, then G^o -RTlnK must be 0.