[hallie3]

Consider the chemical reaction and thermochemical information and initial partial pressures of the reaction components given below:

Cl2(g) + C5H8(g) ⇌ C5H6(g) + 2HCl(g)

ΔHf° (kJ/mol)         S° (J mol-1 K-1)         P (atm)
C5H6   139.00      274.47              0.792
HCl   -92.31      186.90              0.101
Cl2   0.00              223.08              3.93
C5H8   36.00              289.66              7.83

Determine ΔG (in kJ) for this reaction at 761.96 K. Assume ΔH°f and S° do not vary as a function of temperature.

My attempt:
K=[HCl]^2[C5H6] / [Cl2][C5H8]
K=(0.101)^2(0.792) / (3.93)(7.83)
K=(0.08079192) / (30.7719)
K=2.63E-4

Using the formula ΔG = -RTln(k)
ΔG= -((0.008314)(761.96K)(ln(2.63E-4)))
ΔG=52.22 kJ


Am I going about solving this correctly?

[Chymst]

My attempt:
K=[HCl]^2[C5H6] / [Cl2][C5H8]
K=(0.101)^2(0.792) / (3.93)(7.83)
K=(0.08079192) / (30.7719)
K=2.63E-4
[/quote]
The partial pressures stated are probably not equilibrium, so you are actually calculating Q_rxn. 

When substances are in their standard state (usually given as 100 kPa for solids and gases and 1M for aqueous solutions), you calculate free energy by

ΔG°=ΣG°(prod)-ΣΔG°(react) [subtracting the free energy of the reactants from the products]

This gives you the "standard state" free energy at what ever temperature the tables you used were assembled at.

However, in practice, you almost never have standard temperature or pressures.  In that case, there two other calculations or corrections to make:
1. Correct for new temperature.  Calculate ΔS°_rxn and ΔH°_rxn from your data tables and then calculate the reaction free energy at the new temperature:
 :rarrow:ΔG°_rxn=ΔH°_rxn-T·ΔS°_rxn
2. Correct for nonstandard states.  This is where you use Q_rxn
 :rarrow:ΔG_rxn= ΔG°_rxn+RTln(Q_rxn)