Free energy change.. Please help

[dray5150]

This is from EK FL 2f:
2(CH4) +3(O2)------> 2CO (gas) + 4(H20) G = -1088 kJ

CH4 (gas) + 2O2----> CO2 (gas) + 2(H20)(gas) G= -801 kJ

Based upon the reactions given above, what is the free energy change for the rxn

2(CO) (gas) + O2 (gas) -----> 2(CO2) (gas)

A. -1889kJ
B. -514 kJ
C. - 287kJ
D. 287 kJ


Ok so I take -801kJ and multiply it times 2 to get -1600kJ (rounding off)
I reverse the 2nd reaction to match the equation and get + 1088. Now according to (sum of formation of G products) - (sum of formation of G reactants) the answer should be:
(-1600)- (1088) = -2688

What am I doing wrong?
The answer is supposed to be -514kJ

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[Chowdder]

No no no, you got it mixed up. You're talking about sum of formation that is when you have an equation that gives you the G of formation for CO2, CO, and O. Then you do (G product) - (G reactants). Here you are just adding the two equations together. What you did is correct. You double the second equation and add it to the first one. H2O and CH4 will get cancelled out and you're left with the desired reaction.

So it's 1088+(-1602) = -514

Clear??

 

[dray5150]

Yeah oK... man I knew something was wrong. I guess I need to be aware not to confuse those 2 types of questions