Question

Acetylene, C2H2, can be converted to ethane, C2H6, by a process known as hydrogenation. The reaction is:
C2H2(g) + 2H2(g) === C2H6(g)

Given the following, what is the Kp for the reaction?

C2H2(g): 209.2 ΔG˚f (kJ/mol)
H2(g): 0 ΔG˚f (kJ/mol)
C2H6(g): -32.89ΔG˚f (kJ/mol)

ΔG˚ = -RTlnK
ΔG = ΔG˚RTlnQ

I don't what the answer is.
ΔG˚ for reaction = - RT lnK
242,090 J/mol = - (8.314 J/mol-K)(298.15 K) lnK
K = Kp = 3.85E-43

That's what I got but it wouldn't go through. Although that seems to be an extremely small number.

Answer 1

the delta G of the whole reaction you need to use is found by subtracting the delta G for your reactants from the delta G of your product

-32.89 (c2h6) - 209.2 (c2h2) + 0 (h2) = -242.09

then you have to plug it into the equation deltaG = -RTlnK, making sure to convert R to kJ by dividing it by 1000

8.314/1000 = 0.008314

and use the standard temp, 298.15K for T

-242.09 = -0.008314*298.15*lnK

this can be rewritten as

K=e^(G/-RT)

so

K=e^(-242.09/-0.008314*298.15)

K=e^97.713

the answer is 2.73E42

looks like your only problem was keeping your deltaG positive