2H2 (g) + O2 (g) <---> 2H2O (g)

Question

what is the likely magnitude of the equilibrium constant?

a) k < 10 ^-3
b) 10^-3<K<10^3
c)k=0
d)k>10^3

i know that choice c is incorrect because if k=o there would be no need to calculate equilibrium, but how would i go about narrowing down the other answers? thanks.

Answer 1

You have already eliminated c. Since we know this is a spontaneous reaction and occurs with explosions when the two gases are mixed, I wouldn't think a number as small as 0.001 (a) is appropriate so that leave b and d. b means somewhere between 0.001 and 1000, more than likely about <500 or so. That is a relatively small number as K goes, so I would pick >10^3 (d). We can make an educated guess if we look up delta G^o for H2O liquid. My book lists -237.2 kJ/mol.
delta G^o = -RTlnK = -237.2 kJ
Change that to J and we have
-237200 = 8.314*298*ln K.
Solve that for ln K = about 95 (you can do it on your calculator) and K = 3.8 x 10^41 which is a huge number. I would pick d. Check my reasoning.

Answer 2

thanks, i double checked and everything looks right. i'm going with answer d. thank you again!