Sonntag, Borgnakke and van Wylen
15.38
Find the equilibrium constant for the reaction 2NO + O2 ⇔ 2NO2 from the elementary reactions in Table A.11 to answer which of the nitrogen oxides, NO or NO2, is the more stable at ambient conditions? What about at 2000 K? 2 NO + O2 ⇔ 2 NO2 (1) But N2 + O2 ⇔ 2 NO (2) N2 + 2 O2 ⇔ 2 NO2 (3) Reaction 1 = Reaction 3 - Reaction 2 ⇒
0
0
0
∆G1 = ∆G3 - ∆G2
At 25 oC, from Table A.10: or
=>
ln K1 = ln K3 - ln K2
ln K1 = -41.355 - (-69.868) = +28.513
K1 = 2.416×1012
an extremely large number, which means reaction 1 tends to go very strongly from left to right. At 2000 K:
ln K1 = -19.136 - (-7.825) = - 11.311
or K1 = 1.224 × 10-5
meaning that reaction 1 tends to go quite strongly from right to left.