The peak temperature in an automobile engine is around 2300K, and the exhaust temperature typically is 900K. Standard enthalpies, free energies, and equilibrium constants for the breakdown of NO,

2NO +

are given below for these temperatures and room temperature.





Verify for yourself the rather remarkable drop in with decreasing temperature, starting with the values.) Notice that the heat of the reaction is quite insensitive to temperature, remaining constant within 0.06 kcal throughout a temperature range of two thousand degrees. This is reasonable, since the heat arises mainly from making and breaking chemical bonds, and it takes just as much energy to break a bond at 2300K as at 298K.