14. Chemical Equilibrium   Previous PageNext Page

Hydrogen and oxygen gases behave similarly. The reaction:
2H(g) + O(g) 2HO(g)
G = - 109.3 kcal per 2 moles of HO
has a large negative free energy change and threfore is highly spontaneous. Yet we can allow a mixture of hydrogen and oxygen to sit for years without seeing an appreciable reaction. We have only to bring a lighted match up to the mixture, however, for a vivid demonstration of how intrinsically spontaneous the reaction is. The same effect can be produced by a catalyst such as platinum black, a finely divided form of metallic platinum that has a large surface area.

One of the villains in automotive smog is nitric oxide, NO. If we calculate the free energy of decomposition of NO,
2NO(g) N(g) + O(g)
G= -41.4 kcal per 2 moles of NO we arrive at the conclusion that the reaction should be spontaneous. The breakdown of NO to harmless atmospheric gases should be quite complete. Yet any inhabitant of the Los Angeles basin can tell you that this is only wishful thinking. Oxides of nitrogen are among the most difficult components of the smog problem. They do not break down to N and O at an appreciable rate, although breakdown is thermodynamically spontaneous. By analogy with the water reaction, you might expect that a catalyst could be found that would speed up the decomposition of NO, and this is true.

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