Equilibrium rate, surface pressure attainment

The differences in time-dependent adsorption behavior between 99% PVAC at 25° and 50°C demonstrate the influence of intra- and intermolecular hydrogen bonding in the adsorption process. The limiting surface pressure of the hydrophobic water-soluble polymer appears to be 33 mN/m, approximately 7 mN/m below that of commonly used surfactants. The rate of attainment of equilibrium surface pressure values is faster if there is uniformity of the hydrophobic segments among the repeating units of the macromolecule. 

The Vapor Pressure of Liquids. Vapor pressure is defined as the pressure exerted by a vapor in equilibrium with its liquid. Consider a closed, evacuated container which has been partially filled with a liquid. The molecules of the liquid are in constant motion but not all the molecules move with the same velocity and there will be some which possess a relatively high kinetic energy. If one of these fast moving molecules reaches the liquid surface, it may possess sufficient energy to overcome the attractive forces in the liquid and pass into tiie vapor space above. As the number of molecules in the vapor phase increases the rate of return to the liquid phase also increases and eventually a condition of dynamic equilibrium is attained when the number of molecules leaving the liquid is equal to the number returning. The molecules in the vapor phase obviously exert a pressure on the containing vessel and this pressure is known as the vapor pressure. 

In the synthesis of ammonia, under industrial conditions, the reaction normally comes sufficiently close to equilibrium for the applications of thermodjaiamics to prove of immense value, f Thus it will predict the influence of changes of pressure, temperature and composition on the maximum attainable yield. By contrast in the catalytic oxidation of ammonia the yield of nitric oxide is determined, not by the opposition of forward and backward reactions, as in ammonia synthesis, but by the relative speeds of two independent processes which compete with each other for the available ammonia. These are the reactions producing nitric oxide and nitrogen respectively, the latter being an undesired and wasteful product. The useful yield of nitric oxide is thus determined by the relative speeds of these two reactions on the surface of the catalyst. It is therefore a problem of rates and not of equilibria. 

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