Water vapour dissociation

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

The importance of the dissociated hydrogen in contributing to the decarburisation of carbon steels heated in water vapour has already been mentioned. 

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

The solubility of hematite was measured in NaOH/NaCl (0.007-2 m) at between 60 and 300 °C at saturated water vapour pressure with the dissociation reaction being described by ... 

Problem 4 At 1000 K water vapour at 1 atmosphere pressure has been found to be dissociated into hydrogen and oxygen to the extent of 3 x 10 s%. Calculate the free energy decrease of the system in this reaction. (R -1.98 cal/mole/degree). 

Sunlight can produce a slow but appreciable combination between the gases,1 the action being due to the ultra-violet rays. Ultra-violet radiation affects both detonating gas and water-vapour tending m each case to produce an equilibrium between the vapour and the constituent elements 2 with increase m intensity of the illumination the equilibrium shifts m favour of further dissociation, but the proportion of dissociated vapour at the position of equilibrium is very small. The process of combination probably proceeds by the stages 3... 

Water-vapour exerts a considerable influence upon the dissociation of nitrogen trioxide. The effect of prolonged drying of the liquid is apparently to reduce the number of N02 and NO molecules normally present by causing them to combine to N406 molecules. On vaporisation, these latter molecules dissociate to a mixture of N,Oj, NO, and NO molecules, the N203 not undergoing further dissociation.7 Thus... 

In the following table are given the percentage degrees of dissociation of water-vapour at temperatures ranging from 1000° to 3500° abs., and under pressures varying from 0-1 to 100 atmospheres. 

PERCENTAGE DEGREE OF DISSOCIATION OF WATER-VAPOUR AT VARIOUS TEMPERATURES AND PRESSURES.5... 

Qddo,1 in 1915, was led to similar conclusions as to the existence of an equilibrium mixture of mono- and di-hydrol molecules in water-vapour, but concluded that at 32° C. the amount of association is nil, the molecular weight of water-vapour corresponding exactly to 18-016. With rise of temperature association takes place, reaching the high value of 41-41 per cent, at 270° C. Fall of temperature below 32° C. is accompanied by diminution of the molecular -weight in consequence of ionic dissociation. 

This idea has been further developed by Sutherland,2 who concludes that ordinary water vapour is monohydrol, H20 liquid water is an equilibrium mixture of tnhydrol, (H20)3, and dihydrol, (H20)2 whilst ice consists entirely of trihydrol. The high value found for the latent heat of fusion of ice thus receives explanation, for it is not due merely to the heat absorption consequent upon physical change of solid to liquid it is enhanced by the heat required to effect the simultaneous dissociation or depolymerisation of a large proportion of trihydrol molecules to the dihydrol form, as indicated by the thermal equation which Sutherland writes as ... 

The volatilization of ammonium chloride by heat is attended by much dissociation8 in presence of moisture, corresponding with 67 to 63 per cent., and a slight decrease in the extent of the dissociation between 280° and 330° C.4 Absence of water-vapour almost entirely prevents the dissociation of ammonium chloride, and the combination of hydrogen chloride and ammonia.6... 

At the temperature of combustion, there is appreciable dissociation of water vapour into hydrogen and oxygen, or hydrogen and hydroxyl and of CO2 into CO and O2. 

At standard conditions only, pressure = fugacity (Atkins, 1995), and (A.l) applies. The equilibrium mixture is identical to the slightly dissociated water vapour, associated with the water at the cathode of the cell. The equilibrium concentrations are going to be a small amount of reactants, hydrogen and oxygen, relative to a high concentration of H2O ... 

Adsorption on molybdenum disulphide is important because of its effect on lubrication, and Kalamazov and co-workers , studied the adsorption of oxygen, hydrogen, nitrogen and water vapour. They found that after desorption at 900°C and 10 Pa (10 Torr) subsequent re-adsorption was at a lower level, and inferred that active adsorption sites had been destroyed by the vacuum and high temperature. They found that at 700°C adsorbed water vapour was dissociated, causing oxidation and the liberation of hydrogen. 

The liberation of gaseous hydrogen was shown by Kalamazov to occur when water vapour was dissociated on the surface of molybdenum disulphide at 700°C. It has also been shown that hydrogen sulphide was produced during sliding of molybdenum disulphide in moist nitrogen, presumably by the reaction... 

Hg + H20-->Hg + H -t OH Dissociation of water vapour Explosion method Vibration levels of OH H2O2 dissociation Reaction between OH radicals... 

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