Activity of a gas

The energy of activation of a gas-phase reaction where bonds are broken homolytically but no bonds are formed is equal to AH°. 

Salting-out Coefficients at 25°C. The effect of an electrolyte on the solubility or activity of a gas dissolved in an aqueous solution is commonly expressed as a salting out coefficient ... 

Two explanations for the different responses of plants to a particular GA have been suggested (87). The first assumes that the GA must bind to a specific receptor site to give a response, the degree of response being related to the binding efficiency. Variations in the structure of the receptor from one plant species to another would then explain the differences in the bio-activity of a GA in different assays. The second possibility is that the bio-activity of some GAs is due to their metabolism to active products by the assay plants. The presence or absence of bio-activity in an assay would then reflect the ability of the assay to metabolize the applied GA. The structure of the GA-receptor and the ability of the assay plant to metabolize the applied GA probably both influence the result of a bioassay, and it is in fact difficult to distinguish between these possibilities. Thus those C20 GAs which show bio-activity may do so because they are converted to GAs. However, C20 GAs with the 19,20 6-lactone are probably active per se since they are protected from further oxidation at carbon-20 and therefore probably from conversion to GAs. 

From this it can be concluded, that for a selected standard state the activity of a gas equals its fugacity. 

The standard state chosen in the foregoing is a hypothetical one, viz, an actual gas behaving ideally at 1 atm. pressure, which may be difficult to comprehend. Its use may be avoided, however, by means of a reference state which leads to exactly the same results. The reference state chosen is identical with that employed in connection with fugacity, namely, the gas at very low total pressure of the mixture. It is then postulated that the ratio of the ojctivity of any gas to its partial pressure becomes unity in the refer--ence state, i.e., apd)/pi approaches unity as the total pressure becomes very small. It will be evident that this postulate makes the activity of a gas in a mixture identical with its fugacity, just as does the standard state proposed above. 

When the standard state is chosen so as to make the activity of a gas equal to its fugacity, as explained above, this expression also gives the varialion of the a ctivity with the total pressure. 

At low temperatures and total pressures near 1 bar, gases tend to obey the perfect gas law, and the activity of a gas equals its partial pressure (1 bar = 10 pascals = 0.986923 atmosphere = 760 mm Hg at 0°C). Garrels and Christ (1965) offer a more exhaustive discussion of gases. (See also Stumm and Morgan 1981 Wood and Fraser 1976 Henderson 1982). 

Note that, forgetting about activity coefficients for the moment, we have arranged things rather conveniently, such that the activity of pure solids and liquids will be 1.0 (the mole fraction of a pure compound being 1.0) the activity of a solid solution component is its mole fraction the activity of a gas is (numerically equal to) its partial pressure and the activity of an aqueous solute is (numerically equal to) its molality. These are useful approximations for real systems, which can be improved by using the activity coefficients. Note that activities are always dimensionless, though we have not demonstrated this. 

We have seen that the activity of a gas is related to its effective pressure. For liquids, it is the liquid vapour pressure which is important, and for solutions, the vapour pressure is linked to the concentration of the component of interest. The concentration of a solvent or major component is conveniently measured as the mole fraction, x. For ideal vapours and an ideal solution it may be shown that the chemical potential of a component number 1 is ... 

For gases in equilibrium with a solution, i = y/P where P/ is the partial pressure of the gas in atmospheres. As the total pressure decreases, y/ approaches 1. When reactions take place at atmospheric pressure, the activity of a gas can be approximated closely by its partial pressure. 

Fig. 3. Migration activity of a GA through a simple ad hoc network. Numbers represent Jumps value. Arrows with an X at the end mean the GA is killed.

In arc lamps, the emission is obtained by the activation of a gas by collision with accelerated electrons generated by an electric discharge between two electrodes, typically tungsten-made. The type of lamp is often denoted by the gas contained in the bulb including neon, argon, xenon, krypton, sodium, metal halide and mercury. In particular, for mercury lamps, the following classification, based on the Hg pressure, is done ... 

As we normally associate the activity of a gas with its partial pressure Pj, the adsorption isotherm of a gas is a relationship of the type... 

The energy of activation of a gas-phase reaction where bonds are broken homolytically but no bonds are formed is equal to AH° An example of this type of reaction is the chain-initiating step in the chlorination of methane—the dissociation of chlorine molecules into chlorine atoms ... 

In applied ete odiemistry, a number of electrode reactions of interest involve gaseous reactants or products (H, O, Fj, Clj, CH ), The non-ideality of gases is discussed in terms of their fugacity coefficient y which relates the measured parameter-pressure to the fugacity, i.e. activity of a gas. 

Strategy Write the chemical equation for the half-reaction. Then express the reaction quotient in terms of the activities and the corresponding stoichiometric coefficients, with products in the numerator and reactants in the denominator. Pure (and nearly pure) solids and liquids do not appear in Q (because their activities are 1), nor does the electron. The activity of a gas is set equal to the numerical value of its partial pressure in bar (more formally %=pj/p )-... 

---