Gases chemical reactions, involving pure solids

In Chapter 4, we dealt with the thermodynamic, physical and chemical properties of pure liquids. However, in most instances solutions of liquids are used in chemistry and biology instead of pure liquids. In Chapter 5, we will examine the surfaces of mainly nonelectrolyte (ion-free) liquid solutions where a solid, liquid or gas solute is dissolved in a liquid solvent. A solution is a one-phase homogeneous mixture with more than one component. For a two-component solution, which is the subject of many practical applications, the major component of the solution is called the solvent and the dissolved minor component is called the solute. Liquid solutions are important in the chemical industry because every chemical reaction involves at least one reactant and one product, mostly forming a single phase, a solution. In addition, the understanding of liquid solutions is useful in separation and purification of substances. 

In this equation, e is the actual potential and f° is the standard potential, n is the number of electrons involved, and Q is a ratio of concentration terms. Q is equal to the ratio of concentrations of products to concentrations of reactants, each raised to the power corresponding to the coefficient in the balanced chemical equation. Pure solids and liquids and the solvent water are not included in Q their effective concentrations are assumed to be 1. Gas pressures in atmospheres are used instead of concentrations. For a general reaction... 

Finally, if a process involves the sublimation of a pure solid (such as ice or solid CO2) or the evaporation of a pure liquid (such as water) in a different medium such as air. the mole (or mass) fraction of the substance in the liquid or solid phase is simply taken to be 1.0, and the partial pressure and thus the mole fraction of the substance in the gas phase can readily be determined from the saturation data of the substance at the specified temperature. Also, the assumption of thermodynamic equilibrium at the interface is very rca.sonablc for pure solids, pure liquids, and solutions, except when chemical reactions are occurring at the interface. 

When a gas-phase or liquid-phase system involves a solid as a reactant or product, we have a multi-phase system but this calculation is simplified enormously by the fact that solid phases can be treated as pure. If the reaction involves a gas or liquid phase, only the moles of those species that are present in the gas or liquid participate in the equation that determines the equilibrium of the system. The fact that the activity of solids does not depend on the amount of the solid also implies that it is possible for equilibrium to fully consume a solid reactant. This is not possible with reactions that do not involve solids unless the equilibrium is shifted very strongly towards the products. The calculation of chemical equilibrium is demonstrated with the example below. 

For the opposite limit of no bonding ionicity, gas-solid interactions at the surfaces of purely covalent elemental semiconductors are predominantly chemical reactions associated with local chemical bonds involving semiconductor valence states. The oxidation of Si is a good example. Here it is not possible to define a one-electron state associated with the adsorbate. Rather, a new state in the valence band of the semiconductor has been created with a more or less localized wavefimction to... 

Most chemical reactions take place not between pure solids, liquids, or gases, but among ions and molecules dissolved in water or other solvents. In Section 4.1 we noted that a solution is a homogeneous mixture of two or more substances. Because this definition places no restriction on the nature of the substances involved, we can distinguish six types of solutions, depending on the original states (solid, liquid, or gas) of the solution components. Table 13.1 gives examples of each of these types. 

Environmental reactions may involve not only chemicals dissolved in water but also chemicals in solid and gaseous forms. If a pure solid is part of a chemical reaction (i.e., if it is being formed or dissolved), by convention its concentration is entered as one if a gas is part of a reaction, by convention its concentration is represented as its partial pressure, as discussed further in Section 1.7.1. Note that the letters (s) or (g) are often entered after a chemical formula to indicate solid or gas, respectively. 

Rate of Solubility—The rath of solubility of small particles depends on a great number of variables. Eq (12-2) takes into account free surface energy (a) and particle surface (1 /d). These are purely surface considerations, and are scarcely complete in themselves. The shape of the surface and its physical state must also be specified, that is, its relative freedom from contamination which might influence the speed of reaction. The effect of packing density and the extent of agitation imparted to the particles are also important, particularly with regard to exposure of fresh surfaces and formation of possible gas pockets. The liquid and liquid-solid phases jointly are additional important considerations. The volume of the liquid, its temperature, and the amount of dissolved solid already in solution must enter into all calculations. Nor can we ignore the chemical nature of the substances involved in the... 

The sulfur froth is collected in a slurry tank and subsequently processed in a filter or centrifuge to remove the solution remaining in the iioth. It is usually necessary to wash the sulfur-cake with at least 2 pounds of water per pound of sulfur to recover most of the chemicals contained in the solution and to produce relatively pure sulfur. To maintain the system water balance, wash water and water produced by the reaction have to be evaporated, either with the vent gas in the oxidizer or in a separate evaporator, depending on the quantity of water involved. The sulfur-cake, which contains about 50 to 60% solids, may be further processed by melting in an autoclave. In this manner high grade liquid or solid sulfur is produced. If... 

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