Thermodynamic Stability of Natural Aqueous Systems

Thermodynamic Stability of Natural Aqueous Systems

Igor Povar (Institute of Chemistry, Moldova), Oxana Spinu (Institute of Chemistry, Moldova), Tudor Lupascu (Institute of Chemistry, Moldova) and Gheorghe Duca (Institute of Chemistry, Moldova)
DOI: 10.4018/978-1-7998-1241-8.ch004

Abstract

Long-term prediction of the complex chemical processes and their practical consequences is only feasible by using simulation models. The chapter presents a thermodynamic approach to the study of complex chemical equilibria in natural aqueous multicomponent systems. Its quintessence consists in the thermodynamic analysis of the conditions of various processes on the basis of derived overall thermodynamic characteristics. Ways to shift from chemical-analytical, descriptive characteristics in heterogeneous multicomponent systems “mineral – natural aqueous systems” to the quantitative study of involved chemical equilibria by thermodynamic and computer modeling are extensively described. The analyzed chemical processes include the heterogeneous interaction of minerals and aqueous solution, homogeneous reactions of hydrolysis, complex formation, and acid-base equilibria.
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Main Focus Of The Chapter

In this chapter the thermodynamics of the precipitation-dissolution processes of the sparingly soluble salts of different nature and the arbitrary composition under the conditions of the various side reactions between the precipitated ions (solid phase) and the saturated solution components are examined. As side reactions, protonation reactions of the ligand, hydrolysis of the metal ion, formation of neutral, protonated and hydroxocomplex are considered. For this purpose, an original method of analyzing complex chemical equilibria in the presence of solid phases is used. The notion of generalized reaction equation is introduced in order to take into account all conjugated side reactions. This equation is the generalization of a set of common chemical equations, taking into account the simultaneous formation of several different chemical species. The necessity of examining a global process derives from the fact that the nature and ratio of the concentrations of the complex species resulting from the reactions depend on the ratio of the concentrations of the initial substances (metal ion and ligand), temperature and other factors (thermodynamic parameters).

Key Terms in this Chapter

Solubility Product: The solubility product of a compound at a given temperature is equal to the product of the concentrations of its ions in the saturated solution, with each concentration term raised to the power to the number of ions produced on dissociation of one mole of the substance.

Generalized Equation of Reaction: It describes the overall process including many simultaneous reactions. Its stoichiometric coefficients, being partial molar fractions, are variable quantities depending on the chemical composition and temperature of the multicomponent system.

Stoichiometric Coefficient: A number of species appearing before the symbol for each species in the equation for a chemical reaction. By convention, it is negative for reactants and positive for products.

Molar Fraction: It is a unit of concentration, defined to be equal to the number of moles of a component divided by the total number of moles of a solution. Because it is a ratio, mole fraction is a unitless expression. The mole fraction of all components of a solution, when added together, is equal 1.

Chemical Species: Atoms, molecules, molecular fragments, ions, etc., subjected to a chemical process or to a measurement.

Residual Concentration: The difference between the initial concentration of species in the solution and its quantity in mol in the solid phase per one liter of saturated solution. After reaching the equilibrium state in the heterogeneous mixture, the solid phase is separated from the liquid one, and then the residual concentration is measured.

Remediation: Efforts to counteract some or all of the effects of pollution after it has been released into an environment. Also, to clean a site that has been damaged by industry that is physical, chemical, or biological in origin.

Sparingly Soluble Compound: It precipitates up to reaching equilibrium between the produced solid phase and its ions in the solution.

Mineral: A naturally occurring chemical compound, usually of crystalline form and abiogenic in origin. A mineral has one specific chemical composition.

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