Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Black-body radiation wikipedia , lookup
Second law of thermodynamics wikipedia , lookup
Chemical potential wikipedia , lookup
Non-equilibrium thermodynamics wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Thermodynamic system wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Determination of equilibrium constants wikipedia , lookup
CE3503 Expectations – Equilibrium Reactions that proceed to equilibrium slowly (hours, e.g. BOD exertion, to decades, radioisotope decay) require a kinetic approach as introduced during our discussion of kinetics, reactors and mass balance. Those that proceed more rapidly (milliseconds, e.g. dissociation of strong acid, to minutes, e.g. air-water equilibria) may be evaluated using an equilibrium approach, i.e. based on the principles of physical and chemical equilibrium. The latter case is examined here. Our exploration of equilibria assumes a reversible reaction proceeding in a closed system, although we will consider the effect of open system conditions on chemical fate. A generalized reaction, adaptable to several types of equilibria may be written as, aA bB cC dD Note that the equilibrium position is seldom equal parts of A and B on one side and C and D on the other. The equilibrium position for a strong acid, for example, lies well to the right with little undissociated acid present, while that for a weak acid may lie more toward the middle, i.e. equal parts dissociated and undissociated acid. The general form of the equilibrium constant is written as ‘products over reactants’, i.e. C D K a b A B c d If, at any time, the system departs from this ratio, concentrations of products and reactants will adjust until their ratio once again equals K. This is called LeChatelier’s Principle. For example, in the equation below, aA bB cC dD if C is consumed, the reaction will proceed to the right, reducing the quantities of A and B and adding C and D until the ratio is equal to K; if D is added, the reaction will proceed to the left, reducing the quantities of C and D and adding A and B until the ratio is equal to K. Most applications of equilibrium to environmental engineering may be understood on the basis of the equilibrium position, the equilibrium coefficient and LeChatelier’s Principle. Specific examples of the application of equilibrium principles to environmental engineering follow. Volatilization Here, the equilibrium is between the chemical in its liquid form and the chemical in its gaseous (vapor) form, Cl Cg and the equilibrium constant, termed the saturation vapor partial pressure, is, C g K Cl Each chemical has an intrinsic value for the saturation vapor partial pressure, i.e. in a closed system, at equilibrium, the liquid and gas concentrations will equal a certain ratio, K. The value of K typically increases with increasing temperature, i.e. more chemical is present in the gas phase at higher temperatures. Consider the fate of 50 liters of a volatile organic chemical spilled in a closed room compared with the fate of 50,000 liters of that same chemical spilled in a tanker wreck. Describe the fate of the chemical in these two cases on the basis of equilibrium principles, i.e. in terms of the two equations presented above. Air-Water Exchange Here, the equilibrium is between the chemical in an aqueous solution (dissolved in water) and the chemical in air, Caq Cg and the equilibrium constant, termed the Henry’s Law constant, is, C g K Caq Each chemical has an intrinsic value for the Henry’s Law constant, i.e. in a closed system, at equilibrium, the ratio of the concentration in air to the concentration in water will equal K. The value of K typically increases with increasing temperature, i.e. more chemical is present in air than in water at higher temperatures. Consider an attempt to remove (a) oxygen and (b) an organic chemical from water by airstripping. This would work for the volatile organic chemical, but not for oxygen. Explain this result on the basis of equilibrium principles, i.e. in terms of the two equations presented above. Acid-Base Here, the equilibrium is between the chemical in its undissociated form (e.g. HA) and in its dissociated form (H+ and A-), HA H A and the equilibrium constant, termed the dissociation constant, is, H A K HA Each acid (and base) has an intrinsic value for the dissociation constant, i.e. in a closed system, at equilibrium, the ratio of the undissociated acid to the acid in its dissociated form will equal K. Acids with large values of K (strongly dissociated, equilibrium lying to the right) are called strong acids, while acids with small values of K (weakly dissociated, equilibrium lying to the left) are called weak acids. Consider the case of chlorine dissolved in water, forming hypochlorous acid and hypochlorite ion HOCl H OCl In water treatment, we may adjust the pH to increase the amount of the more effective disinfectant present. Explain this approach on the basis of equilibrium principles, i.e. in terms of the three equations presented above. Precipitation-Dissolution Here, the equilibrium is between the chemical in the dissolved form and in the solid (precipitate) form, AB( s ) A B and the equilibrium constant, termed the solubility product, is, A B K AB and, because the concentration of a solid in a solid is equal to 1, K A B Each material that may exist in solid form has an intrinsic value for the solubility product. This means that if the concentration of A+ times the concentration of B- exceeds the value of K, solid AB will form, reducing the concentrations of A+ and B- in solution. Iron can be removed from drinking water through aeration, i.e. oxidation of ferrous (Fe2+) iron to ferric (Fe3+) iron. Comment on what this implies about the solubility products of ferrous versus ferric iron on the basis of equilibrium principles, i.e. in terms of the two equations presented above. Ion Exchange Here, the equilibrium is between ions dissolved in water and ions attached to a resin, A Bresin Aresin B and the equilibrium constant, termed the selectivity coefficient, is, K Aresin B A Bresin Ions are attracted to the resin in proportion to their size and charge. The classic application of ion exchange is for removal of hardness (Ca2+ and Mg2+) from drinking water. Initially, the resin holds sodium (Na+) ions. As water is passed through the resin, Ca2+ and Mg2+ are exchanged (held by the resin), releasing Na+ to the water. Over time, the resin becomes saturated with Ca2+ and Mg2 and must be recharged. Describe the recharge process on the basis of equilibrium principles, i.e. in terms of the two equations presented above. Sorption Here, the equilibrium is between the chemical in an aqueous solution (dissolved in water) and the chemical adsorbed to a solid surface, C( aq ) C( adsorbed ) and the equilibrium constant, termed the sorption coefficient, is, K Cadsorbed Caqueous Each chemical entity has an intrinsic value for the solubility product, i.e. has a tendency to sorb. In general, we expect hydrophobic chemicals (organic substances) to sorb more strongly than hydrophilic chemical (inorganic substances). Consider activated carbon, a strong sorbent, as a means of removing chloride and gasoline components from a contaminated water supply. Comment on the relative effectiveness of activated carbon in removing these, on the basis of equilibrium principles, i.e. in terms of the two equations presented above.