tailieunhanh - Lecture Thermodynamics: An engineering approach (8/e): Chapter 16 - Yunus A. Çengel, Michael A. Boles

Chapter 16 - Chemical and phase equilibrium. The objectives of Chapter 16 are to: Develop the equilibrium criterion for reacting systems based on the second law of thermodynamics; develop a general criterion for chemical equilibrium applicable to any reacting system based on minimizing the Gibbs function for the system; define and evaluate the chemical equilibrium constant;. | Chapter 16 Chemical and Phase Equilibrium Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Consider the combustion of propane with 120 percent theoretical air. The combustion equation is Conservation of mass for each species yields Since the percent theoretical air is known, the N2 balance gives B. The H balance gives G. Then the C and O balances give two equations that relate the remaining three unknowns D, E, and F. Therefore, we need one more equation. To obtain this last equation relating the mole numbers of the products, we assume that the products are in chemical equilibrium. To develop the relations for chemical equilibrium, consider placing the product gases in a system maintained at constant T and P. The constant T and P are achieved if the system is placed in direct contact with a heat reservoir and a work reservoir. In particular, let’s consider that CO2, CO, . | Chapter 16 Chemical and Phase Equilibrium Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Consider the combustion of propane with 120 percent theoretical air. The combustion equation is Conservation of mass for each species yields Since the percent theoretical air is known, the N2 balance gives B. The H balance gives G. Then the C and O balances give two equations that relate the remaining three unknowns D, E, and F. Therefore, we need one more equation. To obtain this last equation relating the mole numbers of the products, we assume that the products are in chemical equilibrium. To develop the relations for chemical equilibrium, consider placing the product gases in a system maintained at constant T and P. The constant T and P are achieved if the system is placed in direct contact with a heat reservoir and a work reservoir. In particular, let’s consider that CO2, CO, and O2 form a mixture in chemical equilibrium. Taking the positive direction of heat transfer to be to the system, the increase of entropy principle for a reacting system is If the reaction takes place adiabatically, then . A reaction taking place adiabatically does so in the direction of increasing entropy. If we apply both the first law and the second law for the fixed mass system of reacting gases for fixed T and P where there is both heat transfer and work, we obtain Now, we define a new (for us anyway) thermodynamic function, the Gibbs function, as The differential of the Gibbs function when T and P are constant is 0 0 The chemical reaction at constant temperature and pressure will proceed in the direction of decreasing Gibbs function. The reaction will stop and chemical equilibrium will be established when the Gibbs function attains a minimum value. An increase in the Gibbs function at constant T and P would be a violation of the second law. The .