Chapter 15 Chemical Equilibrium SY 3/16/11 15-1 Figure to come The heme/O2 equilibrium, with image and models? Caption to come Chapter 15: Chemical Equilibrium Chapter In Context This chapter begins our coverage of chemical equilibria. The nature of a dynamic equilibrium was briefly introduced earlier in our description of phase equilibria (sections 12.X and 12.X). In the chapters that follow this one we will apply the concepts and tools learned in this chapter to acid-base equilibria and the chemistry of insoluble ionic compounds. Chemical equilibrium and the reversibility of chemical reactions plays an important role in one of the most important chemical reactions in the human body, the binding of oxygen molecules to heme groups in the protein hemoglobin. When hemoglobin is exposed to oxygen in the lungs, oxygen molecules attach to the heme groups. When the oxygenated hemoglobin reaches oxygen-depleted cells, the oxygen is released. The process is reversible, and as you will see in this chapter, it is the reversible nature of chemical reactions that is the basis of chemical equilibrium. Reversibility is a central and important aspect of chemical activity in many areas of nature and commerce. For example, - Biology: The oxygenation of heme is crucial to life, but so is the absorption and release of other gases, such as carbon dioxide, from body fluids. Other equilibrium processes that occur in the body include the reversible binding of metal ions by proteins and the acid-base balance of biochemical systems. - Environment: The dissolution of toxic metal ions can be reversibly changed through changes in solution acidity brought on by acid rain or industrial acid discharge. These changes can allow for the transportation of normally insoluble harmful substances into natural water systems, affecting plants, fish, and other aquatic species. - In Your World: The application and removal of floor wax depends on the reversible binding of Zn2+ ions to organic polymer materials. Also, soft drinks, beer and champagne all fizz when opened because of the reversible solubility of CO2 in water. Chapter Goals - - Chapter 15 15.1 The Nature of the Equilibrium State 15.2 The Equilibrium Constant 15.3 Using Equilibrium Constants in Calculations 15.4 Disturbing a Chemical Equilibrium: Le Chatelier’s PrincipleChapter 15 Chemical Equilibrium SY 3/16/11 15-2 15.1 The Nature of the Equilibrium State OWL Opening Simulation 15.1 Microscopic Reversibility In the previous chapter we learned that chemical reactions proceed in a series of steps called a reaction mechanism. The principle of microscopic reversibility tells us that any chemical reaction can proceed in the forward or reverse direction. Thus any series of chemical reactions, the elementary steps in a mechanism, for example, can proceed in either the forward or reverse direction. Consider the two-step mechanism for the reaction of nitrogen dioxide with carbon monoxide. Step 1: NO2(g) + NO2(g)  NO(g) + NO3(g) Step 2: NO3(g) + CO(g)  NO2(g) + CO2(g) Net reaction: NO2(g) + CO(g)  NO(g) + CO2(g) The two elementary steps in the mechanism could also proceed in the reverse direction. Step 1’ NO2(g) + CO2(g)  NO3(g) + CO(g) (Step 2 reversed) Step 2’ NO3(g) + NO(g)  NO2(g) + NO2(g) (Step 1 reversed) The new net reaction is the reverse of the original reaction. New net reaction: NO(g) + CO2(g)  NO2(g) + CO(g) The equilibrium arrow (Æ) is used to indicate that a reaction is reversible. NO2(g) + CO(g) Æ NO(g) + CO2(g) There are occasions when the use of an equilibrium arrow is not appropriate. For example, when hydrogen and oxygen react to form water vapor (Figure 15.X), product formation is very strongly favored and no noticeable amounts of reactants are formed by the reverse reaction. The chemical equation representing this reaction therefore uses a single reaction arrow (). Also, if the reverse reaction cannot occur, such as when one of the reaction products is physically separated from a reaction mixture as a gas (the reaction of a metal carbonate with acid, for example, Figure 15.X), a single reaction arrow is used. The Equilibrium State In Chapter 12, we described the nature of a dynamic equilibrium between a liquid and its vapor in a sealed flask. When equilibrium was reached in the flask, the amount of liquid did not change, but the vaporization and condensation processes continued. When the rate of vaporization is equal to the rate of condensation, a state of dynamic equilibrium between the two phases is reached. We can apply this concept to reversible chemical reactions and use it to describe the nature of a dynamic chemical equilibrium. Consider the reversible reaction between Fe3+ and the thiocyanate ion. Fe3+(aq) + SCN–(aq) Æ FeSCN2+(aq) When the two reactants are mixed, they react in the forward direction to form FeSCN2+ in a second order process. Fe3+(aq) + SCN–(aq)  FeSCN2+(aq) Rate (forward) = kforward[Fe3+][SCN–] Figure 15.X Flashback 12.X Vapor PressureChapter 15 Chemical Equilibrium SY 3/16/11 15-3 As the reaction proceeds, the product concentration increases and the reverse reaction begins to take place. FeSCN2+(aq)  Fe3+(aq) + SCN–(aq) Rate (reverse) = kreverse[FeSCN2+] As the reaction continues, the rate of the forward reaction decreases (because [Fe3+] and [SCN-] decrease) and the rate of the reverse reaction increases (because [FeSCN2+] increases). Eventually, a state of chemical equilibrium is reached where the rate of the forward reaction is equal to the rate of the reverse reaction: Rate (forward) = Rate (reverse) kforward[Fe3+][SCN-] = kreverse[FeSCN2+] When this equilibrium state is achieved, the concentrations of all the species in solution are constant, even though the forward and reverse reactions continue to take place. The equilibrium state can be represented graphically by plotting the concentration of reactants and products over time (Figure 15.X). Notice that during the first stage of the reaction, the forward reaction is faster than the reverse reaction and reactant concentrations decrease while the product concentration increases. When the system reaches equilibrium, the forward