EQUILIBRIUM - CHEMICAL EQUILIBRIUM In this chapter we will...

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1 CHEMICAL EQUILIBRIUM In this chapter we will try to answer the following questions: How does the free energy of a chemical reaction vary with the composition of the reaction mixture? To what extent does a reaction proceed before it reaches equilibrium? How can we describe the concentrations of reactants and products at equilib- rium? What are the factors that affect chemical equilibrium? How can exploit those factors to enhance the occurrence of certain reactions? THE REVERSIBILITY OF REACTIONS A system reaches equilibrium when there is no further tendency for its com- position to change. Such equilibrium is dynamic! Meaning that both the forward and the reverse reactions are occurring simultaneously at the same rate and therefore, one can observe no net change in composition. A + B C + D EQUILIBRIUM AND THE LAW OF MASS ACTION · The law of mass action states: at equilibrium, the composition of a re- action mixture can be expressed in terms of the equilibrium constant. · The equilibrium constant, K, is equal to the activities of the products raised to the power of their coefficients in the balanced chemical equa- tion divided by the activities of the reactants raised to the power of their coefficients. - For pure solids or liquids the activity = 1 (no units). - For components of an ideal solution, the activity of each component is the ratio of its molar concentration to a standard concentration of 1M (therefore, units cancel). - For a mixture of gases, the activity of each component is the ratio of its partial pressure to a standard pressure of 1 atm (therefore, units cancel). - For reactions mixtures that involve ions in solution, we write the equilibrium constant for the net ionic equation using activities of ions.
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1 · Heterogeneous equilibria involve species in more than one phase. While homogeneous equilibria involve all species in the same phase. · For mixtures of gases, K can be referred to as K p . (Partial pressures are always in atmosphere for the calculation of Kp) Examples: Write the expression for the equilibrium constant for the each of the follow- ing reactions: 1- Nitrogen reacts with hydrogen to give ammonia. 2- Silver nitrate reacts with sodium chloride in water to produce silver chloride as a precipitate. 3- 2NH 3 (g) + 3CuO(s) N 2 (g) + 3H 2 O(l) + 3Cu(s) 4- MgCO 3 (s) MgO(s) + CO 2 (g) THE THERMODYNAMIC ORIGIN OF EQUILIBRIUM CONSTANTS
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1 · Every chemical process proceeds spontaneously toward equilibrium. · Depending on its composition, a system moves toward equilibrium by producing more products or more reactants! · If the system does not have enough products, the spontaneous change is toward producing them and the forward reaction is spontaneous. · If the system has excess products, the spontaneous change will be to pro-
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EQUILIBRIUM - CHEMICAL EQUILIBRIUM In this chapter we will...

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