EQUILIBRIUM_S16 - CHEMICAL EQUILIBRIUM In this chapter we...

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CHEMICALEQUILIBRIUMIn this chapter we will try to answer the following questions:To what extent does a reaction proceed before it reaches equilibrium? How can we describe the concentrations of reactants and products at equilibrium?How does the free energy of a chemical reaction vary with the compositionof the reaction mixture? What are the factors that affect chemical equilibrium?How can exploit those factors to enhance the occurrence of certain reactions?THE REVERSIBILITY OF REACTIONSA system reaches equilibrium when there is no further tendency for its composition 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 + BC Changes in concentration for the reaction H2O and CO as equilibrium is reachedZumdahl, S. S. Chemical Principles, 6thed.; Boston: Houghton Mifflin Company,2010.Changes with time in reaction rates of forward and reverse reactions for H2O + CO as equilibrium is reachedZumdahl, S. S. Chemical Principles, 6thed.; Boston: Houghton Mifflin Company,2010.
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EQUILIBRIUM AND THE LAW OF MASS ACTIONThe law of mass action states: at equilibrium, the composition of a reaction 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 equation 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 reference standard concentration of 1M (therefore, units cancel). -For a mixture of gases, the activity of each component is the ratio ofits partial pressure to a reference 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. 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 Kp. (Partial pressures are always in atmosphere for the calculation of Kp)THE EXTENT OF REACTIONThe equilibrium constant, K, indicates the extent to which a reaction can occur. A large equilibrium constant tells us that most of the reactants are converted into products before equilibrium is established. On the other hand, a small equilibrium constant indicates that only small amounts of reactants were converted into products.The value of Kis constant at constant temperature and changes if the temperature changes.The value of K does not depend on the initial concentrations.

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