Dynamic Equilibrium

Dynamic Equilibrium - 1 DYNAMIC EQUILIBRIUM-Products over...

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DYNAMIC EQUILIBRIUM ----------Products over Reactants I. EQUILIBRIUM AND THE EQUILIBRIUM CONSTANT For a chemical reaction of the form Reactants Products and starting with only reactants present, stoichiometry predicts the amount of products that are formed assuming that the reaction goes to completion, i.e., a 100% reaction yield. However, most reactions do not go to completion so that the amounts of products are less that the predictions of stoichiometry. What is occurring in these situations is that the reverse reaction, Reactants Products is also taking place. When the reaction is “over”, the rates of the forward and reverse reactions are the same. Both reactions are still occurring but they “cancel” each other out so that no changes in the properties of the chemicals (concentrations, partial pressures, masses, etc.) take place. Reactions that have reached this point are depicted as Reactants Products with a double arrow and are said to be in a state of dynamic equilibrium or equilibrium for short. Stoichiometry took these reactions into consideration by introducing the reaction yield. However, it turns out that the reaction yield depends on the initial amounts of reactants so that it is not a very general property for a reaction. A much more general property for a reaction is called the equilibrium constant . In defining the equilibrium constant in terms of properties of reactants and products, the general reaction will be considered. + + aA bB cC dD Rx. 1 If all compounds are solutes that are present in a solution with equilibrium concentrations of [A] eq , [B] eq , [C] eq and [D] eq , the equilibrium constant, K c , is defined as K c = [C] eq c [D] eq d [A] eq a [B] eq b (1) 1
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where the subscript “c” on K indicates it is defined in terms of concentrations. By convention, the products are in the numerator and the reactants are in the denominator. In general, the power of each concentration term is given by the stoichiometric coefficient of the corresponding chemical. It is very important to realize that the equilibrium constant does not depend on concentrations in that for any amounts of reactants and/or products initially present, the equilibrium concentrations will still satisfy Eq. (1). This is thereforE much more general that the reaction yield. The equilibrium constant is a positive number and, depending on the reaction, can range from being very small to being very large. If it is very small, the reaction starting with only reactants proceeds to the product side to only a small extend. If it is very large, the reaction will proceed to the product side to a large extent. It seems in Eq. (1) that K c has units of molarity to the power of c+d-a-b but, in actuality, equilibrium constants are unitless. The equilibrium constants actually come from thermodynamics and are defined in terms of dimensionless quantities called “activities”. For solutes, an approximation to activity is given by Molarity divided by 1M which is unitless. The value of 1M corresponds to the standard state of any solute.
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