1212L+LeChatliers+Principle - Chemical Equilibrium An...

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Introduction Stiochiometric equations show the mole ratios of reactants consumed and products formed. They do not show whether the reaction occurs or how complete the change will be if it occurs. Some reactions proceed to completion. Examples of these are acid/base reactions and reactions of active metals with H 2 O. Many other reactions do not go to completion and the amount of product formed is less than the theoretical yield. Reactions that reach equilibrium are reversible reactions and have a mixture of reactants and products present. Consider the reaction, H 2 ( g ) + I 2 ( g ) → 2HI( g ) reaction occurs at 450ºC (Eq. 1) At a given temperature (450ºC ) and pressure (atmospheric), the reaction between H 2 and I 2 does not go to completion. Some H 2 and I 2 remain unreacted. Under the same conditions, the reverse reaction 2HI( g ) → H 2 ( g ) + I 2 ( g ) (Eq. 2) ceases to show activity before all the HI is consumed, likewise giving a mixture of HI, H 2 , and I 2 . Eq. 1 is called the forward reaction and Eq. 2 is called the reverse reaction and at equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction. Three types of general observations are made about reversible reactions. First, reversible reactions reach equilibrium with both products and reactants present. Second, when the reaction reaches equilibrium under specified conditions of temperature and pressure, the ratio of the concentrations of the products to reactants is always constant when expressed in the following way: aA + bB ↔ cC +dD, b a d c eq [B] [A] [D] [C] K = K eq is called the equilibrium constant. The K eq expression for Eq. 1 is: ] ][I [H [HI] K 2 2 2 eq = The form of the equilibrium constant expression is always the product of the molar concentrations of the products divided by the product of the molar concentration of the reactants. The molar concentration of each species is placed in brackets [ ], and the concentration is raised to an exponent equal to the stoichiometric coefficient of that species in the balanced equation. Chemical Equilibrium – An Introduction
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The third observation concerns LeChâtelier’s Principle, which states that when a system at equilibrium is disturbed, the system will respond by favoring the direction of reaction that restores, if possible, equilibrium. The direction of the change will adjust all the concentrations to the levels required by the equilibrium constant. Factors to which the reaction will respond to relieve applied stresses include concentration and temperature changes. A concentration change resulting from the addition or removal of reactant or product causes stress to a system at equilibrium. A net change in concentrations of all species occurs in such a way that equilibrium is reestablished. Changing the volume of the container changes the concentration of all gaseous species. Changes in the partial pressure of a gaseous species are directly proportional to the molarity of that species. Consider the reaction in described in Equation 1.
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1212L+LeChatliers+Principle - Chemical Equilibrium An...

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