chem - Chemical Equilibrium

Chem Chemical - Chemical Equilibrium Le Chteliers Principle Objective To observe and explain changes in equilibrium using Le Chteliers principle

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Chemical Equilibrium: Le Châtelier’s Principle
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Objective: To observe and explain changes in equilibrium using Le Châtelier’s principle after establishing four equilibrium systems and applying stresses through the addition or removal of reactants or products. Theory: Chemistry can be defined as the science of matter, and its interaction with other matter. A chemical reaction is defined as a change in the arrangement of atoms or molecules to yield substances of different composition and properties. Many chemical reactions do not go to completion but rather approach an equilibrium state in which both reactants and products are present. Thus, at equilibrium, the chemical process appears to stop, with a mixture of reactants and products. This condition in which the concentrations of all reactants and products cease to change with time is called chemical equilibrium. The rate at which the products are formed from the reactants equals the rate at which the reactants are formed from the products. In order for equilibrium to occur, neither reactants nor products can escape from the system. Once equilibrium is reached, the ratio of the concentrations or partial pressures of the reactants and products is a constant. It makes no difference if the equilibrium begins with reactants only, products only, or a mixture of both. At equilibrium, the equilibrium mixture remains constant. However, this is not to say that the reactants and products stop reacting. The equilibrium is dynamic. The formation of products and reactants back and forth still occur, but at the exact same rate, therefore causing no net changes in their amounts. To indicate a chemical equilibrium, a double arrow is shown: A ↔ B An extremely common expression used to illustrate chemical equilibrium is the Haber process, the basis for synthesizing ammonia: N 2(g) + 3H 2(g) ↔ 2NH 3(g) Since at equilibrium the concentrations and partial pressures of products and reactants always have a constant value, to systematically change the relative amounts would not affect this constant value. Given a general equilibrium equation: a A + b B ↔ c C + d D, where a , b, c , and d represent the coefficients of the reactants and products A, B, C, and D, respectively, the ratio between reactants and products can be expressed as: K c = [C] c [D] d [A] a [B] b or K p = (P C ) c (P D ) d (P A ) a (P B ) b This relationship is known as the equilibrium-constant expression for the reaction. The constants, K c and K p , are numerical values obtained when actual concentrations or molarities are substituted. The products raised to their coefficients divided by the reactants raised to their coefficients constitute the equilibrium-constant expression. The equilibrium-constant expression depends only on the stoichiometry of the reaction, and not on its mechanism. The value of the equilibrium constant does not depend on the initial amounts of reactants or products. The only factor that can change the equilibrium constant is the temperature. The rates of reaction are dependent upon several factors, such as concentration of
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This note was uploaded on 02/25/2012 for the course CHEM 4385 taught by Professor Staff during the Fall '11 term at Texas State.

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Chem Chemical - Chemical Equilibrium Le Chteliers Principle Objective To observe and explain changes in equilibrium using Le Chteliers principle

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