01 Chatelier Principle - Properties of Systems in Chemical...

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Properties of Systems in Chemical Equilibrium – Le Châtleier’s Principle Chad K. Bush 08 March 2006 Tina Masterson
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Bush 1 Abstract In this lab we will work with several equilibrium systems, and we will alter these systems in various ways, forcing shifts to the right and left by changing concentrations of various ions and temperature. In this way we will explore the various implications of Le Châtelier’s Principle on chemical equilibria. Introduction In this lab we will be exploring the many varied implications of Le Châtelier’s Principle on equilibrium systems and their equilibrium constants (K c and K sp ). Le Châtelier’s Principle states that if you attempt to change a system in chemical equilibrium, it will react in such a way as to counteract the change you attempted. Take for instance the following reaction in equilibrium: A (aq) B (aq) + C (aq), K c = [B][C] (1) [A] Using Le Châtelier’s Principle, we know that if this reaction is proceeding at equilibrium (that is that the rates of the forward and reverse reactions are the same), were we to remove some amount of the product B from the system, the reaction would undergo a shift to the right to compensate and A will be consumed to produce more B and C until equilibrium is reestablished (at which point the values of [A], [B], and [C] when substituted into the equation for K c will equal the value of K c ). If product is added to the system, the reaction will shift to the left to consume the product until equilibrium is reestablished. Thus for any change in the system, the system will attempt to compensate for the change and maintain equilibrium. Le Châtelier’s Principle works not only for concentration changes but also for changes in temperature in a system. Let us continue to consider the above reaction (1). Suppose that H < 0, that is, that the reaction is exothermic. Then, it could be considered that one of the products of
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Bush 2 the reaction is heat, because the standard enthalpy of the system decreases as the system approaches equilibrium. Supposing then that we add heat (raise the temperature) to the system, we should expect to observe a shift to the left (to the endothermic side of the reaction) as the system attempts to compensate for the change in temperature. The same is true if we lower the temperature, but the system would be expected to shift to the left and products (B and C) will be consumed to produce reactants (A) until the value of K e for the new temperature is reached. Thus if H < 0 and heat is added, the value of K c decreases and the reaction shifts toward the reactants, and if H > 0 and heat is added, the value of K c increases and the reaction shifts toward the products. In this lab, we will be applying these concepts of Le Châtelier’s Principle to manipulate various reactions at equilibrium including those of acid-base indicators, solubility equilibria, and complex ion equilibria.
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