Le Chatelier’s principle
and chemical equilibrium systems
Jan Samuel Matuba, Mark Cel Gonzaga
Chemical Engineering Department, College of Engineering, University of the Philippines, Diliman 1100 -------------------------------------------------
January 11, 2012
Reaction rates determine the speed at which products are formed from the reactants. Expressed as concentration divided by time, reaction rates measure the rate of disappearance of the reactants, and, conversely, the rate of formation of the products1. Not all reactions however proceed at the forward direction only. In some reactions, the system appears to have finished the reaction and is composed entirely of the products when in fact, the reaction system achieves equilibrium state2. In equilibrium, the rate of the forward and the reverse reactions are equal. That is, the products are formed from the reactants at the same rate that the reactants are produced from the products2. At this point, the concentrations of the product and the reactants remain unchanged so that the ratio of the molar concentrations of the products over those of the reactants, raised to their stoichiometric coefficients, is a constant3. This constant, the equilibrium constant, Keq,, is also equal to the ratio of the reaction rates of the forward and the reverse reactions. For a hypothetical equilibrium reaction:
where kf is the rate of the forward reaction; kr, the rate of the reverse reaction; [C], [D], [A], [B] the molar concentrations of reagents C, D, A, and B, respectively; and c, d, a, b the stoichiometric coefficients of reagents C, D, A, B, respectively. It is observed that when an equilibrium reaction is disturbed- adding a reactant, altering the volume, applying pressure, etc- the reaction would tend to proceed forward or reverse depending on the conditions....
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