Le Chatlier’s principle

One would normally expect that placing the sample into the concentrated acid would cause the reaction to occur even faster than in the dilute acid, based upon Le Chatlier s principle. In addition, one would not expect that exposure to the concentrated acid would alter the behavior of the Fe in the dilute acid, but it does. This alteration was called passivity by Schoenbien in 1836. The electrochemical explanation of the Faraday experiment is discussed below. While a large body of literature exists concerning the underlying mechanisms, for our purposes only a brief review of the fundamental origins will be required. 

Simple general chemistry principles can be used to help maximize precipitation and product yield. The first is the common ion effect, formally defined as making a weak acid or weak base weaker by the addition of a salt that contains a common ion, a direct consequence of Le Chatlier s principle. The common ion effect can also be used to understand metal complex precipitation. Consider a saturated solution of [Co(NH3)6]C13, equation (1.20). 

Chemical equilibrium responds to a "stress" by moving to the side that relieves the effect of "stress" and returns the system to equilibrium, according to Le Chatlier s Principle. If we add heat to an exothermic reaction it will shift toward the reactants on the left. If we add mass, concentration, or pressure on the reactant side of the equilibrium, the system responds by shifting toward the products. If we can remove products from the reaction zone (the system or control volume), then we also shift the reaction equilibrium to the right. In fact, even if we remove just one of the products from a set of products, the system will shift to the right. 

You may recall from general chemistry that many reactions do not go to completion but only up to a certain point, because they are reversible. How far the reaction goes depends on the concentrations (or partial pressures for a gas) of the products and the reactants, which are related by the reaction stoichiometry and the equilibrium constant K. If we want to increase the amount of conversion, one way we can do this is to separate out the products from the product mixture and re-feed the purified reactants in to the reactor. By Le Chatlier s Principle, this will cause the reaction to continue moving towards the products. 

In order to achieve high conversion, it is often necessary to drive condensation reactions by removal of the small-molecule condensate. This shifts the reaction equilibrium toward the polymer, as dictated by Le Chatlier s principle. In the case of ADMET polymerization with a,(B-dienes, removal of ethylene, the by-product of polymerization, prevents the reverse reaction to re-form the monomer. 

Fig. 1 (a) Single unit cell once electrons are rejected through external circuit by hydrogen oxidation, the reaction shifts in forward direction due to Le Chatlier s principle. GDL represents gas diffusion layer and PEM is proton exchange membrane, (b) Multiple-cell configuration (2-cell) the electrons from cell 2 anode flows into the cell 1 cathode plate enabling series configuration. 

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