Chemical Equilibrium and the How Far Question

See an animation of Catalytic Decomposition of Hydrogen Peroxide at http //brookscole.com/ chemist ry/joesten4 

Chemical reactions establish the same kind of equifibrium. In chemical equilibriiun a chemical reaction and its reverse are occurring at equal rates. Theoretically, all chemical reactions are reversible—able to take place in either direction and, therefore, to come to equilibrium. Studies of many, many chemical reactions have shown that reactions reach equilibrium at a characteristic and predictable point. Understanding what this is provides an answer to the How far question for a reaction. 

Some reactions go virtually to what we call completion—the conversion of such a large quantity of the reactants to products that what is unconverted is not noticeable and is unimportant. The combination of hydrogen and oxygen to form water is a reaction of this kind. Once a spark has gotten the first few molecules over the activation energy hill, the reaction continues rapidly and explosively until one or both reactants are used up. A slow reaction can also go to completion. As time passes, an iron nail exposed to the atmosphere continues to rust away gradually, until only the rust remains. 

You may get the idea that all chemical reactions go to completion when you watch a hydrogen-oxygen explosion or watch a piece of wood burn in the fireplace. Chemicals do not always react to form products with the complete conversion of reactants, however. Whenever the point is reached at which the forward reaction is proceeding at the same rate as the reverse reaction, equilibrium is established and the amounts of reactants and products remain unchanged. But because equilibrium is a dynamic condition, the forward and reverse reactions are still happening so that each reactant or product is replaced as soon as it is consumed. 

For equilibrium to be reached, it is important that none of the reactants or products can escape. An example is provided by the conversion of limestone (calcium carbonate) to lime (calcium oxide). By heating limestone in open pits, early U.S. settlers produced lime for mortar  

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