LeChatelier - Module 28 Equilibrium Module 28 Equilibrium...

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Module 28 — Equilibrium ©2010 ChemReview.net v. 3d Page 817 Module 28 ± Equilibrium Pretests : In this module, if you have had a prior chemistry course, the initial lessons may be review. If you think you are familiar with the topic of a lesson, try a few problems at the end of the last problem set in the lesson. If you get those right, move to the next lesson. * * * * * Introduction Chemical reactions can be divided into three types. 1. Reactions that go nearly 100% to completion. Burning paper is one such reaction. Once the reaction begins, the reaction goes until one of the reactants (the paper or oxygen) is essentially used up. 2. Reactions that don±t go. Trying to convert carbon dioxide and water into paper is very difficult to do in a chemist±s laboratory (though plants are able to accomplish most steps in this reaction by the remarkable process of photosynthesis). 3. Reactions that are reversible and go partially to completion. In reversible reactions, as the reaction proceeds, the reactants are gradually used up. As a result, the forward reaction slows down. As product concentrations increase, they more frequently collide and react to re-form the reactants. Finally, both the forward and reverse reactions are going at the same rate. As long as no substances or energy are added to or removed from the reaction system, the two rates will remain equal and no further reaction seems to take place. The system is said to be at equilibrium . For equilibrium to exist, x all reactants and products must be present in at least small quantities, and x the reaction must be in a closed system: no particles or energy can be entering or leaving the reaction vessel. At equilibrium, no reaction seems to be occurring, but this appearance is deceiving. Equilibrium is dynamic : the forward and reverse reactions continue. However, because the rates of reaction of the forward and reverse reactions are the same, there is no net change. In theory, all reactions that occur are reversible, and all reactions go to equilibrium. In practice, many equilibria favor the products so much that nearly all of the limiting reactant is used up, and the reaction is considered to go ²to completion.³ For those reactions, if the limiting reactant is known, calculation of the amounts of reactants used up and products formed can be done using conversion stoichiometry. When reactions go only partially to completion, there is no limiting reactant completely used up that decides how much of the products form. Reactions that go to equilibrium require an accounting system to track the particles used up and formed.
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Module 28 — Equilibrium ©2010 ChemReview.net v. 3d Page 818 Lesson 28A: Le Châtelier±s Principle Timing : Depending on the sequence in your course, problems based on Le Châtelier±s Principle may be assigned either at the beginning or end of a unit on Equilibrium. This lesson may be completed at either point. When Le Châtelier±s Principle is assigned in your class, read the introduction above and then complete this lesson. Pretest
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LeChatelier - Module 28 Equilibrium Module 28 Equilibrium...

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