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Unformatted text preview: 1 Chemistry II Laboratory Experiment #2 Exploring Le Chatelier's Principle. Read the entire experiment description before attempting the prelab questions. Pre-lab We are going to construct a mechanical analog to an equilibrium state (see below). We have a &quot;see saw&quot; that is balanced (horizontal) with four 1 gram weights on either side. Two other 1gram weights also are available. The balancing of weights is assigned a K eq = 1.0 for the equilibrium between A and B. Our equilibrium system is constrained to always return to this horizontal value by shifting only the weights representing A and B that are on the bar. The final constraint on this system is that it is &quot;closed&quot; - you can only use up to the total of the ten 1gram weights. A B A B K eq = 1.0 1.0 gram weights (total of 10) closed system 1. Case #1. You perturb the system by adding two grams of reactant A to the left side of the fulcrum. Describe the system: give the weights of A and product B after the system regains its equilibrium state. A B I nitial conditions 4 4 S ystem perturbed 4 +2 4 C hange as system approaches equilibrium -x +x E quilibrium attained 6-x 4+x K eq = [B] [A] = ? ? 2. Case #2. Starting with the original system, describe it at equilibrium after deleting 2 grams of B. 3. Case #3. Starting with the original system, describe it at equilibrium after deleting 2 grams of A. 4. Case #4. Starting with the original system, describe it at equilibrium after adding 2 grams of B (right side). 2 Chemistry II Laboratory Experiment #2 Exploring Le Chatelier's Principle. Read Chapter17. Many chemical reactions do not go to completion - they do not convert all of their reactant(s) to product(s). Instead, these reactions attain an equilibrium state in which both product(s) and reactant(s) are present at constant concentrations. These chemical equilibria develop for reversible reactions; at the equilibrium point the rate of the forward reaction balances the rate of the reverse reaction, reactant(s) product(s) + heat (for exothermic reactions) Thus, a reversible chemical reaction is a composite of a forward reaction, reactant(s) product(s) + heat that is occurring concurrently with its reverse reaction, heat + product(s) reactant(s) Since the reaction linking reactant(s) and product(s) goes both forward and backward, any equilibrium or balance point of the forward and reverse reactions can be approached from either direction. In class, you learned to characterize these equilibrium systems by the equilibrium constant , K eq , that is the ratio of the product to reactant concentrations (adjusted for stoichiometric coefficients). Provided that the temperature remains constant, the value for K eq remains invariant. The salient point is that a chemical system (reaction) maintains its equilibrium state for any given set of conditions....
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