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Lecture 38, November 26, 2008

Lecture 38, November 26, 2008 - Equilibrium Concentrations...

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Equilibrium Concentrations The equilibrium concentrations of reactants and products for a chemical reaction can be predicted using the balanced chemical equation and known equilibrium constants. Procedure: 1. Write a balanced chemical equation for the relevant equilibrium or equilibria. 2. Write the corresponding equilibrium expression or expressions. 3. Create a table of concentrations for all reacting species.
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Direction of Reaction Starting only with reactants, it is obvious which direction reaction will go Suppose both reactants & products are present. How can we tell which way reaction would go to reach equilibrium? Use Reaction Quotient (Q) α A + β B γ C + δ D Q = [C] γ [D] δ /[A] [ α B] β
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Reaction Quotient Compare Q and K to see which way reaction will go Q = K: reaction at equilibrium
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LeChatlier’s Principle “When a change (stress) is imposed on a system at equilibrium, the system will react in the direction that reduces the amount of change.” Three common means of introducing change (stress) in chemical equilibria 1.Adding or removing material (change in concentration) 2.Changes in P 3.Changes in T
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Effect of Change in Concentration
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Concentration Changes There are 4 types of concentration changes. For an increase in product concentration, more reactants are formed. For a decrease in product concentration, more products are formed. For an increase in reactant concentration, more products are formed. For a decrease in reactant concentration, more reactants are formed.
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Effect of Change in Concentration For a reaction at equilibrium, increasing the concentration of one of the reactants will shift the equilibrium towards the products. The increase in product concentration for a reactant concentration increase can be rationalized by examining the reaction quotient.
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Reaction Quotient Compare Q and K to see which way reaction will go Q = K: reaction at equilibrium
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Effect of Change in Pressure For reactions involving gases, if the number of moles of gas differs between reactants and products, a shift in pressure (due to a volume change) will result in a change in equilibrium position. For the balanced chemical equation, if the moles product gases is greater than moles reactant gases For an increase in pressure, the equilibrium will shift toward reactants. For a decrease in pressure, the equilibrium will shift toward products
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Effect of Pressure Change 2NO 2 N 2 O 4 Increase pressure, concentration of N 2 O 4 increases, which reduces the total number of moles of gas present
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LeChatlier: K and T Think of “heat” as a reactant or product Exothermic: A + B C + Heat Raising T will shift equilibrium to left Increase the temperature, products are formed.
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