Ch.15 - Chemical Equilibrium
Chapter 15, Problem 5
The reaction A21g2 + B21g2 ∆ 2 AB1g2 has an equilib- rium constant Kc = 9. The following figure represents areaction mixture that contains A2 molecules (red), B2 mol- ecules (blue), and AB molecules. What statement about the mixture is true? (LO 15.5) 
(a) The mixture is at equilibrium, and there will be no net shift in reaction direction.(b) The reaction will shift toward the reactants to reach equilibrium.(c) The reaction will shift toward the products to reach equilibrium.(d) More information is needed to answer this question.
Verified step by step guidance1
Step 1: Identify the number of each type of molecule in the image. Count the number of A2 (red), B2 (blue), and AB (yellow) molecules.
Step 2: Write the expression for the equilibrium constant Kc for the reaction A2(g) + B2(g) ⇌ 2 AB(g). The expression is Kc = [AB]^2 / ([A2][B2]).
Step 3: Calculate the initial concentrations of A2, B2, and AB using the counts from the image. Assume the volume of the container is constant.
Step 4: Substitute the concentrations into the equilibrium constant expression to determine the reaction quotient Qc.
Step 5: Compare Qc to Kc. If Qc < Kc, the reaction will shift toward the products. If Qc > Kc, the reaction will shift toward the reactants. If Qc = Kc, the system is at equilibrium.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chemical Equilibrium
Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. In this state, the system appears static, but molecular activity continues. Understanding this concept is crucial for determining whether a reaction mixture is at equilibrium or if it will shift in response to changes in concentration, temperature, or pressure.
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Chemical Equilibrium Concepts
Equilibrium Constant (Kc)
The equilibrium constant (Kc) quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. A Kc value greater than 1 indicates that products are favored at equilibrium, while a value less than 1 suggests that reactants are favored. In this case, Kc = 9 implies that the formation of AB is favored, which is essential for predicting the direction of the reaction shift.
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Le Chatelier's Principle
Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. This principle helps predict how a reaction will shift in response to changes in concentration, pressure, or temperature. In the context of the question, it is vital for determining whether the reaction will shift toward reactants or products based on the current concentrations of A2, B2, and AB.
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