Ch.15 - Chemical Equilibrium

Problem 42a

Chapter 15, Problem 42a

(a) If Qc > Kc, how must the reaction proceed to reach equilibrium?

Verified step by step guidance

Understand the terms: Qc (reaction quotient) and Kc (equilibrium constant) are both measures of the relative amounts of products and reactants in a reaction mixture. Qc is calculated at any point in time, while Kc is calculated at equilibrium.

Compare Qc and Kc: If Qc > Kc, it means that the concentration of products is greater than what is expected at equilibrium.

Determine the direction of the shift: To reach equilibrium, the reaction must shift in the direction that reduces the concentration of products and increases the concentration of reactants.

Identify the direction: The reaction will proceed in the reverse direction (towards the reactants) to decrease the concentration of products and increase the concentration of reactants.

Conclusion: By shifting towards the reactants, the reaction will eventually reach a state where Qc equals Kc, achieving equilibrium.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Reaction Quotient (Qc)

The reaction quotient, Qc, is a measure of the relative concentrations of products and reactants at any point in a reaction. It is calculated using the same expression as the equilibrium constant (Kc) but with the current concentrations instead of equilibrium concentrations. Qc helps determine the direction in which a reaction will proceed to reach equilibrium.

Equilibrium Constant (Kc)

The equilibrium constant, Kc, is a value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. It is a fixed value for a particular reaction under defined conditions. Comparing Qc to Kc allows us to predict the direction of the reaction: if Qc < Kc, the reaction will proceed forward, while if Qc > Kc, it will shift backward.

Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is disturbed by a change in concentration, temperature, or pressure, the system will adjust to counteract the disturbance and restore a new equilibrium. In the context of Qc and Kc, if Qc > Kc, the system will shift to the left, favoring the formation of reactants to reduce the concentration of products and achieve equilibrium.

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Related Practice

Open Question

Two different proteins X and Y are dissolved in aqueous solution at 37 _x001F_C. The proteins bind in a 1:1 ratio to form XY. A solution that is initially 1.00 mM in each protein is allowed to reach equilibrium. At equilibrium, 0.20 mM of free X and 0.20 mM of free Y remain. What is Kc for the reaction?

Textbook Question

(a) If Q_c < K_c, in which direction will a reaction proceed in order to reach equilibrium?

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Textbook Question

(b) What condition must be satisfied so that Q_c = K_c?

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Textbook Question

(b) At the start of a certain reaction, only reactants are present; no products have been formed. What is the value of Qc at this point in the reaction?

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Textbook Question

At 100°C, the equilibrium constant for the reaction COCl₂(𝑔) ⇌ CO(𝑔) + Cl₂(𝑔) has the value 𝐾_𝑐= 2.19×10⁻¹⁰. Are the following mixtures of COCl₂, CO, and Cl₂ at 100°C at equilibrium? If not, indicate the direction that the reaction must proceed to achieve equilibrium. (a) [COCl₂] = 2.00×10⁻³𝑀, [CO] = 3.3×10⁻⁶𝑀, [Cl₂] = 6.62×10⁻⁶𝑀

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Open Question

As shown in Table 15.2, Kp for the equilibrium N2(g) + 3 H2(g) ⇌ 2 NH3(g) is 4.51 * 10^-5 at 450 _x001F_C. For each of the mixtures listed here, indicate whether the mixture is at equilibrium at 450 _x001F_C. If it is not at equilibrium, indicate the direction (toward product or toward reactants) in which the mixture must shift to achieve equilibrium. (a) 98 atm NH3, 45 atm N2, 55 atm H2. (b) 57 atm NH3, 143 atm N2, no H2. (c) 13 atm NH3, 27 atm N2, 82 atm H2.