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

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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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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?

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] = 6.62×10⁻⁶ M

(b) [COCl₂] = 4.50×10⁻²M, [CO] = 1.1×10⁻⁷M, [Cl₂] = 2.25×10⁻⁶ M