Ch.15 - Chemical Equilibrium

Problem 41b

Chapter 15, Problem 41b

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

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and are both expressions that describe the ratio of the concentrations of products to reactants for a given chemical reaction.

The reaction quotient, , is calculated using the concentrations of the reactants and products at any point in time, not necessarily at equilibrium.

The equilibrium constant, , is calculated using the concentrations of the reactants and products at equilibrium.

For to equal , the system must be at equilibrium, meaning the concentrations of reactants and products have reached a state where they no longer change with time.

Therefore, the condition that must be satisfied for = is that the reaction mixture is at 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 chemical reaction. It is calculated using the same expression as the equilibrium constant but with the current 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 reversible reaction at a given temperature. It is a fixed value for a specific reaction under specific conditions, indicating the extent to which reactants are converted into products at equilibrium.

Dynamic Equilibrium

Dynamic equilibrium occurs in a reversible reaction when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. At this point, Qc equals Kc, indicating that the system is balanced and no net change occurs. This condition is essential for understanding when a reaction has reached 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

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

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

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

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Key Concepts

Reaction Quotient (Q<sub>c</sub>)

Equilibrium Constant (K<sub>c</sub>)

Dynamic Equilibrium

(b) What condition must be satisfied so that Qc = Kc?

Verified video answer for a similar problem:

Key Concepts

Reaction Quotient (Q<sub>c</sub>)

Equilibrium Constant (K<sub>c</sub>)

Dynamic Equilibrium

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