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Ch.15 - Chemical Equilibrium
All textbooksBrown 14th EditionCh.15 - Chemical EquilibriumProblem 42b
Chapter 15, Problem 42b

(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?

Verified step by step guidance
1
Understand that Qc, the reaction quotient, is a measure of the relative amounts of products and reactants present during a reaction at a given point in time.
Recall that the expression for Qc is similar to the equilibrium constant (Kc), but it can be calculated at any point in the reaction, not just at equilibrium.
The formula for Qc is: \( Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b} \), where [C] and [D] are the concentrations of the products, and [A] and [B] are the concentrations of the reactants, with a, b, c, and d being their respective stoichiometric coefficients.
At the start of the reaction, only reactants are present, meaning the concentrations of the products are zero.
Since the concentrations of the products are zero, the numerator of the Qc expression is zero, making Qc equal to zero at the start of the reaction.

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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 (Kc), but for non-equilibrium conditions. Qc helps determine the direction in which a reaction will proceed to reach equilibrium.
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Reaction Quotient Q

Initial Conditions of a Reaction

At the start of a reaction, only reactants are present, meaning the concentrations of products are zero. This initial state is crucial for calculating Qc, as it reflects the system's starting point before any reaction occurs. Understanding these initial conditions allows for predicting how the reaction will shift as it progresses toward equilibrium.
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Equilibrium and Reaction Direction

The value of Qc at any point in a reaction indicates whether the system is at equilibrium, or if it will shift toward products or reactants. If Qc is less than Kc, the reaction will proceed forward to produce more products; if Qc is greater than Kc, the reaction will shift backward to form more reactants. This concept is essential for understanding how reactions progress over time.
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Related Practice
Textbook Question

(a) If Qc < Kc, 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 Qc = Kc?

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Textbook Question

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

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Textbook Question

At 100°C, the equilibrium constant for the reaction COCl2(𝑔) ⇌ CO(𝑔) + Cl2(𝑔) has the value 𝐾𝑐 = 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] = 2.00×10−3 M, [CO] = 3.3×10−6 M, [Cl2] = 6.62×10−6 M

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

(c) [COCl2] = 0.0100 M, [CO] = [Cl2] = 1.48×10−6 M

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