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Ch.15 - Chemical Equilibrium
All textbooksBrown 14th EditionCh.15 - Chemical EquilibriumProblem 77c
Chapter 15, Problem 77c

Consider the hypothetical reaction A(g) ⇌ 2 B(g). A flask is charged with 0.75 atm of pure A, after which it is allowed to reach equilibrium at 0°C. At equilibrium, the partial pressure of A is 0.36 atm. (c) What could we do to maximize the yield of B?

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Identify the reaction and the changes in pressure: The reaction given is A(g) ⇌ 2 B(g). Initially, the pressure of A is 0.75 atm, and at equilibrium, it is 0.36 atm. This indicates that A is converting into B as the reaction proceeds.
Apply Le Chatelier's Principle: This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In this case, we want to shift the equilibrium towards the production of more B.
Increase the total pressure: Increasing the total pressure of the system will shift the equilibrium towards the side with fewer moles of gas. Since the reaction produces more moles of gas on the product side (2 moles of B from 1 mole of A), reducing the volume or increasing the pressure will shift the equilibrium towards the reactants. However, to increase the yield of B, this is not favorable.
Decrease the concentration of A: Removing some of A from the system or decreasing its partial pressure can shift the equilibrium towards the production of more B. This can be achieved by partial removal of A or by dilution.
Add an inert gas: Adding an inert gas at constant volume increases the total pressure without changing the partial pressures of the reacting gases, thus not affecting the equilibrium position directly. However, at constant pressure, adding an inert gas increases the volume, which can shift the equilibrium towards the side with more moles of gas, thus producing more B.

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Key Concepts

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

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. In the context of the reaction A(g) ⇌ 2B(g), if we increase the concentration of A or decrease the volume, the equilibrium will shift to the right, favoring the production of B.
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Equilibrium Constant (Kp)

The equilibrium constant (Kp) for a reaction at a given temperature is a measure of the ratio of the partial pressures of the products to the reactants at equilibrium. For the reaction A(g) ⇌ 2B(g), Kp can be expressed as Kp = (P_B^2) / (P_A). Understanding Kp helps predict how changes in pressure or concentration will affect the equilibrium position.
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Effect of Temperature on Equilibrium

The effect of temperature on equilibrium is governed by the endothermic or exothermic nature of the reaction. For an exothermic reaction, increasing the temperature shifts the equilibrium to the left, favoring reactants, while decreasing temperature shifts it to the right, favoring products. Knowing the heat of the reaction can guide decisions on temperature adjustments to maximize the yield of B.
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Related Practice
Textbook Question

A sample of nitrosyl bromide (NOBr) decomposes according to the equation 2 NOBr(g) ⇌ 2 NO(g) + Br2(g) An equilibrium mixture in a 5.00-L vessel at 100°C contains 3.22 g of NOBr, 3.08 g of NO, and 4.19 g of Br2. (c) What was the mass of the original sample of NOBr?

Textbook Question

As shown in Table 15.2, the equilibrium constant for the reaction N2(g) + 3 H2(g) ⇌ 2 NH3(g) is Kp = 4.34 × 10-3 at 300°C. Pure NH3 is placed in a 1.00-L flask and allowed to reach equilibrium at this temperature. There are 1.05 g NH3 in the equilibrium mixture. (b) What was the initial mass of ammonia placed in the vessel?

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