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Ch.15 - Chemical Equilibrium
Chapter 15, Problem 56a

At 80Β°C, 𝐾𝑐 = 1.87Γ—10βˆ’3 for the reaction PH3BCl3(𝑠) β‡Œ PH3(𝑔) + BCl3(𝑔) (a) Calculate the equilibrium concentrations of PH3 and BCl3 if a solid sample of PH3BCl3 is placed in a closed vessel at 80Β°C and decomposes until equilibrium is reached.

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Key Concepts

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

Equilibrium Constant (Kc)

The equilibrium constant (Kc) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. For the reaction PH₃BCl₃(s) β‡Œ PH₃(g) + BCl₃(g), Kc = [PH₃][BCl₃] / [PH₃BCl₃]. Since solids do not appear in the expression, Kc only involves the gaseous products.
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Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. In this case, if PH₃BCl₃ decomposes, the system will shift to produce more PH₃ and BCl₃ until the concentrations reach a state that satisfies the equilibrium constant.
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ICE Table (Initial, Change, Equilibrium)

An ICE table is a tool used to organize the initial concentrations, the changes in concentrations, and the equilibrium concentrations of reactants and products in a chemical reaction. For this reaction, the initial concentration of PH₃BCl₃ is known, and as it decomposes, the changes in concentrations of PH₃ and BCl₃ can be calculated to find their equilibrium concentrations using the Kc value.
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Related Practice
Textbook Question

For the equilibrium Br2(𝑔) + Cl2(𝑔) β‡Œ 2 BrCl(𝑔) at 400 K, 𝐾𝑐 = 7.0. If 0.25 mol of Br2 and 0.55 mol of Cl2 are introduced into a 3.0-L container at 400 K, what will be the equilibrium concentrations of Br2, Cl2, and BrCl?

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Textbook Question

At 373 K, 𝐾𝑝 = 0.416 for the equilibrium 2 NOBr(𝑔) β‡Œ 2 NO(𝑔) + Br2(𝑔) If the pressures of NOBr(𝑔) and NO(𝑔) are equal, what is the equilibrium pressure of Br2(𝑔)?

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Textbook Question

At 218Β°C, 𝐾𝑐 = 1.2Γ—10βˆ’4 for the equilibrium NH4SH(𝑠) β‡Œ NH3(𝑔) + H2S(𝑔) Calculate the equilibrium concentrations of NH3 and H2S if a sample of solid NH4SH is placed in a closed vessel at 218Β°C and decomposes until equilibrium is reached.

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Textbook Question

At 80Β°C, 𝐾𝑐 = 1.87Γ—10βˆ’3 for the reaction PH3BCl3(𝑠) β‡Œ PH3(𝑔) + BCl3(𝑔) (a) Calculate the equilibrium concentrations of PH3 and BCl3 if a solid sample of PH3BCl3 is placed in a closed vessel at 80Β°C and decomposes until equilibrium is reached. (b) If the flask has a volume of 0.250 L, what is the minimum mass of PH3BCl3(𝑠) that must be added to the flask to achieve equilibrium?

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Textbook Question

At 25Β°C, the reaction CaCrO4(𝑠) β‡Œ Ca2+(π‘Žπ‘ž) + CrO42βˆ’(π‘Žπ‘ž) has an equilibrium constant 𝐾𝑐 = 7.1Γ—10βˆ’4. What are the equilibrium concentrations of Ca2+ and CrO42βˆ’ in a saturated solution of CaCrO4?

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Textbook Question

Consider the following equilibrium, for which Δ𝐻<0

2 SO2(𝑔) + O2(𝑔) β‡Œ 2 SO3(𝑔)

(f) How will each of the following changes affect an equilibrium mixture of the three gases: SO3(𝑔) is removed from the system?

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