Ch.15 - Chemical Equilibrium

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.15 - Chemical Equilibrium

Problem 146a

Chapter 15, Problem 146a

Refining petroleum involves cracking large hydrocarbon molecules into smaller, more volatile pieces. A simple example of hydrocarbon cracking is the gas-phase thermal decomposition of butane to give ethane and ethylene: (a) Write the equilibrium constant expressions for K_p and K_c.

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Identify the balanced chemical equation for the reaction. For the thermal decomposition of butane (C4H10) into ethane (C2H6) and ethylene (C2H4), the equation is: C4H10(g) → C2H6(g) + C2H4(g).

Write the expression for the equilibrium constant K_c using concentrations. K_c is defined as the product of the concentrations of the products raised to the power of their stoichiometric coefficients, divided by the product of the concentrations of the reactants raised to the power of their stoichiometric coefficients. For this reaction, it is: K_c = [C2H6][C2H4] / [C4H10].

Write the expression for the equilibrium constant K_p using partial pressures. K_p is similar to K_c but uses partial pressures instead of concentrations. The expression is: K_p = (P_C2H6)(P_C2H4) / (P_C4H10).

Understand the relationship between K_p and K_c. They are related by the equation K_p = K_c(RT)^Δn, where R is the gas constant, T is the temperature in Kelvin, and Δn is the change in moles of gas (moles of products - moles of reactants). For this reaction, Δn = (1 + 1) - 1 = 1.

Apply the ideal gas law where necessary to convert between concentration and pressure if required, using the formula P = nRT/V, where P is the pressure, n is the number of moles, R is the gas constant, T is the temperature, and V is the volume.

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Key Concepts

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

Equilibrium Constant (K)

The equilibrium constant (K) quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction. For gaseous reactions, K<sub>p</sub> is expressed in terms of partial pressures, while K<sub>c</sub> is based on molar concentrations. Understanding how to derive these constants is essential for analyzing chemical reactions, particularly in the context of cracking hydrocarbons.

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. This principle is crucial in refining processes, as it helps predict how changes in temperature, pressure, or concentration will affect the yield of products like ethane and ethylene during hydrocarbon cracking.

Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into simpler substances when heated. In the context of hydrocarbon cracking, butane decomposes into smaller hydrocarbons, such as ethane and ethylene, under high temperatures. Understanding this process is vital for optimizing conditions in petroleum refining to maximize the production of desired volatile compounds.

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Thermal Equilibrium

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Textbook Question

Refining petroleum involves cracking large hydrocarbon molecules into smaller, more volatile pieces. A simple example of hydrocarbon cracking is the gas-phase thermal decomposition of butane to give ethane and ethylene: (c) A sample of butane having a pressure of 50 atm is heated at 500 °C in a closed container at constant volume. When equilibrium is reached, what percentage of the butane has been converted to ethane and ethylene? What is the total pressure at equilibrium?

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Textbook Question

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Refining petroleum involves cracking large hydrocarbon molecules into smaller, more volatile pieces. A simple example of hydrocarbon cracking is the gas-phase thermal decomposition of butane to give ethane and ethylene: (a) Write the equilibrium constant expressions for Kp and Kc.

Verified video answer for a similar problem:

Key Concepts

Equilibrium Constant (K)

Le Chatelier's Principle

Thermal Decomposition

Refining petroleum involves cracking large hydrocarbon molecules into smaller, more volatile pieces. A simple example of hydrocarbon cracking is the gas-phase thermal decomposition of butane to give ethane and ethylene: (a) Write the equilibrium constant expressions for K_p and K_c.