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Ch.15 - Chemical Equilibrium
All textbooksMcMurry 8th EditionCh.15 - Chemical EquilibriumProblem 154a
Chapter 15, Problem 154a

Acetic acid tends to form dimers, (CH3CO2H2), because of hydrogen bonding: The equilibrium constant Kc for this reaction is 1.51⨉102 in benzene solution but only 3.7⨉10-2 in water solution. (a) Calculate the ratio of dimers to monomers for 0.100 M acetic acid in benzene.

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1
Identify the equilibrium reaction for the dimerization of acetic acid: 2 CH_3CO_2H ⇌ (CH_3CO_2H)_2.
Write the expression for the equilibrium constant K_c: K_c = [(CH_3CO_2H)_2] / [CH_3CO_2H]^2.
Substitute the given K_c value for benzene (1.51 \times 10^2) into the equilibrium expression.
Let x be the concentration of dimers formed at equilibrium. Then, the concentration of monomers at equilibrium is (0.100 - 2x) M.
Solve the equation 1.51 \times 10^2 = x / (0.100 - 2x)^2 to find the ratio of dimers to monomers, which is x / (0.100 - 2x).

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Key Concepts

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

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative atom, such as oxygen or nitrogen, experiences an attraction to another electronegative atom. In the case of acetic acid, the hydrogen bonds between the molecules lead to the formation of dimers, which significantly influences the physical properties and behavior of the substance in different solvents.
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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 reversible chemical reaction. It provides insight into the extent of the reaction; a high Kc value indicates that products are favored, while a low Kc suggests that reactants are favored. In this question, the different Kc values in benzene and water reflect how solvent polarity affects dimerization of acetic acid.
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Concentration and Reaction Quotient

Concentration refers to the amount of a substance in a given volume of solution, typically expressed in molarity (M). The reaction quotient (Q) is calculated similarly to Kc but uses the initial concentrations of reactants and products. By comparing Q to Kc, one can determine the direction in which a reaction will proceed to reach equilibrium. In this case, calculating the ratio of dimers to monomers involves using the concentration of acetic acid and the equilibrium constant.
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Related Practice
Textbook Question

The equilibrium constant Kc for the gas-phase thermal decomposition of cyclopropane to propene is 1.0 ⨉105 at 500 K:

(a) What is the value of Kp at 500 K?

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Textbook Question

The equilibrium constant Kc for the gas-phase thermal decomposition of cyclopropane to propene is 1.0 * 105 at 500 K:

(c) Can you alter the ratio of the two concentrations at equilibrium by adding cyclopropane or by decreasing the volume of the container? Explain.

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Open Question
Acetic acid tends to form dimers, 1CH3CO2H22, because of hydrogen bonding: The equilibrium constant Kc for this reaction is 1.51 * 10^2 in benzene solution but only 3.7 * 10^-2 in water solution. Why is Kc for the water solution so much smaller than Kc for the benzene solution?
Textbook Question

Acetic acid tends to form dimers, (CH3CO2H2), because of hydrogen bonding: The equilibrium constant Kc for this reaction is 1.51⨉102 in benzene solution but only 3.7⨉10-2 in water solution. (b) Calculate the ratio of dimers to monomers for 0.100 M acetic acid in water.

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Open Question
A 125.4 g quantity of water and an equal molar amount of carbon monoxide were placed in an empty 10.0-L vessel, and the mixture was heated to 700 K. At equilibrium, the partial pressure of CO was 9.80 atm. The reaction is CO(g) + H2O(g) ⇌ CO2(g) + H2(g). (a) What is the value of Kp at 700 K? (b) An additional 31.4 g of water was added to the reaction vessel, and a new state of equilibrium was achieved. What are the equilibrium partial pressures of each gas in the mixture? What is the concentration of H2 in molecules/cm³?
Textbook Question
A 79.2 g chunk of dry ice (solid CO2) and 30.0 g of graphite (carbon) were placed in an empty 5.00-L container, and the mixture was heated to achieve equilibrium. The reaction is CO 1g2 + C s ∆ 2 CO g (b) What is the value of Kp at 1100 K if the gas density at 1100 K is 16.9 g/L?
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