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Ch.15 - Chemical Equilibrium
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.15 - Chemical EquilibriumProblema 154a
Capitolo 15, Problema 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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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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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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Hydrogenation Reactions

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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Equilibrium Constant Expressions

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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Reaction Quotient Q
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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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