14.2
La costante di equilibrio
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Q1: How is the equilibrium constant expression written for a chemical reaction?
The equilibrium constant expression is the ratio of molar concentrations of products divided by molar concentrations of reactants, each raised to their stoichiometric coefficients. For a reaction aA + bB ⇌ cC + dD, the expression is K = [C]^c[D]^d / [A]^a[B]^b. The subscript c in Kc indicates the constant is based on molar concentration. This unitless ratio quantifies the relative amounts of reactants and products at equilibrium.
Q2: What does a large equilibrium constant value tell you about a reaction?
A large equilibrium constant much greater than 1 indicates that the numerator (product concentrations) is greater than the denominator (reactant concentrations), meaning equilibrium lies toward the products and favors the forward reaction. Conversely, a small equilibrium constant much less than 1 means reactant concentrations exceed product concentrations, favoring the reverse reaction. An equilibrium constant near 1 indicates neither direction is favored.
Q3: How does reversing a chemical equation affect its equilibrium constant?
When a chemical reaction is reversed, the equilibrium constant for the reverse reaction becomes the reciprocal of the forward reaction's constant. If the forward reaction has equilibrium constant x, the reversed reaction has equilibrium constant 1/x. This reciprocal relationship reflects the swap of reactants and products in the reversed equation.
Q4: What happens to the equilibrium constant when stoichiometric coefficients are multiplied by a factor?
When stoichiometric coefficients in a chemical equation are multiplied by a factor n, the equilibrium constant is raised to that same factor. If the original equilibrium constant is x, the new equilibrium constant becomes x^n. This exponential relationship ensures the equilibrium expression remains consistent with the modified stoichiometry.
Q5: How do you find the equilibrium constant for coupled reactions?
When two or more equilibrium reactions are added together, the equilibrium constant for the overall reaction is the product of the individual equilibrium constants. If reaction 1 has constant y and reaction 2 has constant z, the combined reaction has equilibrium constant y × z. This multiplicative relationship allows prediction of complex equilibrium systems from simpler component reactions.
Q6: Why are pure solids and pure liquids excluded from heterogeneous equilibrium constant expressions?
Pure solids and pure liquids are excluded from heterogeneous equilibrium constant expressions because their relative molar concentrations remain constant during the reaction. Since the equilibrium constant depends only on concentrations that change, including constant values would be redundant. For example, in ammonium nitrate decomposition, the solid ammonium nitrate is excluded from the expression.
Q7: Does the equilibrium constant indicate how fast a reaction reaches equilibrium?
No, the magnitude of the equilibrium constant does not indicate the reaction rate or how quickly equilibrium is reached. Some equilibria establish nearly instantaneously, while others proceed so slowly that no perceptible change occurs over days, years, or longer. The equilibrium constant only reflects the composition of the reaction mixture at equilibrium, not the speed of reaching that state.