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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblema 16
Capitolo 21, Problema 16

In excess of NH3(aq), Zn2+ forms a complex ion, [Zn(NH3)4]2+ which has a formation constant Kf = 7.8 x 10^8. Calculate the concentration of Zn2+ in a solution prepared by adding 1.00 x 10^-2 mol Zn(NO3)2 to 1.00 L of 0.250 M NH3. (a) 7.9 x 10^-4 M (b) 2.8 x 10^-6 M (c) 3.9 x 10^-9 M (d) 6.4 x 10^-11 M

Guida verificata passo dopo passo
1
Identify the reaction: Zn^{2+} + 4NH_3 \(\rightleftharpoons\) [Zn(NH_3)_4]^{2+}.
Write the expression for the formation constant: K_f = \(\frac{[[Zn(NH_3)_4]^{2+}\)]}{[Zn^{2+}][NH_3]^4}.
Determine initial concentrations: [Zn^{2+}]_0 = 1.00 \(\times\) 10^{-2} \(\text{ M}\), [NH_3]_0 = 0.250 \(\text{ M}\).
Assume x is the change in concentration of Zn^{2+} that forms the complex, then [Zn^{2+}] = 1.00 \(\times\) 10^{-2} - x and [NH_3] = 0.250 - 4x.
Substitute into the K_f expression and solve for x, assuming x is small compared to initial concentrations.

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Complex Ion Formation

Complex ions are formed when metal ions coordinate with ligands, such as ammonia (NH3), to create a stable structure. In this case, Zn2+ ions react with NH3 to form the complex ion [Zn(NH3)4]2+. Understanding this process is crucial for calculating the concentration of free Zn2+ ions in solution, as the formation of the complex significantly affects the equilibrium concentrations.
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Complex Ions and Formation Constant

Formation Constant (Kf)

The formation constant (Kf) quantifies the stability of a complex ion in solution. A high Kf value, like 7.8 x 10^8 for [Zn(NH3)4]2+, indicates that the complex is favored over the free metal ion and ligands. This constant is essential for determining the equilibrium concentrations of the species involved, allowing us to calculate the concentration of uncomplexed Zn2+ ions in the solution.
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Complex Ions and Formation Constant

Equilibrium Calculations

Equilibrium calculations involve applying the principles of chemical equilibrium to determine the concentrations of reactants and products at equilibrium. In this scenario, we set up an equilibrium expression based on the formation constant and the initial concentrations of Zn2+ and NH3. By using stoichiometry and the Kf value, we can solve for the concentration of free Zn2+ ions remaining in the solution after complex formation.
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Equilibrium Constant Calculation
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