Problem 66 - Chemistry: Atoms First

Question

Sometimes equilibria for complex ions are described in terms of dissociation constants, \(K_{\mathrm{d}}\). For the complex ion AIF \(_{6}^{3-}\) the dissociation reaction is: \(\mathrm{AlF}_{6}^{3-} \rightleftharpoons \mathrm{Al}^{3+}+6 \mathrm{F}^{-}\) and \(K_{\mathrm{d}}=\frac{\left[\mathrm{Al}^{3+}\right]\left[\mathrm{F}^{-}\right]^{6}}{\left[\mathrm{AlF}_{6}^{3-}\right]}=2 \times 10^{-24}\) Calculate the value of the formation constant, \(K_{\mathrm{f}}\), for \(\mathrm{AlF}_{6}^{3-}\).

Step-by-Step Solution

Answer

\(K_{\mathrm{f}} = 5 \times 10^{23}\)

Step 1: Understanding the Relationship Between Dissociation and Formation Constants

The dissociation constant, \(K_{\mathrm{d}}\), is related to the formation constant, \(K_{\mathrm{f}}\), by the following mathematical relationship: \(K_{\mathrm{f}} = \frac{1}{K_{\mathrm{d}}}\) since the formation reaction is the reverse of the dissociation reaction. By knowing the dissociation constant, we can find the formation constant by taking its reciprocal.

Step 2: Calculating the Formation Constant

Given the dissociation constant \(K_{\mathrm{d}} = 2 \times 10^{-24}\), we calculate the formation constant \(K_{\mathrm{f}}\) by finding the reciprocal of the \(K_{\mathrm{d}}\). Thus, \(K_{\mathrm{f}} = \frac{1}{K_{\mathrm{d}}} = \frac{1}{2 \times 10^{-24}}\).

Step 3: Simplify and Write Down the Final Answer

We have \(K_{\mathrm{f}} = \frac{1}{2 \times 10^{-24}}\). Simplifying this we get \(K_{\mathrm{f}} = 5 \times 10^{23}\). This is the value of the formation constant for the complex ion \(\mathrm{AlF}_{6}^{3-}\).

Key Concepts

Understanding the Dissociation Constant (Kd)

The dissociation constant, commonly represented as K_d, plays a crucial role in the study of chemical equilibrium, particularly in the behavior of complex ions. It measures the propensity of a complex ion to separate into its constituent ions in solution. A lower K_d value signifies a more stable complex as it indicates that the complex ion is less likely to dissociate.

Chemically, the dissociation constant is defined for the equilibrium of a complex ion breaking down into its components. The general form of the equilibrium reaction for a complex ion, AB, dissociating into A and B, can be represented as:
AB \rightleftharpoons A⁺ + B^-
And the dissociation constant for this reaction is given by:
K_d = \( \frac{[A^{+}][B^{-}]}{[AB]} \)

It is inversely related to the formation constant, meaning that a complex with a high formation constant will have a low dissociation constant, indicating it is largely present as a complex in solution.

Complex Ion Equilibrium

At the heart of complex ion chemistry lies the concept of equilibrium between the complex ion and its individual ions. Complex ion equilibrium involves the reversible reaction where a central metal ion binds with several ligands to form a complex ion. This kind of equilibrium can be described both in terms of the dissociation constant K_d and the formation constant K_f.

For example, in the equilibrium reaction where \( \mathrm{M} \rightleftharpoons \mathrm{M}^{x+}+y\mathrm{L}^{-} \), M represents the central metal ion, L represents the ligands, and the charges x and y indicate the ion charges.
The equilibrium constant for such a reaction represents the ratio of the product concentrations to reactant concentrations at equilibrium. The constant helps predict how much of a complex is formed under certain conditions and is essential for understanding many biochemical, environmental, and industrial processes where complex formation is involved.

Calculation of Equilibrium Constants

Equilibrium constants are central to predicting the behavior of a system at equilibrium. To calculate the formation constant K_f from the dissociation constant K_d, one must understand their inverse relationship. Given the dissociation constant, the formation constant can be found simply by taking its reciprocal. The mathematical process involves flipping the expression of the K_d and, consequently, inverting its numerical value.

In practical terms, if the dissociation of a complex is represented by the equilibrium equation:
\( \mathrm{Complex} \rightleftharpoons \mathrm{Metal}^{n+} + x\mathrm{Ligand}^{-} \),
with the dissociation constant,
\(K_d = \frac{[\mathrm{Metal}^{n+}][\mathrm{Ligand}]^{x}}{[\mathrm{Complex}]}\),
then the formation constant is simply
\( K_f = \frac{1}{K_d} \).

The calculation of K_f provides valuable insight into the stability of a complex in solution. A high K_f suggests a strong tendency to form the complex, making it an essential parameter in areas such as metal extraction, drug design, and the treatment of metal poisonings.