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15.3 Coupled Equilibria

2 min read•june 25, 2024

Solubility and explore how interconnected chemical reactions affect each other's balance. These concepts help explain why adding salt to water changes how much will dissolve, or why some metals dissolve better in certain solutions.

Understanding these ideas is key to predicting chemical behavior in real-world situations. From explaining why tap water leaves mineral deposits to optimizing industrial processes, mastering solubility and coupled equilibria unlocks practical chemistry knowledge.

Solubility and Coupled Equilibria

Effects of coupled equilibria

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  • Involve two or more interconnected chemical reactions that share a common species and influence each other's equilibrium state
  • Can affect solubility of ionic compounds through where solubility decreases when a common ion is added to the solution ([NaCl](https://www.fiveableKeyTerm:NaCl)\text{[NaCl](https://www.fiveableKeyTerm:NaCl)} and [HCl](https://www.fiveableKeyTerm:HCl)\text{[HCl](https://www.fiveableKeyTerm:HCl)})
  • Formation of complex ions can increase solubility by removing free metal ions from solution and shifting equilibrium towards dissolved state (Ag+\text{Ag}^+ and NH3\text{NH}_3 forming Ag(NH3)2+\text{Ag}(\text{NH}_3)_2^+)
  • explains how systems at equilibrium respond to changes in conditions

Calculations for interconnected equilibria

  • Write balanced chemical equations for each equilibrium reaction
  • Identify common species shared between reactions
  • Set up expressions (KspK_{\text{sp}}, KaK_{\text{a}}, KbK_{\text{b}}) for each reaction using equilibrium constant values and initial concentrations
  • Solve system of equations simultaneously to determine concentrations of all species at equilibrium using techniques like substitution, elimination, or mathematical software
  • Use the () to calculate the solubility of sparingly soluble salts

pH impact on solubility

  • Affects solubility of ionic compounds containing acidic or basic anions
    • Acidic anions (CO32−\text{CO}_3^{2-}, PO43−\text{PO}_4^{3-}) become protonated at low pH increasing solubility
    • Basic anions (S2−\text{S}^{2-}, OH−\text{OH}^-) become deprotonated at high pH increasing solubility
  • Changes pH can also affect formation of complex ions
    • Ligands that are weak bases (NH3\text{NH}_3) are more effective at forming complex ions at higher pH values
  • Analyze impact by:
    1. Identifying acidic or basic anions in ionic compound
    2. Determining effect of pH on protonation state of anions
    3. Considering formation of complex ions and their stability at different pH values
    4. Predicting overall impact on solubility based on combined effects of protonation and complex ion formation
  • Equilibrium constant: Quantifies the extent of a reaction at equilibrium
  • : Process of forming complex ions, which can significantly affect solubility
  • : Resist changes in pH and can influence solubility of pH-sensitive compounds

Key Terms to Review (22)

Ag(NH3)2+: Ag(NH3)2+ is a complex ion formed by the coordination of a silver(I) ion (Ag+) with two ammonia (NH3) ligands. This complex ion is an important species in the context of coupled equilibria, as it is involved in various equilibrium reactions.
Ag+: Ag+ is the chemical symbol and ionic form of the silver element. It is a positively charged silver ion that plays a crucial role in various chemical processes and reactions, particularly in the context of writing and balancing chemical equations, precipitation and dissolution, Lewis acids and bases, and coupled equilibria.
Average rate: The average rate of a chemical reaction is the change in concentration of a reactant or product per unit time over a specific time interval. It gives an overall sense of how quickly the reaction proceeds during that period.
Base-ionization constant (Kb): The base-ionization constant (Kb) quantifies the strength of a base in a solution. It is the equilibrium constant for the dissociation of a base into its conjugate acid and hydroxide ion.
Buffer Solutions: Buffer solutions are aqueous solutions that resist changes in pH upon the addition of small amounts of an acid or base. They maintain a relatively stable pH within a specific range, even when other substances are added that would normally alter the pH of the solution.
CO32-: The carbonate ion (CO32-) is a polyatomic anion consisting of one carbon atom and three oxygen atoms. It is an important species in various chemical equilibria, particularly in the context of coupled equilibria.
Common Ion Effect: The common ion effect is a principle in chemistry that describes the influence of a common ion on the solubility of a salt or the position of a chemical equilibrium. It is a fundamental concept that underlies various equilibrium processes in chemistry, including equilibrium calculations, hydrolysis of salts, buffer solutions, precipitation and dissolution, and coupled equilibria.
Complexation: Complexation is the process in which a central metal ion or atom forms a coordinate covalent bond with surrounding ligands, creating a complex species. This concept is particularly relevant in the context of coupled equilibria, as complexation reactions can influence the equilibrium of other chemical processes.
Coupled Equilibria: Coupled equilibria refer to the interconnected nature of chemical equilibria, where the concentrations of reactants and products in one equilibrium system are dependent on the concentrations in another related equilibrium system. This concept is crucial in understanding the behavior of complex chemical systems and their interactions.
Equilibrium Constant: The equilibrium constant is a quantitative measure of the extent of a chemical reaction at equilibrium. It represents the ratio of the concentrations of the products to the reactants, raised to their respective stoichiometric coefficients, and is a fundamental concept in understanding the behavior of chemical systems at equilibrium.
Equilibrium constant, K: The equilibrium constant, $K$, is a ratio that quantifies the concentrations of reactants and products in a chemical reaction at equilibrium. It provides insight into the position of the equilibrium and the extent to which reactants are converted into products.
HCl: HCl, or hydrogen chloride, is a chemical compound consisting of one hydrogen atom and one chlorine atom. It is a colorless, corrosive gas that has a wide range of applications and plays a crucial role in various chemical processes and reactions.
Ka: Ka, or the acid dissociation constant, is a quantitative measure of the strength of an acid in a solution. It represents the equilibrium constant for the dissociation of an acid into its constituent ions, providing insight into the extent to which an acid ionizes in water.
Kb: Kb, or the base dissociation constant, is a measure of the strength of a base in aqueous solution. It quantifies the extent to which a base dissociates or ionizes in water, providing insight into the relative strengths of different bases.
Ksp: Ksp, or the solubility product constant, is a fundamental equilibrium constant that describes the extent of dissolution of a slightly soluble ionic compound in water. It quantifies the equilibrium between the dissolved ions and the solid compound, providing a measure of the solubility of the substance.
Le Chatelier's Principle: Le Chatelier's Principle states that when a system at equilibrium is subjected to a change in one of the conditions (concentration, temperature, or pressure) affecting that equilibrium, the system will shift to counteract the change and re-establish equilibrium.
NaCl: NaCl, commonly known as sodium chloride, is a chemical compound that consists of one sodium atom (Na) and one chlorine atom (Cl) bonded together. It is an ionic compound that is essential for various biological and industrial processes, and it is the primary component of table salt.
NH3: NH3, also known as ammonia, is a colorless, pungent-smelling gas that is composed of one nitrogen atom and three hydrogen atoms. This simple inorganic compound is an important chemical in various contexts, including molecular structure, acid-base chemistry, and the nitrogen cycle.
OH-: OH- is the hydroxide ion, a negatively charged particle composed of one oxygen atom and one hydrogen atom. This ion is a key component in understanding acid-base chemistry, as it plays a central role in the concepts of pH, pOH, relative strengths of acids and bases, hydrolysis of salts, buffers, acid-base titrations, Lewis acids and bases, and coupled equilibria.
PO43-: PO43- is the phosphate ion, a polyatomic ion with a negative three charge. It is an important chemical species involved in various equilibrium processes, particularly in the context of coupled equilibria.
S2-: S2- is the chemical notation for the sulfide ion, which consists of one sulfur atom with a -2 charge. This ion is formed when sulfur gains two electrons, resulting in its ability to participate in ionic bonding with various cations, leading to the formation of ionic compounds. The presence of S2- influences both the properties of these compounds and the equilibria that can exist in chemical reactions involving sulfides.
Solubility Product: The solubility product, denoted as $K_{sp}$, is an equilibrium constant that describes the solubility of a slightly soluble ionic compound in water. It represents the product of the concentrations of the ions in a saturated solution of the compound, each raised to the power of their stoichiometric coefficients.
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