Glossary

5.1 Solubility product constant (Ksp) and molar solubility

3 min read•july 22, 2024

The () is a key concept in understanding how ionic compounds dissolve in water. It helps us predict and compare the solubility of different salts, which is crucial in many chemical processes and environmental systems.

Ksp calculations allow us to determine and compare relative solubilities of compounds. pH can greatly affect salt solubility, with some salts becoming more soluble in acidic conditions and others in basic conditions.

Solubility Product Constant (Ksp) and Molar Solubility

Definition of solubility product constant

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  • Quantitative measure of the equilibrium between a sparingly soluble and its dissolved ions in an aqueous solution
  • Calculated as the product of the concentrations of the dissolved ions raised to the power of their stoichiometric coefficients in the balanced chemical equation
  • Represents the maximum concentration of ions that can be present in a at a given temperature
  • Denoted by the symbol Ksp and expressed without units (e.g., Ksp for is 1.8×10−101.8 \times 10^{-10})
  • Smaller Ksp values indicate lower solubility while larger Ksp values indicate higher solubility of the compound (e.g., has a lower solubility than CuCl)

Calculations with Ksp

  • Calculate Ksp from by writing the balanced dissolution reaction, expressing ion concentrations in terms of molar solubility, and substituting into the
    • For example, for CaF2CaF_2: CaF2⇌Ca2++2F−CaF_2 \rightleftharpoons Ca^{2+} + 2F^-, if molar solubility is s, then Ksp=[Ca2+][F−]2=s(2s)2=4s3Ksp = [Ca^{2+}][F^-]^2 = s(2s)^2 = 4s^3
  • Determine molar solubility from Ksp by writing the balanced dissolution reaction, expressing Ksp in terms of molar solubility, and solving for molar solubility
    • For example, for Ag2CrO4Ag_2CrO_4: Ag2CrO4⇌2Ag++CrO42−Ag_2CrO_4 \rightleftharpoons 2Ag^+ + CrO_4^{2-}, if Ksp is known, then Ksp=[Ag+]2[CrO42−]=(2s)2(s)=4s3Ksp = [Ag^+]^2[CrO_4^{2-}] = (2s)^2(s) = 4s^3, solve for s

Relative solubility from Ksp

  • Compare solubilities of compounds based on their Ksp values at a given temperature
    • Compounds with similar Ksp values have similar solubilities (e.g., AgClAgCl and PbCl2PbCl_2)
    • Solubility decreases as Ksp value decreases (e.g., BaSO4BaSO_4 is less soluble than CaSO4CaSO_4)
    • Solubility increases as Ksp value increases (e.g., CuClCuCl is more soluble than AgClAgCl)
  • Rank compounds in order of increasing or decreasing solubility based on their Ksp values

pH effects on salt solubility

  • Solubility of some salts affected by solution pH due to presence of ions that are conjugate bases of weak acids or conjugate acids of weak bases
  • Salts with anions that are conjugate bases of weak acids (e.g., CO32−,PO43−CO_3^{2-}, PO_4^{3-}) exhibit decreased solubility as pH increases (more basic) and increased solubility as pH decreases (more acidic)
    • For example, solubility of CaCO3CaCO_3 decreases in basic solutions
  • Salts with cations that are conjugate acids of weak bases (e.g., NH4+NH_4^+) show increased solubility as pH increases (more basic) and decreased solubility as pH decreases (more acidic)
    • For example, solubility of AgClAgCl increases in the presence of NH3NH_3
  • explains pH effects on solubility: adding or removing ions (by changing pH) shifts equilibrium to counteract the change and restore balance

Key Terms to Review (25)

AgCl: AgCl, or silver chloride, is a white crystalline solid that is sparingly soluble in water and is primarily known for its role in the formation of precipitates in various chemical reactions. This compound is particularly significant in the context of solubility product constants and molar solubility, as it helps illustrate the principles governing the solubility of ionic compounds in aqueous solutions.
CaF2: Calcium fluoride (CaF2) is an inorganic compound that consists of calcium and fluorine ions. It is commonly known for its role in the context of solubility product constant (Ksp) and molar solubility, as it serves as a classic example of a sparingly soluble ionic compound in water. The understanding of CaF2 helps in grasping the concept of equilibrium in solutions and the calculations involving Ksp values.
Calcium Fluoride: Calcium fluoride is an ionic compound with the formula CaF₂, formed from calcium ions (Ca²⁺) and fluoride ions (F⁻). It is a sparingly soluble salt that plays a significant role in the understanding of solubility and equilibrium in chemical solutions, particularly in determining the solubility product constant (Ksp) and molar solubility.
Common ion effect: The common ion effect refers to the reduction in the solubility of an ionic compound when a solution already contains one of the ions present in that compound. This phenomenon occurs due to Le Chatelier's principle, where the addition of a common ion shifts the equilibrium position, favoring the formation of solid precipitate rather than keeping the ionic compound dissolved in solution. It is significant in understanding precipitation reactions, calculating solubility product constants, and determining equilibrium concentrations.
Dynamic Equilibrium: Dynamic equilibrium is a state in a reversible reaction where the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products over time. This concept is crucial for understanding how systems respond to changes in conditions and is essential for grasping precipitation reactions, solubility product constants, and the influence of external factors on chemical systems.
Enthalpy of solution: The enthalpy of solution is the heat change that occurs when a solute dissolves in a solvent, which can be either exothermic or endothermic. This value provides insight into the energy dynamics of the dissolution process and is essential for understanding how solubility interacts with temperature and concentration, especially when calculating the solubility product constant (Ksp) and determining molar solubility.
Equilibrium concentration: Equilibrium concentration refers to the concentrations of reactants and products in a chemical reaction when the rates of the forward and reverse reactions are equal, resulting in no net change in their amounts over time. This state is critical for understanding how substances behave under specific conditions, particularly when discussing solubility and equilibrium systems. It allows chemists to predict how changes in conditions can shift the balance of reactions, such as when a system reaches solubility limits or calculating how much of a substance will dissolve.
Gibbs Free Energy: Gibbs Free Energy is a thermodynamic potential that measures the maximum reversible work obtainable from a closed system at constant temperature and pressure. It connects the concepts of enthalpy, entropy, and temperature to determine whether a process is spontaneous or non-spontaneous, making it essential for understanding chemical reactions, equilibrium, and thermodynamic stability.
Ionic compound: An ionic compound is a chemical compound composed of ions held together by electrostatic forces known as ionic bonds. These compounds typically form between metals and nonmetals, where metals lose electrons to become positively charged cations and nonmetals gain those electrons to become negatively charged anions. The resulting ionic bonds create a stable structure, which plays a crucial role in solubility and reactions in aqueous solutions.
Ksp: Ksp, or the solubility product constant, is an equilibrium constant that quantifies the solubility of a sparingly soluble ionic compound in water. It represents the product of the molar concentrations of the ions involved in the dissolution process, each raised to the power of their respective stoichiometric coefficients. The Ksp value helps predict whether a solution will be saturated, unsaturated, or supersaturated with respect to a particular salt.
Ksp expression: The Ksp expression, or solubility product expression, is a mathematical representation of the equilibrium between a solid and its dissolved ions in a saturated solution. This expression is crucial for understanding how the solubility of a compound is influenced by various factors, including the presence of common ions and the conditions of precipitation reactions. The Ksp value quantifies the solubility of ionic compounds, allowing for predictions about whether a precipitate will form when two solutions are mixed.
Le Chatelier's Principle: Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change and re-establish equilibrium. This principle is crucial for understanding how changes in concentration, temperature, and pressure affect chemical systems and their equilibria.
Molar solubility: Molar solubility is the number of moles of a solute that can dissolve in one liter of solution at a given temperature, expressed in units of moles per liter (mol/L). This concept is crucial for understanding how solutes dissolve in solvents and is directly related to the solubility product constant (Ksp), which quantifies the equilibrium between solid and dissolved species in a saturated solution.
Molar solubility: Molar solubility is the number of moles of a solute that can dissolve in one liter of solution at equilibrium. This concept is crucial when dealing with sparingly soluble salts and connects directly to the solubility product constant (Ksp), which quantifies the extent to which a compound can dissociate into its constituent ions in a saturated solution. Understanding molar solubility allows for predictions about precipitation and helps in calculating the concentrations of ions in solution.
PH effect on solubility: The pH effect on solubility refers to how the acidity or alkalinity of a solution can influence the solubility of certain compounds, particularly salts and metal hydroxides. Changes in pH can alter the ionization of solutes, affecting the concentration of ions in solution, which in turn impacts the solubility product constant (Ksp) and molar solubility of these compounds. Understanding this relationship is crucial for predicting how solubility will vary under different conditions.
Pressure Effect: The pressure effect refers to the influence of external pressure on the solubility of a substance in a solvent, especially for gases. Increased pressure typically enhances the solubility of gases in liquids, which is important when considering precipitation reactions and the solubility product constant, as it can shift equilibrium conditions and impact the concentration of ions present in a solution.
S = molar solubility: Molar solubility, represented by 's', is the maximum amount of a solute that can dissolve in a given volume of solvent at equilibrium, typically expressed in moles per liter (mol/L). This concept is crucial in understanding how substances interact in a solution and how their solubility can be quantified through the solubility product constant (Ksp). Knowing the molar solubility helps predict how much of a substance can dissolve and informs decisions in various chemical processes.
Saturated solution: A saturated solution is a solution that contains the maximum amount of solute that can be dissolved in a solvent at a given temperature and pressure. In this state, any additional solute will not dissolve and may remain undissolved at the bottom of the container. Understanding saturated solutions is crucial for grasping concepts like solubility product constants and molar solubility, as these parameters describe the equilibrium between solute and solvent in such solutions.
Saturation Concentration: Saturation concentration refers to the maximum amount of solute that can dissolve in a solvent at a given temperature and pressure, resulting in a saturated solution. This concept is crucial when understanding solubility, as it establishes the point at which the solution can no longer accommodate additional solute. When the saturation concentration is reached, any further addition of solute will not dissolve but will remain as a solid.
Silver chloride: Silver chloride is a chemical compound with the formula AgCl, formed when silver ions react with chloride ions. This compound is known for its low solubility in water, which is significant in understanding precipitation reactions and the solubility product constant.
Solubility in acidic solutions: Solubility in acidic solutions refers to the increased ability of certain salts and compounds to dissolve when the pH of the solution is lowered, typically due to the presence of hydrogen ions (H+). This phenomenon often occurs because the acidic environment can shift equilibrium reactions, particularly for salts containing basic anions, thus enhancing their solubility. Understanding this concept is crucial when analyzing how different compounds behave in varying pH levels and how this affects the solubility product constant and molar solubility.
Solubility Product Constant: The solubility product constant, or Ksp, is an equilibrium constant that measures the extent to which a sparingly soluble ionic compound can dissolve in water. It provides a quantitative way to express the solubility of salts and the relationships between the concentrations of the ions produced in a saturated solution. Ksp is crucial for understanding how changes in conditions, like temperature or pressure, can affect solubility and precipitation reactions.
Supersaturated solution: A supersaturated solution is a state of a solution that contains more solute than it can theoretically hold at a given temperature and pressure, often achieved by dissolving the solute at an elevated temperature and then slowly cooling the solution. This unique state is unstable, meaning that any disturbance can cause the excess solute to crystallize out, allowing for further insights into solubility dynamics and the interactions between solutes and solvents.
Temperature effect: The temperature effect refers to how changes in temperature can influence the solubility of ionic compounds in a solution and the equilibrium position of precipitation reactions. When the temperature rises, the kinetic energy of the particles increases, which can lead to higher solubility for many solids and affect the extent of precipitation reactions, shifting equilibrium states based on Le Chatelier's principle.
Unsaturated solution: An unsaturated solution is a solution that contains less solute than can be dissolved at a given temperature and pressure, meaning it has the capacity to dissolve more solute. This type of solution is significant as it allows for the potential to increase solute concentration without altering the temperature or pressure. Understanding unsaturated solutions is crucial in determining the maximum solubility of substances and in calculating the solubility product constant (Ksp).
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