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15.1 Precipitation and Dissolution

3 min read•june 25, 2024

and are key concepts in chemistry. They explain how substances dissolve and form solid precipitates in solutions. Understanding these processes helps predict chemical reactions and their outcomes in various settings.

Calculations using solubility product constants allow us to determine solubilities and ion concentrations. By comparing reaction quotients to values, we can predict when precipitation will occur. Factors like common ions and solution conditions affect solubility in important ways.

Solubility Equilibria and Precipitation

Chemical equations for solubility equilibria

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  • Solubility equilibrium occurs when the rate of of a solid equals the rate of precipitation in a
  • Involves the dissociation of a slightly soluble ionic compound (e.g., \ce[AgCl](https://www.fiveableKeyTerm:AgCl)\ce{[AgCl](https://www.fiveableKeyTerm:AgCl)}) into its constituent ions (e.g., \ce[Ag+](https://www.fiveableKeyTerm:Ag+)\ce{[Ag+](https://www.fiveableKeyTerm:Ag+)} and \ce[Cl−](https://www.fiveableKeyTerm:Cl−)\ce{[Cl-](https://www.fiveableKeyTerm:Cl-)}) in a saturated solution
  • General form of a solubility equilibrium equation: \ceAxBy(s)<=>xAy+(aq)+yBx−(aq)\ce{A_xB_y(s) <=> xA^{y+}(aq) + yB^{x-}(aq)}
    • \ceAxBy(s)\ce{A_xB_y(s)} represents the slightly soluble ionic compound (e.g., \ce[Ca3(PO4)2](https://www.fiveableKeyTerm:Ca3(PO4)2)\ce{[Ca3(PO4)2](https://www.fiveableKeyTerm:Ca3(PO4)2)})
    • \ceAy+(aq)\ce{A^{y+}(aq)} and \ceBx−(aq)\ce{B^{x-}(aq)} represent the dissociated ions in the saturated solution (e.g., \ceCa2+\ce{Ca^2+} and \ce[PO43−](https://www.fiveableKeyTerm:PO43−)\ce{[PO4^3-](https://www.fiveableKeyTerm:PO4^3-)})
  • (KspK_{sp}) is the equilibrium constant for a solubility equilibrium
    • General expression: Ksp=[\ceAy+]x[\ceBx−]yK_{sp} = [\ce{A^{y+}}]^x[\ce{B^{x-}}]^y
      • [\ceAy+][\ce{A^{y+}}] and [\ceBx−][\ce{B^{x-}}] are the molar concentrations of the dissociated ions at equilibrium
      • xx and yy are the stoichiometric coefficients of the ions in the balanced chemical equation
  • help predict whether a compound will dissolve or in solutions

Calculations with solubility product constants

  • Solubility is the maximum amount of a that can dissolve in a given amount of at a specific temperature
    • is solubility expressed in moles of per liter of solution ( or )
  • Calculating solubility from KspK_{sp} involves the following steps:
    1. Write the balanced solubility equilibrium equation
    2. Set up the KspK_{sp} expression using the equilibrium concentrations of the ions
    3. Substitute the molar solubility (s) for the concentration of each ion, considering their stoichiometric coefficients
    4. Solve the resulting equation for s
  • Calculating ion concentrations from solubility is done by multiplying the molar solubility by the stoichiometric coefficient of each ion in the balanced chemical equation
    • Example: For \ceAgCl\ce{AgCl}, if the molar solubility is ss, then [\ceAg+]=[\ceCl−]=s[\ce{Ag+}] = [\ce{Cl-}] = s

Prediction of precipitation reactions

  • occurs when an insoluble solid () forms upon mixing two solutions containing soluble compounds
    • Example: Mixing \ce[AgNO3](https://www.fiveableKeyTerm:AgNO3)\ce{[AgNO3](https://www.fiveableKeyTerm:AgNO3)} and \ce[NaCl](https://www.fiveableKeyTerm:NaCl)\ce{[NaCl](https://www.fiveableKeyTerm:NaCl)} solutions forms a white \ceAgCl\ce{AgCl} precipitate
  • ([Q](https://www.fiveableKeyTerm:Q)[Q](https://www.fiveableKeyTerm:Q)) is the ratio of the product of the concentrations of the products raised to their stoichiometric coefficients divided by the product of the concentrations of the reactants raised to their stoichiometric coefficients
    • For a solubility equilibrium: Q=[\ceAy+]x[\ceBx−]yQ = [\ce{A^{y+}}]^x[\ce{B^{x-}}]^y
  • Predicting precipitation involves comparing QQ to KspK_{sp}:
    1. If Q<KspQ < K_{sp}, the solution is unsaturated, and no precipitation will occur
    2. If Q=KspQ = K_{sp}, the solution is saturated, and no net precipitation will occur
    3. If Q>KspQ > K_{sp}, the solution is supersaturated, and precipitation will occur until Q=KspQ = K_{sp}
  • occurs when the presence of a common ion (an ion that appears in the solubility equilibrium expression) decreases the solubility of the slightly soluble compound
    • Example: Adding \ceNaCl\ce{NaCl} to a saturated \ceAgCl\ce{AgCl} solution will decrease the solubility of \ceAgCl\ce{AgCl} due to the increased \ceCl−\ce{Cl-} concentration
  • can be applied to understand how changes in concentration, temperature, or pressure affect solubility equilibria

Factors affecting solubility

  • of a solution influences the solubility of ionic compounds
  • accounts for the non-ideal behavior of ions in solution, especially at higher concentrations
  • is a technique used to selectively precipitate ions from a mixture based on their different solubilities

Key Terms to Review (39)

Activity Coefficient: The activity coefficient is a measure of the deviation of a substance's behavior from ideal behavior in a solution or mixture. It is a dimensionless quantity that describes the non-ideal interactions between the solute and the solvent, affecting the chemical potential and equilibrium of the system.
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.
AgCl: AgCl, or silver chloride, is a chemical compound composed of one silver (Ag) atom and one chlorine (Cl) atom. It is an important compound in the context of both writing and balancing chemical equations as well as precipitation and dissolution reactions.
AgNO3: AgNO3, or silver nitrate, is an inorganic compound with the chemical formula AgNO3. It is a white, crystalline solid that is soluble in water and is commonly used in various chemical applications, including precipitation reactions and as a precursor for other silver compounds.
Aqueous: The term 'aqueous' refers to a solution or mixture in which water is the solvent. It describes a system where substances are dissolved or suspended in water, creating a homogeneous liquid phase. This term is particularly relevant in the context of precipitation and dissolution reactions, as the aqueous environment plays a crucial role in the formation and solubility of various compounds.
Aqueous solution: An aqueous solution is a solution in which water is the solvent. Commonly used in chemistry to describe reactions occurring in water.
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.
Ca2+: Ca2+ is the calcium ion, a positively charged ion with a charge of +2. It is an essential mineral that plays crucial roles in various biological processes, including ionic bonding and precipitation and dissolution reactions.
Ca3(PO4)2: Ca3(PO4)2, commonly known as calcium phosphate, is a chemical compound that consists of calcium and phosphate ions. This compound plays a crucial role in biological systems, particularly in the formation of bones and teeth, and is also significant in various industrial applications, particularly in fertilizers and food additives.
Cl-: Cl- is the chemical symbol for the chloride ion, which is a negatively charged ion consisting of a chlorine atom that has gained an extra electron. This ion is an important component in various chemical processes and contexts, including writing and balancing chemical equations, ionic bonding, electrolytes, and precipitation and dissolution.
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.
Dissolution: Dissolution is the process by which a solute (a substance that is dissolved) disperses and incorporates into a solvent (the dissolving medium) to form a homogeneous solution. This term is central to understanding the fundamental concepts of solubility and precipitation in chemistry.
Fractional Precipitation: Fractional precipitation is a technique used in analytical chemistry to separate and purify different ions or compounds from a solution by taking advantage of their varying solubilities. It involves the selective precipitation of certain species from a solution, allowing for the separation and isolation of individual components.
Ionic Strength: Ionic strength is a measure of the concentration of ions in a solution and their ability to influence the behavior of other dissolved species. It is a crucial parameter in understanding various chemical processes, including buffer systems and precipitation reactions.
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.
M: M is a widely used term in chemistry that represents various important concepts, including molarity, stoichiometry, effusion and diffusion, rate laws, and precipitation and dissolution. This versatile term is crucial for understanding and applying fundamental chemical principles across multiple topics in the field of chemistry.
Mol/L: Mol/L, also known as molarity, is a measure of the concentration of a substance in a solution. It represents the number of moles of a solute dissolved in one liter of the solution. Molarity is a fundamental concept in chemistry that is essential for understanding chemical reactions, equilibrium, and precipitation processes.
Molar Solubility: Molar solubility is a measure of the maximum amount of a substance that can dissolve in a given volume of solvent at a specific temperature, expressed in moles per liter. It is a fundamental concept in understanding precipitation and dissolution processes in chemistry.
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.
PO4^3-: PO4^3- is the phosphate ion, a polyatomic ion consisting of one phosphorus atom covalently bonded to four oxygen atoms. It is a key component in many chemical and biological processes, particularly in the context of polyprotic acids and precipitation reactions.
Precipitate: A precipitate is a solid that forms and separates from a liquid mixture during a chemical reaction. It often occurs when two aqueous solutions react to form an insoluble product.
Precipitate: A precipitate is a solid material that forms and separates from a liquid solution during a chemical reaction or a change in solution conditions. It is the result of a chemical process where dissolved substances come out of the solution and form a solid phase.
Precipitation: Precipitation is the process by which a solid or liquid substance separates from a solution or suspension and forms a distinct phase. This phenomenon is commonly observed in the context of chemical reactions and the solubility of substances in various media.
Precipitation reaction: A precipitation reaction occurs when two aqueous solutions react to form an insoluble solid, known as the precipitate. This type of reaction is a double displacement reaction where the ions in solution exchange partners.
Q: Q, also known as the equilibrium constant, is a measure of the relative concentrations of reactants and products at equilibrium in a chemical reaction. It is a fundamental concept in understanding the behavior of chemical systems and is essential in the study of equilibrium constants, equilibrium calculations, and precipitation and dissolution processes.
Reaction Quotient: The reaction quotient, denoted as Q, is a measure of the relative concentrations of the products and reactants in a chemical reaction at any given time, regardless of whether the system has reached equilibrium or not. It is a useful tool for understanding the direction and extent of a reaction as it progresses towards equilibrium.
Reaction quotient (Q): The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction mixture at any given point in time. It is calculated using the same expression as the equilibrium constant but with current concentrations or partial pressures.
Saturated Solution: A saturated solution is a solution in which the maximum amount of a substance has been dissolved at a given temperature. The solution is in equilibrium with undissolved particles of the solute, and no more solute can be dissolved without the precipitation of the excess.
Selective precipitation: Selective precipitation is a technique in which specific ions are precipitated out of a solution by carefully choosing the precipitating agent and conditions to target only those ions.
Solubility Equilibria: Solubility equilibria refers to the dynamic balance that exists between the dissolved ions or molecules of a substance and the undissolved solid form of that substance in a saturated solution. This concept is central to understanding precipitation and dissolution processes in chemistry.
Solubility Product Constant: The solubility product constant, Ksp, is an equilibrium constant that quantifies the extent of dissolution of a slightly soluble ionic compound in water. It represents the product of the concentrations of the constituent ions raised to their respective stoichiometric coefficients at equilibrium.
Solubility Rules: Solubility rules are a set of guidelines that predict the solubility of ionic compounds in water. They help determine whether a salt will dissolve or precipitate when mixed with another solution.
Solute: A solute is a substance that is dissolved in a solvent to form a solution. It can be in any phase: solid, liquid, or gas.
Solute: A solute is the substance that is dissolved in a solution, typically in smaller quantities compared to the solvent. It is the component of a solution that is present in a lesser amount and is dispersed throughout the solvent.
Solvent: A solvent is a substance, typically a liquid, that dissolves a solute to form a solution. The solvent is usually present in greater amount compared to the solute.
Solvent: A solvent is a liquid, solid, or gas that can dissolve other substances, known as solutes, to form a solution. Solvents play a crucial role in various chemical processes, including chemical reactions, extraction, and purification.
Supersaturated Solution: A supersaturated solution is a solution that contains a higher concentration of dissolved solute than the normal saturation point of the solvent at a given temperature. This metastable state is characterized by the presence of excess solute that remains dissolved despite being above the normal saturation level.
Unsaturated Solution: An unsaturated solution is a solution in which the solute concentration is less than the maximum amount that can be dissolved in the solvent at a given temperature and pressure. This term is particularly relevant in the context of precipitation and dissolution, as it describes a solution that has the capacity to dissolve additional solute.
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Glossary
Glossary
15.2 Lewis Acids and Bases