14.4 Hydrolysis of Salts
3 min read•june 25, 2024
is a crucial concept in acid-base chemistry. It explains why some salt solutions aren't neutral, despite being made from acids and bases. The of these solutions depends on the strengths of the acid and base that formed the salt.
Hydrolysis reactions involve water splitting ions from salts. This process can create acidic or basic solutions, depending on the salt's components. Understanding salt hydrolysis helps predict and control pH in various chemical and biological systems.
Hydrolysis of Salts
pH prediction for salt solutions
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- Salts formed from the neutralization of a strong acid and a strong base yield neutral solutions with a pH of 7 (, , )
- Salts formed from the neutralization of a weak acid and a strong base produce basic solutions with a pH greater than 7 due to the anion hydrolyzing and generating ions ( )
- Salts formed from the neutralization of a strong acid and a weak base result in acidic solutions with a pH less than 7 because the cation hydrolyzes and releases ions ( )
- Salts formed from the neutralization of a weak acid and a weak base have a pH that depends on the relative strengths of the acid and base components
- If the acid dissociation constant () is greater than the base dissociation constant (), the solution is acidic with a pH below 7
- If Kb exceeds Ka, the solution is basic with a pH above 7
- If Ka equals Kb, the solution is neutral with a pH of 7
- The concept of is crucial in understanding the behavior of these salt solutions
Ion concentrations in hydrolyzed salts
- For salts of weak acids and strong bases (CH3COONa), follow these steps:
- Write the balanced equation for the hydrolysis reaction: + ⇌ + OH-
- Set up an to track the Initial, Change, and Equilibrium concentrations
- Use the base dissociation constant (Kb) to calculate the concentration of ions:
- For salts of strong acids and weak bases (NH4Cl), follow a similar procedure:
- Write the balanced equation for the hydrolysis reaction: + H2O ⇌ +
- Set up an ICE table
- Use the acid dissociation constant (Ka) to calculate the concentration of ions:
- The can influence the hydrolysis of salts in solution
Hydrated metal ions as acids
- Hydrated metal ions, such as , behave as acids in aqueous solutions by polarizing the O-H bond in the water molecules, making the hydrogen atoms more acidic and prone to dissociation
- The hydrated metal ion donates a proton (H+) to the solution, increasing the H+ ion concentration and decreasing the pH
- The acidity of hydrated metal ions is influenced by the charge and size of the metal ion, with higher charge and smaller size resulting in greater acidity
- Example: Al(H2O)63+ + H2O ⇌ + H3O+ demonstrates the hydrolysis of the hydrated aluminum ion producing H3O+, which makes the solution acidic
- This process can be understood using the
Equilibrium and pH Control
- helps predict how the hydrolysis equilibrium will shift in response to changes in concentration, temperature, or pressure
- can be created using certain combinations of weak acids or bases and their salts to resist pH changes
- Understanding these concepts is essential for controlling and predicting pH in various chemical and biological systems
Key Terms to Review (30)
Acid anhydrides: Acid anhydrides are chemical compounds derived from acids by the removal of water. They typically react with water to form corresponding acids.
Al(H2O)5OH2+: Al(H2O)5OH2+ is a complex ion that forms when aluminum (Al3+) ions interact with water molecules. This complex ion is an important concept in the context of hydrolysis of salts, as it demonstrates how the presence of water can affect the behavior of ionic species in aqueous solutions.
Al(H2O)63+: Al(H2O)63+ is the hexaaquaaluminum(III) ion, a complex ion formed when aluminum (Al3+) ions are surrounded by six water molecules in an octahedral arrangement. This complex ion is an important species in the context of hydrolysis of salts.
Ammonium Chloride: Ammonium chloride is a chemical compound with the formula NH4Cl. It is a colorless crystalline salt that is highly soluble in water and is commonly used in various industrial and household applications.
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.
CH3COO-: CH3COO- is the acetate ion, which is the conjugate base of acetic acid. It plays a significant role in various chemical reactions, particularly as an electrolyte in solution and in the hydrolysis of salts formed from weak acids and strong bases. Understanding acetate’s behavior helps explain its interactions in biochemical processes and its applications in various fields.
CH3COOH: CH3COOH, also known as acetic acid, is a weak organic acid that plays a crucial role in the context of electrolytes and the hydrolysis of salts. It is a common compound found in various chemical and biological processes.
CH3COONa: CH3COONa, also known as sodium acetate, is a chemical compound that is formed when the weak acid acetic acid (CH3COOH) reacts with the strong base sodium hydroxide (NaOH). It is a common salt that is widely used in various applications, including in the context of the hydrolysis of salts.
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.
Conjugate Acid-Base Pairs: Conjugate acid-base pairs are related chemical species that differ by the presence or absence of a single proton (H+). When an acid donates a proton, it becomes a conjugate base, and when a base accepts a proton, it becomes a conjugate acid. These pairs are fundamental to understanding the Brønsted-Lowry theory of acids and bases, as well as the concepts of pH, relative acid-base strengths, hydrolysis, polyprotic acids, and acid-base titrations.
H+: H+ is the chemical symbol for the hydrogen ion, which is a proton that has been separated from a hydrogen atom. This hydrogen ion plays a crucial role in various chemical processes, including the topics of hydrolysis of salts, buffers, and acid-base titrations.
H2O: H2O, or water, is a chemical compound consisting of two hydrogen atoms covalently bonded to a single oxygen atom. It is a colorless, odorless, and tasteless liquid that is essential for life on Earth, playing crucial roles in various chemical and physical processes across multiple topics in chemistry.
H3O+: H3O+ is the hydronium ion, a positively charged species formed when a proton (H+) interacts with a water molecule (H2O). It is a crucial concept in understanding the behavior of acids, bases, and the pH of aqueous solutions.
Hydrolysis: Hydrolysis is a chemical reaction where a compound is broken down by the addition of water, resulting in the formation of new products. This process is fundamental in various chemical and biological contexts, including the digestion of food, the breakdown of salts, and the interconversion of organic compounds.
ICE Table: An ICE table, also known as an Initial, Change, and Equilibrium table, is a tool used in chemistry to organize and analyze the concentrations of reactants and products in a chemical equilibrium system. It provides a structured way to visualize and calculate the changes in concentrations as a reaction approaches equilibrium.
K2SO4: K2SO4, or potassium sulfate, is a chemical compound consisting of two potassium atoms and one sulfur atom bonded to four oxygen atoms. It is an ionic salt that is commonly used in various industrial and agricultural applications due to its unique properties.
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.
KNO3: KNO3, also known as potassium nitrate, is an inorganic compound that plays a significant role in the context of hydrolysis of salts. It is a crystalline salt that is widely used in various applications, including as an oxidizing agent, a food preservative, and in the production of pyrotechnics and fertilizers.
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.
Lewis Acid-Base Theory: The Lewis acid-base theory is a model that defines acids and bases based on the ability to accept or donate electron pairs, rather than the traditional Arrhenius definition of acids and bases. It provides a more general framework for understanding chemical reactions involving the transfer of electron pairs.
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.
NH4+: NH4+ is the ammonium ion, a positively charged species composed of one nitrogen atom and four hydrogen atoms. It is an important ion in the context of molecular structure, polarity, and the hydrolysis of salts.
NH4Cl: NH4Cl, or ammonium chloride, is a chemical compound composed of the ammonium cation (NH4+) and the chloride anion (Cl-). It is a white crystalline salt that is commonly used in various applications, including as a component in the hydrolysis of salts.
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.
PH: pH, or potential of hydrogen, is a measure of the acidity or basicity of a solution. It is a scale that ranges from 0 to 14, with 7 being neutral, values less than 7 being acidic, and values greater than 7 being basic or alkaline. The pH of a solution is directly related to the concentration of hydrogen ions (H+) present, and it is a critical factor in many chemical and biological processes.
Salt: Salt, scientifically known as sodium chloride (NaCl), is an ionic compound composed of sodium (Na+) and chloride (Cl-) ions. It is an essential mineral that plays a crucial role in various physiological processes within the human body, and it is also a widely used seasoning and preservative in food preparation.
Sodium Acetate: Sodium acetate is a chemical compound with the formula CH3COONa. It is a salt formed by the neutralization of acetic acid (CH3COOH) with sodium hydroxide (NaOH). Sodium acetate is commonly used in various applications, including as a food preservative, a buffer solution, and a deicing agent.
In the context of the chapter on 14.4 Hydrolysis of Salts, sodium acetate is an important example of a salt that undergoes hydrolysis, a chemical reaction where a salt reacts with water to produce an acidic or basic solution.