14.5 Polyprotic Acids
3 min read•june 25, 2024
are fascinating molecules that can donate multiple protons. They ionize in steps, each with its own equilibrium constant. This stepwise process affects their behavior in solution and their role in acid-base chemistry.
Understanding polyprotic acids is key to grasping complex acid-base interactions. Their unique properties make them useful in various applications, from creating to analyzing with multiple equivalence points.
Polyprotic Acids
Stepwise ionization of polyprotic acids
Top images from around the web for Stepwise ionization of polyprotic acids
Polyprotic Acids (14.5) – Chemistry 110 View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
Polyprotic Acids (14.5) – Chemistry 110 View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
1 of 3
Top images from around the web for Stepwise ionization of polyprotic acids
Polyprotic Acids (14.5) – Chemistry 110 View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
Polyprotic Acids (14.5) – Chemistry 110 View original
Is this image relevant?
The intrinsic view of ionization equilibria of polyprotic molecules - New Journal of Chemistry ... View original
Is this image relevant?
1 of 3
- Polyprotic acids can donate multiple protons per molecule (, )
- involves dissociation in multiple steps, losing one proton at a time
- Each step has its own distinct equilibrium constant ()
- Sulfuric acid () ionization steps:
- (Ka1 = very large)
- (Ka2 = 1.2 × 10^-2)
- Phosphoric acid () ionization steps:
- (Ka1 = 7.5 × 10^-3)
- (Ka2 = 6.2 × 10^-8)
- (Ka3 = 4.8 × 10^-13)
- These reactions demonstrate in each step
Equilibrium calculations for polyprotic acids
- Calculate equilibrium concentrations using equilibrium constant expressions and
- Make simplifying assumptions based on relative magnitudes of Ka values
- If Ka1 >> Ka2, assume first dissociation step goes to completion before second step begins
- Allows calculation of [H+] and concentrations of various species in solution
- Example calculation for H2SO4 solution:
- Assume first dissociation step goes to completion due to large Ka1
- Calculate [HSO4-] based on initial concentration of H2SO4
- Use Ka2 expression to calculate [H+] and [SO4^2-]
- Example calculation for H3PO4 solution:
- Use Ka1 expression and initial concentration of H3PO4 to calculate [H+] and [H2PO4-]
- If needed, use Ka2 and Ka3 expressions to calculate concentrations of other species
- These calculations often involve the to express acidity
Relative strengths of successive ionizations
- In polyprotic acids, successive ionization steps become progressively weaker
- Harder to remove proton from negatively charged species than neutral molecule
- Ka1 > Ka2 > Ka3 > ... for a polyprotic acid
- Compare relative strengths of ionization steps using values
- pKa = -log(Ka); smaller pKa indicates stronger acid
- Sulfuric acid: pKa1 < 0, pKa2 = 1.92
- Phosphoric acid: pKa1 = 2.12, pKa2 = 7.21, pKa3 = 12.32
- Trend in pKa values shows each successive ionization step is weaker than previous one
- Similar trend observed in , where successive protonation steps become progressively weaker
- Kb1 > Kb2 > Kb3 > ... for a polyprotic base
Acid-Base Behavior and Applications
- Polyprotic acids play a crucial role in , explaining complex acid-base interactions
- Buffer solutions often involve polyprotic acids, utilizing their multiple dissociation steps
- Titration curves for polyprotic acids show multiple equivalence points, corresponding to each ionization step
Key Terms to Review (30)
Acid-Base Theory: Acid-base theory is a fundamental concept in chemistry that describes the behavior of acids and bases, their properties, and their interactions. It provides a framework for understanding the transfer of protons (H+ ions) in chemical reactions and the resulting changes in pH.
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.
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.
Diprotic acids: Diprotic acids are acids that can donate two protons (hydrogen ions) per molecule in a reaction. They undergo two dissociation steps, each with its own equilibrium constant.
Diprotic base: A diprotic base is a base that can accept two protons (H\(^+\)) per molecule. It typically undergoes two stages of protonation, each with its own equilibrium constant.
Dissociation Constants: Dissociation constants are equilibrium constants that quantify the extent of dissociation of a polyprotic acid into its constituent ions. They are essential in understanding the behavior and properties of polyprotic acids, which are acids that can donate multiple protons in aqueous solutions.
Equilibrium Calculations: Equilibrium calculations involve determining the concentrations of reactants and products at equilibrium in a chemical reaction. This concept is crucial when dealing with reactions that can proceed in both the forward and reverse directions, particularly in the context of polyprotic acids, which can donate more than one proton. Understanding these calculations helps predict how changes in conditions will affect the position of equilibrium, guiding insights into acid-base behavior.
H2PO4-: H2PO4- is the dihydrogen phosphate ion, a polyatomic ion with a charge of negative one. It is a key component in the chemical equilibrium of polyprotic acids, which are acids that can donate more than one proton.
H2SO4: H2SO4, or sulfuric acid, is a strong, corrosive mineral acid that plays a crucial role in both polyprotic acid chemistry and the occurrence, preparation, and compounds of oxygen. It is a colorless, odorless, and dense liquid that is widely used in various industrial and chemical processes.
H3PO4: H3PO4, also known as phosphoric acid, is a polyprotic acid that plays a significant role in various chemical processes, particularly in the context of polyprotic acids, the occurrence and properties of phosphorus, and the occurrence and compounds of oxygen.
Henderson-Hasselbalch equation: The Henderson-Hasselbalch equation is a mathematical relationship that describes the pH of a solution containing a weak acid or a weak base. It is a fundamental concept in understanding the relative strengths of acids and bases, as well as the behavior of polyprotic acids and buffers.
HPO4^2-: HPO4^2- is the dihydrogen phosphate ion, a polyatomic ion with a charge of -2. It is an important component in the context of polyprotic acids, which are acids that can donate more than one proton when dissociating in water.
HSO4-: HSO4- is the hydrogen sulfate ion, a polyatomic ion consisting of one hydrogen atom, one sulfur atom, and four oxygen atoms. It is an important species in the context of polyprotic acids, which are acids that can donate more than one proton.
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.
Mass Balance Equations: Mass balance equations are a fundamental tool in chemistry that describe the conservation of mass within a system. These equations are used to analyze and understand the flow of matter in various chemical processes, such as reactions, separations, and transport phenomena.
Monoprotic acids: Monoprotic acids are acids that can donate only one proton (hydrogen ion) per molecule in an aqueous solution. Examples include hydrochloric acid (HCl) and acetic acid (CH3COOH).
PH Scale: The pH scale is a measure of the acidity or basicity of a solution, ranging from 0 to 14. It is a logarithmic scale that quantifies the concentration of hydrogen ions (H+) in a solution, with lower values indicating higher acidity and higher values indicating higher basicity or alkalinity. The pH scale is a fundamental concept in understanding Brønsted-Lowry acids and bases, as well as the relationship between pH and pOH, and the behavior of polyprotic acids.
Phosphoric Acid: Phosphoric acid, also known as orthophosphoric acid, is a chemical compound with the formula H3PO4. It is a colorless, odorless, and non-volatile acid that plays a crucial role in various chemical and biological processes, particularly in the context of polyprotic acids, the structure and properties of nonmetals, and the occurrence, preparation, and properties of phosphorus.
PKa: pKa is a measure of the strength of an acid, representing the pH at which a weak acid is 50% dissociated. It is a critical parameter that helps determine the relative strengths of acids and bases, the behavior of polyprotic acids, the effectiveness of buffers, and the progress of acid-base titrations.
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.
Polyprotic Acids: Polyprotic acids are chemical compounds that can donate more than one proton (H+) to a base during an acid-base reaction. These acids have multiple ionizable hydrogen atoms, allowing them to undergo multiple deprotonation steps, resulting in the formation of different conjugate bases at different pH levels.
Polyprotic Bases: Polyprotic bases are chemical compounds that can accept more than one proton (H+) during an acid-base reaction, forming multiple conjugate bases. These bases have the ability to deprotonate in a stepwise fashion, exhibiting multiple dissociation constants and pH-dependent speciation.
SO4^2-: The sulfate ion (SO4^2-) is a polyatomic ion consisting of one sulfur atom covalently bonded to four oxygen atoms. It is a common anion found in many chemical compounds and plays a crucial role in various chemical and biological processes, particularly in the context of polyprotic acids.
Stepwise ionization: Stepwise ionization is the process by which a polyprotic acid loses its protons one at a time in sequential steps. Each step has its own distinct ionization constant.
Stepwise Ionization: Stepwise ionization refers to the process by which a polyprotic acid, an acid with multiple ionizable hydrogen atoms, releases its protons in a step-by-step manner. This sequential dissociation of protons allows the acid to form multiple ionic species, each with a distinct charge, as the pH of the solution changes.
Sulfuric Acid: Sulfuric acid (H2SO4) is a highly corrosive and versatile inorganic compound that plays a crucial role in various chemical processes, including catalysis, polyprotic acid behavior, and the general properties of nonmetals.
Titration Curves: A titration curve is a graphical representation of the changes in pH that occur during a titration process. It depicts the relationship between the volume of a titrant added and the resulting pH of the solution, providing valuable information about the acid-base properties of the system being analyzed.
Triprotic acid: A triprotic acid is an acid capable of donating three protons or hydrogen ions per molecule in aqueous solution. Examples include phosphoric acid ($H_3PO_4$) and citric acid.