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Position of Equilibrium in Acid-Base Reactions02:05

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In any solution, the value of pKa indicates whether an acid is completely dissociated or not. A negative pKa corresponds to a stronger acid, whereas a positive pKa corresponds to a weaker acid. Consider the reaction between ammonia and an ethoxide ion. In this reaction, ethanol with a pKa of 15.9 is a stronger acid than ammonia with a pKa of 38. Recall that the strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base. Hence, the ethoxide ion is a weak base.
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This lesson delves into a critical aspect of the relative strengths of acids and bases. The strength of an acid is evaluated by the acid dissociation into its conjugate base and a hydronium ion in water. The complete dissociation of a strong acid is confirmed with a very high concentration of hydronium ions. As a result, an incomplete dissociation process affirms a weak acid. Therefore, the equilibrium is in the forward direction for strong acids and backward for weak acids in these reactions.
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Extraction: Effects of pH00:53

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Consider a neutral form of an amine, B, with a partition coefficient, K, in a liquid mixture containing organic and aqueous phases. The pH of the aqueous phase affects the charge on acidic and basic solutes, and the charged form is usually more soluble in the aqueous phase. Suppose the conjugate acid form of the amine is soluble only in the aqueous phase while the base form is soluble in both phases. Then the distribution coefficient, D, can be given as the ratio of amine concentration in the...
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The pH of a salt solution is determined by its component anions and cations. Salts that contain pH-neutral anions and the hydronium ion-producing cations form a solution with a pH less than 7. For example, in ammonium nitrate (NH4NO3) solution, NO3− ions do not react with water whereas NH4+ ions produce the hydronium ions resulting in the acidic solution.  In contrast, salts that contain pH-neutral cations and the hydroxide ion-producing anions form a solution with a pH greater than 7. For...
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The Electrical Double Layer01:30

The Electrical Double Layer

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In the region where two bulk phases meet, an intricate electric charge distribution arises due to charge transfer, ion adsorption, molecular orientation, and charge distortion. This complex distribution is commonly referred to as the electrical double layer.When a solid electrode interfaces with ions in an electrolyte solution, the speed of electron transfer dictates the rates of oxidation and reduction. The electrode acquires a charge through the escape of atoms into the solution as cations or...
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Titration of Polyprotic Acids with a Strong Base01:23

Titration of Polyprotic Acids with a Strong Base

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Titration of a polyprotic acid, which contains multiple ionizable protons, involves distinct dissociation steps, each with its own dissociation constant (Ka). Each successive Ka is weaker than the previous one. In the titration of a polyprotic acid like sulfurous acid with a strong base such as sodium hydroxide, the base first neutralizes the initial ionizable proton, forming an intermediate species (e.g., hydrogen sulfite ions). This step's titration curve resembles that of a weak...
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pKa at Quartz/Electrolyte Interfaces.

Morgane Pfeiffer-Laplaud1, Marie-Pierre Gaigeot1, Marialore Sulpizi2

  • 1LAMBE CNRS UMR8587, Université d'Evry val d'Essonne , Blvd F. Mitterrand, Bât Maupertuis, 91025 Evry, France.

The Journal of Physical Chemistry Letters
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Summary
This summary is machine-generated.

The acidity of quartz surfaces changes with salt solutions, following a specific ion order. This behavior is explained by how ions and water interact at the quartz-water interface.

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Area of Science:

  • * Physical Chemistry
  • * Materials Science
  • * Computational Chemistry

Background:

  • * Understanding the acidity of mineral-water interfaces is crucial for various geochemical and industrial processes.
  • * The Hofmeister series describes ion-specific effects on the properties of aqueous solutions, but its application to solid-liquid interfaces requires detailed investigation.

Purpose of the Study:

  • * To calculate the acidity (pKa) of silanol sites at the crystalline quartz/aqueous electrolyte interface.
  • * To investigate the influence of different electrolytes (NaCl, NaI, KCl) on interfacial acidity.
  • * To rationalize the observed ion-specific effects at the molecular level.

Main Methods:

  • * Utilized ab initio molecular dynamics (AIMD) simulations to model the quartz-electrolyte interface.
  • * Calculated pKa values for silanol groups under varying electrolyte conditions.
  • * Analyzed microscopic solvation structures and ion-surface interactions.

Main Results:

  • * Calculated pKa values exhibit a trend consistent with a combination of cationic and anionic Hofmeister series: pKa(neat solution) < pKa(NaCl) < pKa(NaI) < pKa(KCl).
  • * The observed ranking is rationalized by changes in local solvation of silanols and silanolates (SiO(-)).
  • * Both water restructuring induced by electrolytes and specific cation-silanolate interactions contribute to the pKa shifts.

Conclusions:

  • * Ab initio molecular dynamics simulations provide atomistic insights into interfacial chemical reactivity.
  • * The study demonstrates the necessity of molecular modeling for understanding complex interfacial phenomena.
  • * Ion-specific effects at the quartz-aqueous interface are governed by solvation dynamics and direct ion-surface interactions.