3.6
Composition de la solution pendant les titrages d'acide/base monoprotique
Le titrage d’un acide faible avec une base forte entraîne la formation d’eau et de la base conjuguée de l’acide. Par exemple, le titrage de l’acide ac…
Rappelons que le titrage de l’acide acétique avec de l’hydroxyde de sodium produit de l’acétate de sodium et de l’eau, ce qui permet d’obtenir un tampon.
La concentration relative des différentes espèces dans la solution tampon en différents points du titrage est indiquée par les valeurs alpha.
Alors que la ligne courbe représente le titrage de l’acide acétique et de l’hydroxyde de sodium, les lignes droites étiquetées alpha-noeud et alpha-un désignent la concentration d’équilibre relative des ions acide acétique et acétate, respectivement.
Initialement, la valeur du nœud alpha de 0,987 suggère que 98,7 % de l’acide acétique est indissocié, les 1,3 % restants constituant des ions acétate.
Au point de croisement ou au point de demi-équivalence, le pH est égal à pKa. En conséquence, la solution a une concentration égale d’acide acétique et d’ions acétate.
Au fur et à mesureque le titrage se poursuit, la concentration de l’acide acétique, alpha-noeud, diminue jusqu’à presque zéro au point d’équivalence. Alternativement, la concentration des ions acétate, alpha-one, augmente et se rapproche de l’unité.
Q1: What happens to acetic acid and acetate ions during titration with sodium hydroxide?
During titration of acetic acid with sodium hydroxide, the weak acid is gradually converted to its conjugate base, sodium acetate, forming a buffer solution. The alpha values track this composition change: initially, alpha-0 (acetic acid) is 0.987 and alpha-1 (acetate ions) is 0.013. As titration proceeds, alpha-0 decreases while alpha-1 increases, reaching equal concentrations at the half-equivalence point where pH equals pKa.
Q2: What do alpha values represent in an acid-base titration?
Alpha values represent the relative equilibrium concentration or fractional composition of species in a buffer solution during titration. Alpha-0 (α0) indicates the fraction of undissociated weak acid, while alpha-1 (α1) indicates the fraction of conjugate base. These values are expressed as decimals or percentages, allowing visualization of how solution composition changes throughout the titration process.
Q3: Why is the half-equivalence point significant in weak acid titrations?
At the half-equivalence point, the pH of the solution equals the pKa of the weak acid. At this critical point, the concentrations of acetic acid and acetate ions are equal, with alpha-0 and alpha-1 both equal to 0.5. This relationship between pH and pKa makes the half-equivalence point essential for understanding buffer behavior and predicting solution pH during titration.
Q4: What is the composition of the solution at the equivalence point?
At the equivalence point, the titration is complete and nearly all acetic acid has been converted to acetate ions. The alpha-0 value drops to nearly zero, while alpha-1 approaches unity (1.0), indicating the solution is predominantly sodium acetate. This conjugate base solution exhibits basic properties due to hydrolysis of acetate ions.
Q5: How does the buffer solution form during the titration of acetic acid?
As sodium hydroxide is added to acetic acid, a buffer forms because both the weak acid (acetic acid) and its conjugate base (acetate ions) are present in solution. The buffer composition changes continuously: initially dominated by acetic acid with trace acetate, then becoming increasingly acetate-rich. This coexistence of weak acid and conjugate base maintains relatively stable pH across much of the titration curve.
Q6: What percentage of acetic acid remains undissociated before titration begins?
Before any sodium hydroxide is added, the alpha-0 value is 0.987, meaning 98.7% of the acetic acid remains undissociated. Only 1.3% of the acetic acid has dissociated into acetate ions and hydrogen ions. This initial composition reflects the weak nature of acetic acid and its limited tendency to ionize in aqueous solution.
Q7: How do alpha values change as the titration progresses toward equivalence?
As titration proceeds, alpha-0 (acetic acid fraction) continuously decreases from 0.987 toward zero, while alpha-1 (acetate fraction) increases from 0.013 toward unity. The rate of change accelerates near the equivalence point. This inverse relationship between the two alpha values reflects the conversion of weak acid to conjugate base and is fundamental to understanding buffer capacity and solution composition changes.