8.5
Titrations conductométriques : titrations acide-base faible et base acide forte
Lorsqu’un acide faible comme l’acide acétique est titré par rapport à une base forte comme l’hydroxyde de sodium, la conductance initiale est relative…
Considérez la titration d’un acide faible, l’acide acétique, contre une base forte, le NaOH. Au départ, la conductance de l’acide acétique est faible en raison de sa dissociation faible.
À mesure que le NaOH est ajouté, l’acétate de sodium hautement ionisé est produit, augmentant la conductance.
Une fois tout l’acide neutralisé, toute base supplémentaire introduit des ions OH− à mouvement rapide, entraînant une forte augmentation de la conductance.
Le graphique de conductance par rapport au volume de la base montre deux droites qui se croisent, le point d’intersection indiquant l’extrémité.
Inversement, lorsqu’un acide fort, HCl, est titré contre une base faible, NH4OH, la conductance diminue initialement en raison du remplacement des ions H3O+ rapides par les ions NH4+ plus lents de la base.
La conductance diminue jusqu’à la fin de la neutralisation acide, après quoi il n’y a pas de changement significatif de conductance attribué à la faible dissociation de la base.
Le point final de la titration est déterminé en identifiant le point d’intersection de deux droites sur le graphique de conductance.
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Q1: Why does conductance increase when a weak acid is titrated with a strong base?
When acetic acid is titrated with NaOH, the initial conductance is low due to weak dissociation of acetic acid. As NaOH is added, highly ionized sodium acetate forms, significantly increasing conductance. After neutralization, excess OH− ions further boost conductance sharply due to their high mobility and fast movement through the solution.
Q2: What happens to conductance during a strong acid-weak base titration?
When HCl is titrated against NH4OH, conductance initially decreases because fast-moving H3O+ ions are replaced by slower-moving NH4+ ions from the weak base. This decline continues until neutralization is complete. After that, conductance remains relatively stable due to the weak dissociation of the base producing few additional mobile ions.
Q3: How is the endpoint determined in conductometric titrations?
The endpoint in conductometric titrations is identified by plotting conductance against the volume of titrant added. Two intersecting lines appear on the graph, and their intersection point indicates the equivalence point where the titration is complete. This graphical method provides a clear visual marker for determining when neutralization occurs.
Q4: Why do fast-moving and slow-moving ions affect conductance differently?
Ions have different mobilities based on their size and charge. Fast-moving ions like H3O+ and OH− contribute more to conductance than slower ions like NH4+. When fast ions are replaced by slower ones during titration, overall conductance decreases, creating the characteristic curve shape observed in strong acid-weak base systems.
Q5: What is the role of sodium acetate in weak acid-strong base titrations?
Sodium acetate is the product formed when acetic acid reacts with NaOH during titration. Unlike acetic acid, sodium acetate is highly ionized in solution, producing mobile ions that significantly increase conductance. This sharp increase in conductance after the equivalence point helps identify the titration endpoint clearly on the conductance plot.
Q6: How do conductometric titrations differ between strong acid-weak base and weak acid-strong base systems?
In weak acid-strong base titrations, conductance increases after the endpoint due to excess OH− ions. In strong acid-weak base titrations, conductance decreases until neutralization, then plateaus because the weak base produces few mobile ions. Both systems show two intersecting lines on conductance plots that mark the equivalence point.
Q7: Why is conductance initially low when titrating acetic acid?
Acetic acid is a weak acid that only partially dissociates into ions in solution. Since conductance depends on the concentration of mobile ions, the low degree of dissociation results in low initial conductance. This changes dramatically when the strong base NaOH is added and produces highly ionized sodium acetate with mobile ions.