8.4
Titrations conductométriques : titrations acido-base fortes et acide-base faibles
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Q1: Why does conductance decrease when NaOH is added to HCl during a strong acid-base titration?
During strong acid-base titration, conductance decreases because fast-moving H3O+ ions are gradually replaced by slower-moving Na+ ions. Although chloride ion concentration remains unchanged, the substitution of highly mobile hydrogen ions with less mobile sodium ions reduces overall electrical conductance until the acid is completely neutralized.
Q2: How does the conductance curve change after the equivalence point in a strong acid-strong base titration?
After the equivalence point, adding excess NaOH introduces fast-moving OH− ions into the solution, causing conductance to increase. This post-neutralization increase in conductance creates a distinct linear segment on the conductance versus volume plot, allowing the equivalence point to be identified at the intersection of two straight lines.
Q3: What is the order of neutralization when titrating a mixture of HCl and acetic acid with NaOH?
HCl neutralizes first because it is a stronger acid than acetic acid. Only after HCl is completely neutralized does the titration of acetic acid begin. The conductance plot shows three linear segments, with distinct endpoints representing the neutralization volumes of each acid.
Q4: Why does conductance increase during the titration of acetic acid with NaOH after HCl is neutralized?
As acetic acid is titrated, it forms highly ionized sodium acetate, which increases conductance. The acetate ions contribute to electrical conductivity, causing the conductance to rise during this phase. This behavior differs from the initial HCl titration phase, creating a distinct linear segment on the conductance curve.
Q5: What are the advantages of using conductometric titrations over traditional volumetric methods?
Conductometric titrations can titrate colored solutions that traditional indicator-based methods cannot analyze. They also work effectively with extremely diluted solutions and weak acids or bases. Additionally, the endpoint is graphically determined, eliminating the need for careful observation near the equivalence point, simplifying the titration process.
Q6: How is the equivalence point determined in a conductometric titration?
The equivalence point is determined graphically by plotting conductance against the volume of titrant added. Two straight lines form on the plot, and their intersection indicates the exact volume of titrant required to neutralize the acid. This graphical method provides a precise endpoint without relying on color indicators.
Q7: What fundamental principle underlies conductometric titrations?
Conductometric titrations rely on the principle that a solution's electrical conductance depends on the number and mobility of its ions. As ions are replaced during titration, changes in ion concentration and mobility alter the overall conductance, allowing the neutralization process to be tracked and the equivalence point to be identified.