8.5
Conductometric Titrations: Strong Acid-Weak Base and Weak Acid-Strong Base Titrations
When a weak acid such as acetic acid is titrated against a strong base like sodium hydroxide, the initial conductance is relatively low due to the wea…
Consider the titration of a weak acid, acetic acid, against a strong base, NaOH. Initially, the conductance of acetic acid is low due to its weak dissociation.
As NaOH is added, the highly ionized sodium acetate is produced, increasing the conductance.
Once all the acid is neutralized, any additional base introduces fast-moving OH− ions, leading to a sharp increase in conductance.
The plot of conductance against the volume of the base shows two intersecting lines, with the intersection point indicating the endpoint.
Contrarily, when a strong acid, HCl, is titrated against a weak base, NH4OH, the conductance initially decreases due to the replacement of fast-moving H3O+ ions by the slower-moving NH4+ ions of the base.
The conductance decreases until the completion of acid neutralization, after which there is no significant change in conductance attributed to the weak dissociation of the base.
The endpoint of the titration is determined by identifying the intersection point of two lines on the conductance plot.
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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.