8.5
전도 조절 조절: 강산-약염기 및 약산-강염기 적정
아세트산과 같은 약한 산을 수산화나트륨과 같은 강한 염기와 함께 조절할 때, 초기 전도도는 아세트산의 약한 해리로 인해 상대적으로 낮습니다. 하지만 수산화나트륨이 용액에 첨가되면 아세트산과 반응하여 고이온화된 아세테이트나트륨이 생성되어 전도도가 증가합니다. 모든 아세트…
약한 산인 아세트산을 강한 염기인 NaOH와 적정하는 것을 생각해 보세요. 초기에는 약한 해리 때문에 초산의 전도도가 낮습니다.
NaOH가 첨가되면 고이온화된 아세테이트 나트륨이 생성되어 전도도가 증가합니다.
모든 산이 중화되면, 추가로 염기가 빠르게 이동하는 OH− 이온을 도입하여 전도도가 급격히 증가합니다.
기저 부피에 대한 전도도 도는 두 개의 교차하는 선을 나타내며, 교차점은 끝점을 나타냅니다.
반대로, 강한 산인 HCl이 약한 염기인 NH4OH에 대해 조절할 때, 빠르게 움직이는 H3O+ 이온이 느리게 움직이는 NH4+ 이온으로 대체되어 전도도가 처음에 감소합니다.
전도도는 산성 중화가 완료될 때까지 감소하며, 그 이후에는 염기의 약해리로 인한 전도도 변화가 거의 없습니다.
적정의 끝점은 전도도 그래프에서 두 선의 교차점을 식별하여 결정됩니다.
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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.