8.2
Número de Transporte
O número de transporte é a fração da corrente total transportada por um íon em uma solução eletrolítica. É definido como a razão entre a corrente tran…
O número de transporte é a fração da corrente total transportada por cátions ou ânions em uma solução eletrolítica.
O método de Hittorf explica os números de transporte examinando a migração de íons em uma célula dividida em compartimentos anódico, central e catódico.
Quando uma diferença de potencial é aplicada, os cátions se movem em direção ao cátodo e os ânions em direção ao ânodo, causando mudanças de concentração próximas a cada eletrodo. O compartimento central serve como uma região de referência menos afetada pelas reações de eletrodos.
A razão de mobilidade cátion-aniônica é igual à queda na concentração ao redor do ânodo em relação à do cátodo.
O número de transporte de cátions é definido como a queda de concentração ao redor do ânodo dividida pela queda total em ambos os eletrodos, enquanto o número de transporte de ânions é igual a um menos o número de transporte de cátion.
Uma abordagem alternativa é o método da fronteira móvel, no qual uma corrente elétrica desloca a fronteira entre o eletrólito principal HCl e o eletrólito indicador CdCl₂.
Se um potencial aplicado Q move a fronteira por l cm, deslocando lA cm 3 contendo equivalentes em c gramas de HCl, o número de transporte dos íons H⁺ pode ser calculado matematicamente.
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Q1: What is transport number and why does it matter in electrochemistry?
Transport number is the fraction of total current carried by a specific ion in an electrolyte solution. It represents the ratio of current carried by that ion to the total current flowing through the solution. Understanding transport numbers is essential for predicting ionic mobility and how fast ions move under an electric field, directly connecting to electrochemical reaction behavior.
Q2: How does Hittorf's method measure transport numbers?
Hittorf's method divides an electrolyte solution into anodic, central, and cathodic compartments. When potential is applied, cations migrate toward the cathode and anions toward the anode, causing measurable concentration changes near each electrode. The central compartment serves as a reference region unaffected by electrode reactions, allowing independent measurement of concentration changes that reflect ion mobility ratios.
Q3: What is the relationship between cation mobility and concentration changes in Hittorf's method?
The cation-to-anion mobility ratio equals the concentration fall around the anode to that around the cathode. The cation transport number is defined as the concentration fall around the anode divided by the total fall around both electrodes. This relationship allows researchers to calculate transport numbers from measured concentration changes in gram equivalents.
Q4: How does the moving boundary method determine transport numbers?
The moving boundary method tracks the movement of a sharp boundary between a principal electrolyte and an indicator electrolyte under an applied electric field. For example, to measure H⁺ transport in HCl, CdCl₂ serves as the indicator because its cation has lower mobility than H⁺ and shares the same anion. By measuring boundary displacement over time, the transport number can be calculated mathematically.
Q5: Why is the central compartment important in Hittorf's method?
The central compartment in Hittorf's method serves as a reference region that remains relatively unaffected by electrode reactions. This allows researchers to measure concentration changes in the anodic and cathodic compartments independently and accurately. The central compartment's stability ensures that observed concentration changes reflect only ion migration caused by the applied potential.
Q6: How is anion transport number calculated from cation transport number?
Anion transport number is obtained by subtracting the cation transport number from unity. Since cations and anions together carry all the current in an electrolyte solution, their transport numbers must sum to one. This relationship provides a simple way to determine anion transport once cation transport is measured or calculated.
Q7: What role does ionic mobility play in transport number determination?
Ionic mobility describes how fast an ion moves under an electric field influence. The extent of concentration change near electrodes in Hittorf's method directly reflects the relative mobilities of cations and anions. Transport numbers quantify this mobility difference, connecting the physical behavior of ions in solution to the electrical transport properties measured during electrochemical reactions.