9.5
Potenciais de junção em células galvânicas
A equação de Nernst, derivada sob a suposição de equilíbrio termodinâmico, calcula a força eletromotriz (fem) como a soma das diferenças de potencial…
A equação de Nernst prevê o potencial da célula sob condições não padrão.
No entanto, em muitas células galvânicas, o potencial medido frequentemente difere do valor previsto pela equação de Nernst.
Para entender isso, vamos considerar uma célula galvânica contendo duas meias-células com soluções eletrolíticas diferentes. Quando entram em contato, íons começam a se difundir através da fronteira. Como íons têm mobilidades diferentes, eles se movem em velocidades diferentes.
Esse movimento desigual dos íons cria uma pequena separação de carga na interface entre as duas soluções. Isso desenvolve um potencial adicional chamado potencial de junção líquida, EJ.
Como resultado, a fem medida inclui tanto o potencial de Nernst quanto o potencial de junção.
Para reduzir esse efeito, usa-se uma ponte salina. A ponte salina normalmente contém gel de agar misturado com um cloreto de potássio concentrado semelhante a eletrólitos.
Como íons potássio e cloreto têm mobilidades semelhantes, eles se difundem em taxas semelhantes. Sua difusão ajuda a minimizar o potencial de junção e melhora a precisão da medição.
View the full transcript and gain access to JoVE Core videos
Q1: Why does the measured potential in a galvanic cell differ from the Nernst equation prediction?
The measured potential differs because ions in the two half-cell electrolyte solutions have different mobilities and diffuse at unequal rates across the boundary. This creates a charge separation at the interface, generating an additional potential called the liquid junction potential. The total measured emf includes both the Nernst potential and this junction potential.
Q2: What is a liquid junction potential and how does it form?
A liquid junction potential is a small potential difference that develops at the interface between two electrolyte solutions due to different ion diffusion rates. Since ions have different mobilities, they move at different speeds across the boundary, creating unequal charge separation. This additional potential must be accounted for in accurate electrochemical measurements.
Q3: How does a salt bridge minimize junction potential in galvanic cells?
A salt bridge contains agar gel mixed with an electrolyte like concentrated potassium chloride. Potassium and chloride ions have similar mobilities, so they diffuse at comparable rates. This balanced diffusion minimizes the junction potential at both ends of the salt bridge, causing them to largely cancel each other out and improve measurement accuracy.
Q4: Can salt bridges completely eliminate liquid junction potentials?
No, salt bridges can minimize but not eliminate liquid junction potentials. Although the junction potentials at both ends of the salt bridge largely cancel each other due to similar ion mobilities, a small residual potential remains. This is why junction potentials are considered significant for accurate electrochemical work.
Q5: What role do ion mobilities play in creating junction potentials?
Ion mobilities determine the diffusion rates of ions across electrolyte solution boundaries. When ions have different mobilities, they move at unequal speeds, creating uneven charge distribution at the interface. This charge separation generates the liquid junction potential. Selecting ions with similar mobilities, as in salt bridges, reduces this effect.
Q6: How does the total emf of a cell with a liquid junction relate to the Nernst equation?
The total emf of a cell with a liquid junction equals the sum of the Nernst potential and the liquid junction potential. The Nernst equation calculates the potential under thermodynamic equilibrium conditions without a junction. Adding the junction potential accounts for the real-world charge separation that occurs when different electrolyte solutions meet.
Q7: Why are junction potentials important in electrochemical measurements?
Junction potentials are important because they contribute to the total measured emf and can introduce significant errors in electrochemical work. Although small, they affect measurement accuracy in applications of emf measurements. Understanding and minimizing junction potentials through salt bridges ensures reliable and precise electrochemical data.