9.5
Potentiels de jonction dans les cellules galvaniques
L’équation de Nernst, dérivée sous l’hypothèse d’équilibre thermodynamique, calcule la force électromotrice (fem) comme la somme des différences de po…
L’équation de Nernst prédit le potentiel de la cellule dans des conditions non standard.
Cependant, dans de nombreuses cellules galvaniques, le potentiel mesuré diffère souvent de la valeur prédite par l’équation de Nernst.
Pour comprendre cela, considérons une cellule galvanique contenant deux demi-cellules avec des solutions électrolytiques différentes. Lorsqu’ils entrent en contact, les ions commencent à se diffuser à travers la frontière. Comme les ions ont des mobilités différentes, ils se déplacent à des vitesses différentes.
Ce mouvement inégal des ions crée une petite séparation de charge à l’interface entre les deux solutions. Cela développe un potentiel supplémentaire appelé potentiel de jonction liquide, EJ.
En conséquence, la fem mesurée inclut à la fois le potentiel de Nernst et le potentiel de jonction.
Pour réduire cet effet, un pont à sel est utilisé. Le pont salant contient généralement un gel d’agar mélangé à un chlorure de potassium concentré semblable à un électrolyte.
Comme les ions potassium et chlorure ont des mobilités similaires, ils diffusent à des vitesses comparables. Leur diffusion aide à minimiser le potentiel de jonction et améliore la précision des mesures.
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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.