18.7
Piles de concentration
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Q1: What is a concentration cell and how does it differ from a standard galvanic cell?
A concentration cell is a voltaic cell with two identical half-cells using the same electrode and half-reaction, differing only in the concentration of one redox species. Unlike standard galvanic cells with different half-reactions, concentration cells generate potential solely from concentration differences. The cell potential depends on how much the ion concentrations differ between the two half-cells.
Q2: How does electron flow occur in a concentration cell with silver electrodes?
Following Le Châtelier's principle, electrons flow spontaneously from the dilute half-cell to the concentrated half-cell. In the dilute cell, the silver electrode oxidizes to form silver ions. In the concentrated cell, silver ions reduce to solid silver. This concentration gradient drives the spontaneous electron transfer until equilibrium is reached.
Q3: What happens to a concentration cell when ion concentrations become equal?
When ion concentrations in both half-cells become equal, the concentration cell reaches equilibrium and its cell potential becomes zero. At this point, the cell is pronounced 'dead' because no concentration gradient remains to drive electron flow. The driving force for the redox reaction is completely eliminated.
Q4: How is the cell potential of a concentration cell calculated?
The cell potential of a concentration cell is calculated using the Nernst equation, which accounts for the concentration difference of the redox species. The equation shows that the cell potential depends logarithmically on the ratio of ion concentrations between the two half-cells. A positive cell potential indicates the overall reaction is spontaneous.
Q5: How do pH meters use the concentration cell principle?
pH meters operate as concentration cells with a glass electrode filled with a known hydrogen ion concentration. When immersed in a solution with different hydrogen ion concentration, a measurable potential difference forms across the glass. This potential difference correlates to pH: high potential indicates acidic solutions, zero potential indicates neutral solutions, and low potential indicates basic solutions.
Q6: What real-world applications rely on concentration cell principles?
Concentration cells operate in pH meters, ion channels in nerve cell membranes, and cardiac muscle cells in the human body. These biological systems use concentration gradients of ions to generate electrical signals and drive cellular processes. The principle of potential difference from concentration differences is fundamental to cellular function and laboratory measurement.
Q7: Why does concentration affect cell potential in electrochemical cells?
Concentration affects cell potential because it determines the driving force for redox reactions. Higher ion concentrations in one half-cell create a greater thermodynamic driving force for reduction compared to lower concentrations. This concentration gradient generates an electrical potential that can be measured and used to perform work or drive reactions.