8.1
Electrical Transport
The electrical transport property of a material is defined by its resistance and conductivity. Resistance is the measure of a material's ability to re…
Conductance G is the inverse of resistance R. It shows how easily electricity flows through a solution, depending on the geometry of the measurement cell.
To remove the effect of cell geometry, conductance is multiplied by the cell constant Kcell, defined as the distance between electrodes divided by their cross-sectional area. This gives the specific conductance, κ, which represents the conductance of a one-meter cube of the solution.
Equivalent conductance Λ relates conductance to the amount of electrolyte. It represents the conductance of ions produced by one gram equivalent of an electrolyte and equals κ multiplied by the volume containing one gram equivalent.
Meanwhile, molar conductance Λm describes the conductance due to ions from one mole of electrolyte and is obtained by dividing κ by the molar concentration of the added electrolyte.
As an electrolyte's dilution increases, its Λm increases due to greater electrolyte dissociation, while κ decreases since the total number of ions per unit volume drops.
As the concentration approaches zero, the Λm of strong electrolytes reaches a limiting value, called the molar conductance at infinite dilution, Λ°m.
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Q1: What is the difference between conductance and specific conductance?
Conductance (G) measures how easily electricity flows through a solution and depends on cell geometry. Specific conductance (κ) removes geometric effects by multiplying conductance by the cell constant, which is the electrode distance divided by cross-sectional area. Specific conductance represents the conductance of a one-meter cube of solution, measured in Siemens per meter (S/m).
Q2: How does molar conductance change with dilution?
As electrolyte dilution increases, molar conductance (Λm) increases due to greater electrolyte dissociation and increased ion mobility. However, specific conductance (κ) decreases because the total number of ions per unit volume drops despite higher dissociation. At infinite dilution, strong electrolytes reach a limiting molar conductance value (Λ°m).
Q3: What is the cell constant and why is it important?
The cell constant (Kcell) is defined as the distance between electrodes divided by their cross-sectional area. It removes the effect of cell geometry from conductance measurements, allowing conductance values to be compared across different measurement setups. Multiplying conductance by the cell constant yields specific conductance, which is independent of apparatus design.
Q4: How does equivalent conductance relate to molar conductance?
Equivalent conductance (Λ) represents the conductance of ions from one gram equivalent of electrolyte, while molar conductance (Λm) represents conductance from one mole of electrolyte. Both are calculated from specific conductance (κ), but molar conductance is more useful because it reveals how ion mobility and electrolyte dissociation vary with concentration in theory of strong electrolytes.
Q5: Why does specific conductance decrease while molar conductance increases during dilution?
During dilution, electrolyte dissociation increases, raising molar conductance. However, the rise in ion numbers is significantly less than the increase in solution volume. Consequently, the total ion concentration per unit volume decreases, causing specific conductance to decline despite improved ion mobility and higher molar conductance values.
Q6: What is molar conductance at infinite dilution and when is it used?
Molar conductance at infinite dilution (Λ°m) is the limiting value that strong electrolytes approach as concentration approaches zero. This value is used in conductometric titrations strong acid base and weak acid base titrations to determine the degree of dissociation and ion mobility under ideal conditions where electrostatic interactions between ions are minimized.
Q7: How is molar conductance calculated from specific conductance?
Molar conductance (Λm) is calculated by dividing specific conductance (κ) by the molar concentration of the electrolyte in the solution. This relationship allows researchers to determine how conductivity changes with electrolyte concentration, revealing information about ion mobility and dissociation behavior across different concentration ranges.