Tiêu đề 3.ELECTROCHEMISTRY.pdf ✅

ChemContent | Electrochemistry ChemContent 1 Conductivity of Solutions What is Conductivity? • Electrical resistance is represented by the symbol ‘R’, and it is measured in ohm (Ω). • The electrical resistance of any object is directly proportional to its length (l) and inversely proportional to its area of cross-section (A), i.e. where the constant of proportionality ρ is called resistivity (specific resistance). • The inverse of resistance, R, is called conductance, G, and we have the relation where the constant is called conductivity (specific conductance). • The SI unit of conductance is Siemens, represented by the symbol ‘S’, and it is equal to ohm−1 (also known as mho) or Ω−1 . The SI unit of conductivity ( ) is S m−1 . Conductivity of Electrolytic (Ionic) Solutions • Very pure water has small amounts of hydrogen and hydroxyl ions (~10−7M) which lend it very l o w conductivity (3.5 × 10−5 S m−1 ). • When electrolytes are dissolved in water, they dissociate to give their own ions in the solution; hence, its conductivity also increases. Measurement of Conductivity of Ionic Solutions • We first need to find the resistance of an ionic solution to measure conductivity. • We face two problems when measuring the resistance of an ionic solution: 1. Passing direct current (DC) changes the composition of the solution 2. A solution cannot be connected to the bridge like a metallic wire or other solid conductor Electrolytic or ionic conductance: Conductance of electricity by ions present in solutions
ChemContent | Electrochemistry ChemContent 2 • The first difficulty is resolved by using an alternating current (AC) source of power. Thesecond problem is solved by using a specially designed vessel called conductivity cell. ➢ Conductivity Cell: • It consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically). • These have area of cross-section equal to ‘A’ and are separated by distance ‘l’. The resistance of such a column of solution is then given by the equation: • The quantity ‘l/A’ is called cell constant and is denoted by the symbol G*. It depends on the distance between the electrodes and their area of cross-section and has the dimension [L−1]. • Hence, the cell constant G* is given by the equation: • When the cell constant is determined, we can use it for measuring the resistance or conductivity of any solution. • The set up for the measurement of the resistance is nothing but the well-known Wheatstone bridge. • It consists of two resistances R3 and R4, a variable resistance R1 and the conductivity cell having the unknown resistance R2.
ChemContent | Electrochemistry ChemContent 3 • The Wheatstone bridge is fed by an oscillator O (a source of AC power in the audio frequency range 550−5000 cycles per second). • P is a suitable detector (a headphone or other electronic device). • The bridge is balanced when no current passes through the detector. • Under these conditions, • When the cell constant and the resistance of the solution in the cell are determined, the conductivity of the solution is given by the equation: Molar Conductivity • Conductivity of solutions of different electrolytes in the same solvent and at a given temperature differs due to 1. Charge and size of ions in which they dissociate 2. Concentration of ions or ease with which the ions move under a potential gradient • Therefore, it becomes necessary to define a quantity called molar conductivity denoted by the symbol (λm). It is related to the conductivity of the solution by the equation: • Unit of λm is in S m2 mol−1 . • Hence, molar conductivity can be given by the formula Variation of Conductivity and Molar Conductivity with Concentration • Both conductivity and molar conductivity change with the concentration of the electrolyte. • Conductivity always decreases with a decrease in concentration for both weak and strong electrolytes. It is because the number of ions per unit volume which carry the current in a solution decreases on dilution. • Molar conductivity increases with a decrease in concentration. This is because the total volume (V) of solution containing one mole of electrolyte also increases. • The decrease in  on dilution of a solution is more than compensated by an increase in itsvolume. • When concentration approaches zero, the molar conductivity is known as limiting molar conductivity Molar conductivity (λm): Conductance of the electrolytic solution kept between the electrodes of a conductivity cell at unit distance but having area of cross section large enough to accommodate sufficient volume of solution which contains one mole of the electrolyte.
ChemContent | Electrochemistry ChemContent 4 m and is represented by the symbol λm o . • The variation in λm with concentration is different for strong and weak electrolytes. ➢ Strong Electrolytes: ▪ λ increases slowly with dilution and can be represented by the equation: ▪ It can be seen that if we plot λm against c1/2, we obtain a straight line with intercept equal to λ o m and slope equal to ‘−A’. ▪ The value of the constant ‘A’ for a given solvent and temperature depends on the type of electrolyte. ➢ Weak Electrolytes: ▪ Weak electrolytes such as acetic acid have a lower degree of dissociation at higher concentrations. Hence, for such electrolytes, the change in λm with dilution is due to 1. Increase in the degree of dissociation 2. The number of ions in total volume of solution which contains 1 mol of electrolyte ▪ In such cases, λm o increases steeply on dilution, especially near lower concentrations. Therefore, it cannot be obtained by extrapolating λm to zero concentration. ▪ At infinite dilution, electrolyte dissociates completely (α = 1), but at such low concentration, the conductivity of the solution is so low that it cannot be measured accurately. Therefore, λm o for weak electrolytes is obtained by using Kohlrausch’s law of independent migration of ions. ▪ Thus, at any concentration c, if α is the degree of dissociation, then it can be approximated to the ratio of molar conductivity, λm, at the concentration c tolimiting molar conductivity λm o . m