41. Conductivity measurement, determination of weak acid dissociation Flashcards
What is Electrical conduction?
the ordered movement of charged particles in an electric field
Which method that measure conductivity of an electrolyte solution?
by the conductometric method.
What is conductivity?
the reciprocal of resistance (I/U), the unit of measurement is siemens (S).
2 things that conductivity depend on
(1) The intrinsic nature of the anions and cations they contain, i.e. their mobility and charge
(2) concentrations of anions and cations
can degree of dissociation be measured from conductivity measurement?
YES
can degree of dissociation be measured from conductivity measurement?
YES
The actual concentrations of the ions, in turn, are dictated by the extent to which the electrolyte ______
dissociates
e meas- urement of conductivity is suitable for determining the degree of ___.
dissociates
Summary of conductometry
- Immersing 2 metal plates with same size into an electrolyte solution
- Applying a voltage
- Measuring the current generated
Conductometry
What does current generated depend on? (chemical factors and geometric factors)
- Chemical nature and amount of ions
- Surface and distance of metal plates (geometric factors)
- By convention, metal plates with surface areas of 1cm2 are fixed at a distance of 1cm from each other => 1 cm3 solution is tested
Conductometry
What does current generated depend on? (chemical factors and geometric factors)
- Chemical nature and amount of ions
- Surface and distance of metal plates (geometric factors)
- By convention, metal plates with surface areas of 1cm2 are fixed at a distance of 1cm from each other => 1 cm3 solution is tested
Conductometry
Unit of the specific conductivity of the solution
κ (kappa), in S/cm.
Conductometry
When solutions of different concentrations and electrolytes are prepared and tested, the measured specific conductivity of the more dilute solution is always ((1)___(Lower/higher), and that of the more concentrated solution is always (2)___(Lower/higher
- Lower
- Higher
Conductometry
When solutions of different concentrations and electrolytes are prepared and tested, the measured specific conductivity of the more concentrated solution is always higher.
Question: Why?
- Lower
- Higher
Does the amount of ions depend on degree of dissociation?
YES
Explain the concept of equivalent conductivity?
The extent in the difference of conductivities between the particular concentrations is distinct for each substance
→ It is due to the difference in the initial concentrations can be eliminated
What is the symbol and unit for quivalent conductivity
Symbol - A (capital lambda)
Unit: Scm2/mol.
For substances that dissociate into monovalent ions, what is the equivalent conductivity?
the equivalent conductivity is the total conductivity of a portion of the solution that contains exactly 1 mole of solute
Describe dissociation of strong electrolytes
Strong electrolytes always dissociate completely regardless of the concentration
Describe dissociation in infinitely dilute solutions
The dissociarion is always complete, regardless of the strength of electrolyre
What is the equivalent conductivity of infinitely dilute solutions?
The equivalent conductivity of such a solution (Λ∞) is a constant
Is there solute in the infinitely dilute solution,? What is the consequence?
In fact, there is no solute in the infinitely dilute solution, so the conductivity of such a solution cannot be determined directly in practice.
How to calculate the conductivity of multicomponent electrolyte solution?
the sum of the conductivity values of the individual ions
The conductivity of a solution containing a reasonable amount of a __ (strong/weak) electrolyte is measurable
strong
For weak electrolytes, how to calculate the equivalent conductivity?
the equivalent conductivity is directly proportional to the degree of dissociation, which in turn increases with dilution
→ the degree of dissociation of the electrolyte solution can be calculated from the equivalent conductivity of a given concentration c (Λc) and the infinitely dilute solution (Λ∞)
in concentrated solutions, the equivalent conductivity of strong electrolytes is ___(higher/lower) than in dilute solutions.
lower
in concentrated solutions, the equivalent conductivity of strong electrolytes is lower than in dilute solutions.
WHY IS THAT?
because of the congestion
→ the distance between the oppositely charged ions decreases
→ an attractive electrostatic interaction develops between them
→ arrange in pairs and are less willing to separate and move in opposite directions → their mobility decreases.
Describe the measuring equipment
- A Jenway 3540 conductivity (and pH) meter, to which a bell-shaped conductivity cell is connected
- The conductivity cell consists of a glass bell hiding two platinum plates in the standardized arrengement.
- The plates form the conductivity cell with the electrolyte solution to be measured between them
What are the materials and equipments for the conductometry (except Jenway)
− 0.01 M CH3COOH solution
− 0.05 M CH3COOH solution
− 0.1 M CH3COOH solution
− 4 beakers
8 steps of conductometry with CH3COOH solutions
- Fill separate beakers with three CH3COOH solutions of different concentrations.
- Immerse the bell electrode in the first beaker (0.01 M acetic acid) so that the solution covers the top hole of the glass bell.
- Read the specific conductivity () of the solution measured from the conductometer.
- Rinse the bell electrode by sinking-lifting it a couple of times into-from a beaker filled with distilled water before placing it in a new solution.
- Similarly measure the conductivity of the other two solutions.
- Determine how the specific and equivalent conductivity and the degree of dissociation changed with dilution. (For acetic acid = 344 Scm2/mol).
- Calculate the dissociation constant of the acetic acid using the
