ELECTRICAL PROPERTIES OF THE small molecule ELECTROLYTES
A solute which produces sufficient ions to make a conductive solution.
- Each molecule prior to dissociation is electrically neutral.
- The concentration and nature of the ions characterize the electrical conductivity of an electrolytic solution.
- Dissociation is characterized dissociation coefficient.
- The anion and cation are uniformly distributed throughout the volume of a solution : [(charge (+) = charge(-)] principle of electro-neutrality.
THE SEPARATION FACTOR :
α = number of molecules dissociated / Total nb of the initial molecules
From where 0 < a <1
a = 1 —> total dissociation (életrolyte fort)
a < 1 —> partial dissosiation
a = 0 —> no dissotiation (neutral solution)
EQUIVALENT CONDUCTIVITY LIMITED :
Limited the equivalent conductivity is the equivalent conductivity that corresponds to a total dissociation in an electrolytic solution from which a = ???/????? = ???/ ???∞
The determination of the conductivity limit aeqli permits the classification of the electrolyte into two categories:
STRONG ELECTROLYTE :
- Any electrolyte that dissociates completely in water (NaCl, NaOH, KOH, HCl)
- In the solution, found only majority ions (provided by the dissociation of crystal or molecule) and molecules of the solvent.
AxBy -> far– + yB+
t = 0 : [c] = C -> 0 + 0 = [C]
at t [c] = 0 -» x.C + y.C = [C]
WEAK ELECTROLYTES :
- The ionization of the solute is partial.
- The solution contains the solute ions, molecules of the solute and the solvent those exp : CH3COOH acetic acid in water.
AxBy xA– + yB+
(1- a).Cm a.x.Cm + a.y.Cm / ? = ??? /???∞
A- LAW OWSTWALD :
– For an electrolyte (????) partially dissociated at temperature T with a dissociation rate ?1 and a rate of deformation of the ions (??−,B?+) ?2
– Equilibrium ?1=?2 ⇒ ?1???? = ?2??- ??+
(K: dissociation constant)
- This relationship expresses the Ostwalt dilution law.
- Independent electrical activities.
- Increases with temperature: increased thermal agitation promotes the dissociation effect.
- Depends solute.
- Depends on the nature of the solvent.
B- ACT KALLRAUCH:
- Is a weak electrolyte : ?? ⇌ ?− +?+
- The conductivity limit of a low electrolyte is the sum of each boundary conductivity of these ions.
- one can determine the conductivity of a low electrolyte from the conductivity of a strong electrolyte :
??? = ? ×???∞
?∝(??) = ?∝(?−) + ?∝(?+)
EQUILIBRIUM CONSTANT AND CONSTANT ACIDITY (ELECTROLYTE LOW BIT AB) :
|FROM + H20 = A– + B+|
|Etat initial||Cm 0 0|
|Etat final||(1-a). Cm a.Cm a.Cm|
dissolve 0,1 mol weak acid, AH noted in a volume V = 1l water. The of this acid dissociation coefficient a = 0,08. Determine :
- the acidity constant K
- l’osmolarité ? of the solution
?? + ?2? = ?− + ?3?+
??= 0,1 ???/?
- K = ??.??/?−? = 0.1.(?.??)?/?−?.??= 6,96.10−4
- ? =?? = ??(1-a ) + a ?? + a ?? = ??(1+ a ) =0,1 × (1 + 0,08) = 0,108 ?????/? = 108 mosmll
The equilibrium constant K :
- depends on the solute
- depends on the nature of the solvent
- Increases with temperature
DETERMINATION OF DISSOCIATION RATE :
- If a "1 we can write K = Ca2 therefore α = √ ? / ??
- If a is not small front 1, we must solve the equation of 2nd degree has :
??a? + ?a - ? = ?
ELECTROLYTE DU TYPE BA2
BA2 ⇌ B2 + + 2A-
?? (1 – a ) ⇌ ?? a + 2 ??
VARIATION DE αA
• Change in dissociation coefficient for an electrolyte of the type BA2
BALANCE DONNAN :
There's macromolecules side an excess of & rsquo; diffusible ions, d & rsquo; where a higher pressure than normal osmotic pressure, this new pressure carries the name oncotic pressure. The two compartments contain an ionic solution containing & rsquo; water and ions can freely pass through the membrane (diffusible ions).
Is opposed through a membrane macromolecules dialysanteune solution ionized KPR a KCI aqueous solution e.g., ion concentrations on both diffusible & rsquo; across the membrane can not s & rsquo; even because of the presence of charged macromolecules that do not broadcast.
- L & rsquo; balance is disturbed by the presence in the compartment 1 d & rsquo; a charged macromolecule M nondiffusible.
POTENTIAL DONNAN :
- Due to the asymmetric distribution of ions on either & rsquo; across the membrane, there is a potential difference called Donnan potential (between the two compartments (potential which allows & rsquo; cancel the diffusive flux of ions).
- Because ion concentration imbalance, osmotic pressure is greatly reduced, c & rsquo; is & rsquo; Donnan effect.
CALCULATE THE POTENTIAL DIFFERENCE :
♦ When metal plunged in a solution containing the & rsquo; a salt, a potential difference appears.
♦ At & rsquo; balance, the thermodynamic work of dissolution is equal to the electrical work recombination of ions on the & rsquo; electrode : ?.?.????/??= ?.?.?
C2 : concentration of the medium 2
C1 : concentration of the medium 1
ℱ : faraday
V : potential
♦ The potential difference between the realized 2 solutions is given by the law of Nerst:
? = ??/?? .?? ??/??
- To clear the & rsquo; Donnan effect, we must increase the salt concentration severable.
- L & rsquo; Donnan effect is significant in the case of blood capillaries and in kidney dialysis to :
- to control the salt trade
- minimize osmotic pressure.
- d & rsquo; avoid high migration & rsquo; water through the membrane.
PROPERTIES COLLIGATIVESDES ELECTROLYTES
ELECTROLYTE SOLUTIONS :
- Dissociation and dissociation rates have a significant effect on colligative properties of electrolyte solutions.
- One factor has been proposed to account for these deviations :
ʋ = measured colligative property / valeu expected for non - electrolyte
where o is the coefficient of Van'tHoff.
LE COEFFICIENT DE VAN'THOFF :
- To account for the presence of an electrolyte, must be modified mathematical expressions of colligative properties by multiplying the concentration of solute by the coefficient.
- For non-electrolyte solutes, the coefficient of equal to Van'tHoffest 1.
- For electrolyte solutes, u coefficient is greater than 1. ʋ = 1 + a (i-1)
a : The rate or dissociation coefficient.
i : number of free molecules dissociated ions of the solute.
ʋ : name d'osmoles.
FACTOR-VAN'THOFF EXAMPLE :
NaCl solid dissociates 2 ions in water: NaCl -> Na+ + Cl–
For each mole of dissolved solid, one obtains two osmoles ion in solution.
Colligative properties depend on the concentration of particles in solution; lowering the freezing point of a NaCl aqueous solution should be twice that of an aqueous solution of a non-electrolyte having the same molality. o should be near 2 for NaCl in solution (for a total dissociation).
COEFFICIENT VAN'THOFF-Equations :
D????? = ʋ?? × ??
D?is? = ʋ?? × ?is?
? = ʋ.?/?.??
(n : amount of material of non-diffusively substances)
ELECTROLYTES AND BIOLOGY :
Often the biological action of a compound occurs only if it is dissociated.
- Potassium cyanide is dangerous dissociated,
- morphine salts have a particularly powerful action that they are ionized.
- heavy metal cations are highly toxic;
Generally any ion imbalance in the body causes serious disorders.
A- DRUG DIÉLECTROLYSE :
It uses the action of the electric fields to enter the body ionized anti-inflammatory drugs.
exp : at a joint rheumatology and sports medicine
Leduc's experience has shown the role of an electric field to penetrate the ions in the tissues in the early century.
(rabbit connected to the anode attracting CN- dies, while the rabbit connected to the cathode K + attracting survives).
B- conductometric titration :
♦ can be studied a neutralization reaction (acid-base reaction) by chemical titration conductiméctrique; the neutralization point corresponds to the minimum conductivity of the solution.
♦ Effectively, this is the point of neutralization as the more mobile H + ions and OH- are at the lowest concentration.
C- DOSAGE CONDUCTIMÉTRIQUE :
• The conductivity is determined by the total concentration of monovalent ions (Cl-, na +, K+, CO3H-) blood plasma.
• concentration of proteins pondéraleCpp (g/l) slowing down the ions travel,
it is necessary to calculate the corrected conductivity from the conductivity measured by the following empirical formula
???? = ???? × ???/??? − ?,????
Course of Dr Allouache – Faculty of Constantine