General characteristics of nonelectrolytes solutions. Water solutions of nonelectrolytes. Acid-base characteristics of solutions

1.  GENERAL CHARACTERISTICS OF NONELECTROLYTES SOLUTIONS

Main laws and ratios of solutions’ theory are correct for model systems called the ideal solutions.

     Ideal solutions name solutions at which formation neither thermal, nor volumetric changes in system do not occur.  Characteristics of ideal solutions don’t depend on the nature of a dissolvent  and the soluble substance but depend just on the quantity of particles (It is accepted that interaction of particles of dissolvent and the soluble substance are absent).  The laws of ideal solutions can be applied to real ones, extrapolating obtained data to maximal dilution of these solutions, for example, working with the diluted solutions of nonelectrolytes.  In such solutions the distance between molecules of soluble substance is big and its mutual influence can be excluded.   Only for such very diluted solutions  it was possible to deduce quantitative ratios between characteristics of solutions and their concentration.

     The laws describing such important characteristics of solutions as a boiling-point and freezing-point  of solutions, a size of steam pressure above solutions etc. are received for the first time for solutions of nonelectrolytes. As it is seen from the common enumeration these characteristics are connected with a change of a phase condition of systems, transition of solutions’ components from one aggregate condition to another.

     Later on when we will study water solutions we always should remember that ratios and equations we use are true only for ideal and proper solutions, that is for solutions at which the soluble substance is in the form of molecules to an ion in which size does not exceed  10-8cm.  Further we will always examine solutions of fixed, solid and liquid substance which fumes above the solutions we can neglect.

1.1Raul’s law

Raul’s law describes a change of the pressure of saturated steam of dissolvent above the solution.   If we place a liquid to a closed vessel then an evaporation of liquid will take place till you set the balance between the quantity of particles transforming from liquid phase into gas phase (evaporation rate) and the quantity of particles transforming from gas phase (vaporous) into liquid phase (condensation rate).  Steam pressure of a dissolvent above a liquid which appear at this is a constant at given temperature and has got a name pressure (or elasticity) of saturated steam.

At dilution of any solid (fixed) substance in a liquid the concentration of molecules of dissolvent will reduce, the number of molecules transforming into unit time from liquid phase to vaporous phase will decrease. As a result the balance between liquid and its steam will be set at the less concentration of steam (less pressure).  Thus, a soluble substance reduce steam pressure of dissolvent above the solution.

In 1866 French scientist Raul F. studied solutions of different fixed liquids and solid substances and formulated the following law:

“FOR DILUTED SOLUTIONS OF NONELECTROLYTES RELATIVE PRESSURE DECLINE OF SATURATED STEAM OF DISSOLVENT ABOVE THE SOLUTION EQUALS TO MOLAL PART OF THE SOLUBLE SUBSTANCE”

Raul’s law in analytic way:

P0-P  =∆P/ P0 = N2                          N2=n2/ n2+ n1

P

P0 – pressure of saturated steam above the pure dissolvent;

P – pressure of saturated steam of dissolvent above the solution at the same temperature;

P0-P     relative steam pressure decline;

P

N2 – molal part of soluble substance

Raul’s law is true for diluted solutions when the number of  mols of soluble substance (n2)  is very small. (n1>> n2)

Physiological solutions the water is as a dissolvent with molecular weight M=18, then n1=1000/18 and n1>> n2.

∆P/ P0= N2 = n2/ n2+ n1= n2/1000/48= 0.018 n2

1.2. DECLINE OF FREEZING POINT AND INCREASE OF BOILING POINT OF SOLUTIONS

Freezing points and boiling points of fixed substance depend on the pressure of saturated steam of the solution.  The liquid freezes when the pressure of saturated steam above the liquid will be equal to the pressure of steam above the same substance in solid condition and it boils when the pressure of its steam becomes equal to external pressure. For example, at 0°C the pressure of  ice steam and liquid water is 4,6 mmHg.  Since the pressure of  solution steam is lower than the pressure of pure dissolvent steam then solutions boil at higher temperature and freeze at lower temperature than pure dissolvent.

The consequence of Raul’s law is the decline of freezing point (∆tf) and increase of  boiling point (∆tb) of  solution comparing to pure dissolvent is proportionate to molar concentration of soluble substance.

∆tf = KCm    ∆tb=ECm

where ∆tb=∆tb - ∆t°b  - difference of boiling points of solution (t) and pure dissolvent (t°);

∆tf=∆t°f - ∆tf – difference of freezing points of pure dissolvent (t°) and solution (t);

K – cryoscopic constant;

E – ebullioscopic constant;

Cm – molar concentration of solution

Experimentally established that at dilution of 1 mol of nonelectrolyte (for example glucose, sucrose, urea and etc.) in 1000 grams of water  freezing point declines on 1,86° and boiling point increases on 0,52°.  The first size is called cryoscopic constant of water and the second - ebullioscopic constant of water.  Values of these constants correspond with molar decline of boiling point for ideal one molar solution. Values of cryoscopic and ebullioscopic constants depend on the nature of dissolvent and don’t depend on the nature of soluble substance (Table 1).

The possibility of precise measurement of freezing point decline (∆tf) and boiling point increase (∆tb) lets to calculate the meanings of molar mass of different substances

∆tb = 1000*E*mb/M*mp                           ∆tf  = 1000*K* mb/M*mp

where M – molar mass of soluble substance, gram/mol;

mp – dissolvent mass, gram;

mb – soluble substance mass, gram;

Table 1

Cryoscopic and Ebullioscopic constants

Dissolvent	Boiling point	E. °C/mol	Freezing point	K.  °C/mol
Water	100	0,513	0	1,66
Acetone	56	1,71	-	-
Benzol	80,15	2,53	5,48	5,12
Ethyl alcohol	78,40	1,22	114,6	1,99
Acetic acid	118,1	2,93	17	3,9
Chloroform	61,26	3,63	-63,5	4,66

Problem: Solution containing 30 grams of substance in 1000 grams of water freezes at -0,31°C.  Define molar mass of soluble substance

Solution: Cryoscopic constant for water is equal to 1,86°.

Inserting data of the problem into corresponding equation we find molar mass of soluble substance:

M=1,86*30/0,31 = 180 gram/mol

Method of definition of molar mass of the substance at decline of freezing point of solution is called cryoscopic, at increase of boiling point – ebullioscopic.