SURFACE TENSION AND CHEMICAL CONSTITUTION PARACHOR

Surface tension to due to an inward force acting on the molecules at the surface of a liquid and is, therefore, considered to be dependent on the structure of molecules

The Parachor

In 1923, D.B. Macleod suggested an empirical relationship between the surface tension and density of a liquid, which may be stated as

                   

γgamm+a

𝛾

D-d=C

 where D and d are the densities and its vapour, respectively, y is the surface tension at the same temperature and C is a characteristic constant of the liquid                                               

S. Sugden (1924) obtained a relationship by multiplying Macleod equation with the molecular mam. M. of the liquid, and called the new constant as Parachor [P] 

 SURFACE TENSION AND CHEMICAL CONSTITUTION PARACHOR

Surface tension to due to an inward force acting on the molecules at the surface of a liquid and is, therefore, considered to be dependent on the structure of molecules

The Parachor

In 1923, D.B. Macleod suggested an empirical relationship between the surface tension and density of a liquid, which may be stated as                   

γgamm+a

𝛾

D-d=C

 where D and d are the densities and its vapour, respectively, y is the surface tension at the same temperature and C is a characteristic constant of the liquid

 . Sugden (1924) obtained a relationship by multiplying Macleod equation with the molecular mam. M. of the liquid, and called the new constant as Parachor [P] 

                 MY1-4/D-d=MC=[p]

At ordinary temperature, the density of vapour, d, is negligible as compared with D for the liquid, the equation (8) redues to

                 MY1-4/D=[p]

Or.            VmY1-4=[P]      (since M/D=Vm)

where Vm is the molar volume of the liquid. If surface tension y is unity (ie., gamma = 1 ) then equation (10) may be written as

                Vm=[p]

Thus, the Parachor [P] may be defined as the molar volume of a liquid at a temperature at which its surface tension is unity. It is approximately independent of temperature. It was shown by Sugden that Parachor is both additive and constitutive property and its value for any compound can be expressed as the sum of two sets of constants, one depending on the atoms present and the other upon the structural factor. The former is called atomic structural parachor and the latter is called structural parachor. 

For two liquids 1 and 2

             M1Y1 ¼ /D1.   = [p1]             (11)

             M2Y2  ¼ D2.  =[p2]               ( 12)

Divided equation(12)by Eq.(11),ifY1=Y2,we get

        [P1]/[P2]=M1Y1 ¼/D1÷M2Y2 ¼ /D2=M1/D1÷M2/D2=( Vm)1÷(Vm)2.                                      (13)

Thus, a comparison of parachor means, the comparison of molar veiumes under such conditions that the liquids have the same surface tensions.

Parachor and Chemical Constitution-Uses of Parachor in Elucidating Structures 

The comparison of experimental parachor values with the theoretically calculated calues of a compound helps us to decide about its chemical constitution as illustrated by the Following examples

Deciding constitution-Structure of Benzene If the Kekule formula for benzene be aneepted, the value of ita parachor can be calculated using Vogel's data

 

 6 carbon atoms.  6×8.6 = 51.6

 

 6 hydrogen atom.   6x15.7 = 94 2

 

 3 double bonds.   3x19.9 =59.7

 

 6 membered ring.              =1.4

 

Calculated parachor value for benzene =  206.9

 

 The experimental parachor value for benzene is 206.2. which is, therefore, in agreement with Kekule's formula.

 

Kekule formula:

           

        

Deciding the Nature of Valency Bonds.

 The parachor has also been found useful in providing information regarding the nature of bonds in certain groups. The nitre group (NO₂) for example, may be represented

      O                 O               0

_N \\            _N//             _N\

      \\                 \ O              \O

       O

1.                  II                  III

[P]= 98.9         [P]=74.1      [49.3]

The experimental value of parachor for - N * O_{2} group has been found to be 73.0. which is obviously in favour of the structure II.

 Existence of singlet linkage

Sugden suggested the existence of singlet linkage in compounds like PCI, and S*F_{0} A singlet linkage is a coordinate linkage formed by the donation of one electron onlythus in such a case we have the sharing of a single electron instead of the usual lone pair.

 

    (5 covalent linkage)        3 covalent and 2 single linkage

     [P]=316.9                      [P]=284

 

The experimental value for the parachor of PCls is 282.5, which is in agreement with the proposed structure (II) involving two singlet linkage and confirms the existence of two single-electron linkages in PCl, molecule. This prediction is supported by the observation that two of the chlorine atoms are easily eliminated on heating. 1

 

                    PCI→PCl3+ Cl₂

 

The Position of Substituent in an Aromatic Ring does not change the parachor value of the compound. The observed value of o-chlorotoluene is 280.8 and for p-chlorotoluene is 283.6. The theoretically calculated value for both the isomers is the same and is 283.3. Hence, the positional isomerism does not affect the parachor value.

 

 

 

                 MY1-4/D-d=MC=[p]

At ordinary temperature, the density of vapour, d, is negligible as compared with D for the liquid, the equation (8) redues to

                 MY1-4/D=[p]

 

Or.            VmY1-4=[P]      (since M/D=Vm)

 

where Vm is the molar volume of the liquid. If surface tension y is unity (ie., gamma = 1 ) then equation (10) may be written as

                Vm=[p]

 

Thus, the Parachor [P] may be defined as the molar volume of a liquid at a temperature at which its surface tension is unity. It is approximately independent of temperature. It was shown by Sugden that Parachor is both additive and constitutive property and its value for any compound can be expressed as the sum of two sets of constants, one depending on the atoms present and the other upon the structural factor. The former is called atomic structural parachor and the latter is called structural parachor.

 

For two liquids 1 and 2

             M1Y1 ¼ /D1.   = [p1]             (11)

             M2Y2  ¼ D2.  =[p2]               ( 12)

Divided equation(12)by Eq.(11),ifY1=Y2,we get

        [P1]/[P2]=M1Y1 ¼/D1÷M2Y2 ¼ /D2=M1/D1÷M2/D2=( Vm)1÷(Vm)2.                                      (13)

 

Thus, a comparison of parachor means, the comparison of molar veiumes under such conditions that the liquids have the same surface tensions.

 

 

 

 

 

Parachor and Chemical Constitution-Uses of Parachor in Elucidating Structures

 

The comparison of experimental parachor values with the theoretically calculated calues of a compound helps us to decide about its chemical constitution as illustrated by the Following examples

 

Deciding constitution-Structure of Benzene If the Kekule formula for benzene be aneepted, the value of ita parachor can be calculated using Vogel's data

 

 6 carbon atoms.  6×8.6 = 51.6

 

 6 hydrogen atom.   6x15.7 = 94 2

 

 3 double bonds.   3x19.9 =59.7

 

 6 membered ring.              =1.4

 

Calculated parachor value for benzene =  206.9

 

 The experimental parachor value for benzene is 206.2. which is, therefore, in agreement with Kekule's formula.

 

Kekule formula:

           

                 

 

Deciding the Nature of Valency Bonds.

 

 The parachor has also been found useful in providing information regarding the nature of bonds in certain groups. The nitre group (NO₂) for example, may be represented

      O                 O               0

_N \\            _N//             _N\

      \\                 \ O              \O

       O

1.                  II                  III

 

[P]= 98.9         [P]=74.1      [49.3]

 

The experimental value of parachor for - N * O_{2} group has been found to be 73.0. which is obviously in favour of the structure II.

 

Existence of singlet linkage

Sugden suggested the existence of singlet linkage in compounds like PCI, and S*F_{0} A singlet linkage is a coordinate linkage formed by the donation of one electron onlythus in such a case we have the sharing of a single electron instead of the usual lone pair.

 

    (5 covalent linkage)        3 covalent and 2 single linkage

     [P]=316.9                      [P]=284

 

s

 

The experimental value for the parachor of PCls is 282.5, which is in agreement with the proposed structure (II) involving two singlet linkage and confirms the existence of two single-electron linkages in PCl, molecule. This prediction is supported by the observation that two of the chlorine atoms are easily eliminated on heating. 1

 

                    PCI→PCl3+ Cl₂

 

The Position of Substituent in an Aromatic Ring does not change the parachor value of the compound. The observed value of o-chlorotoluene is 280.8 and for p-chlorotoluene is 283.6. The theoretically calculated value for both the isomers is the same and is 283.3. Hence, the positional isomerism does not affect the parachor value.