Modelo Mental Inexacto

This theory is introduced by This theory is introduced by FriteFrite London in 1930. It is known London in 1930. It is known

as London dispersion forces. as London dispersion forces.

In (a) the nonIn (a) the non--polar molecule which does not have a dipole polar molecule which does not have a dipole

In (a) the nonIn (a) the non--polar molecule which does not have a dipole polar molecule which does not have a dipole

within the molecule begin to fluctuate and thus forming a within the molecule begin to fluctuate and thus forming a

“temporary” dipole as in (b). Thus the forces of attraction will “temporary” dipole as in (b). Thus the forces of attraction will formed between the temporary dipole and this forces is

formed between the temporary dipole and this forces is named as

7.2 Effect of the intermolecular forces ( Van

7.2 Effect of the intermolecular forces ( Van derder waalswaals ) on the ) on the physical properties of the molecules

physical properties of the molecules

H H _{vapourisationvapourisation} give a quantitative measurement of strength of give a quantitative measurement of strength of

attractive forces present in liquid. So,

attractive forces present in liquid. So, 

H H _{vapourisationvapourisation} , ,  thethe boiling point ,

boiling point ,  the intermolecular forces among its molecules. the intermolecular forces among its molecules.

When a molecule increase in size, the number of electron also When a molecule increase in size, the number of electron also

increase, so the

increase, so the attraction between the electron valence and attraction between the electron valence and nucleus become less

nucleus become less. This distortion of electron cloud can easily . This distortion of electron cloud can easily occur and increase the

occur and increase the polarisabilitypolarisability of the negative ion. of the negative ion. occur and increase the

occur and increase the polarisabilitypolarisability of the negative ion. of the negative ion.

This can be relating with the dispersion forces among molecules This can be relating with the dispersion forces among molecules

therefore

therefore

H H _{vapourisationvapourisation}  , e.g. : Value of boiling point of , e.g. : Value of boiling point of halogen gas increase.

halogen gas increase. ( from F( from F_{2 2}  II₂₂ ))

In hydrocarbon, boiling point increase with relative molecular In hydrocarbon, boiling point increase with relative molecular

mass (RMM). Molecule with higher RMM will have a higher mass (RMM). Molecule with higher RMM will have a higher boiling point.

boiling point.

The effect of branched chain in hydrocarbon will also affect the The effect of branched chain in hydrocarbon will also affect the

boiling point of hydrocarbon involved boiling point of hydrocarbon involved

Structure RMM Boiling point (°C) 2,2–dimethyl propane 72 4 2-methylbutane 72 18

This is due to a larger surface area in a straight chain of This is due to a larger surface area in a straight chain of

hydrocarbon, and allows greater forces between the hydrocarbon, and allows greater forces between the molecules

molecules –– giving larger Van giving larger Van derder Waals forces Waals forces –– compare to compare to branch chain hydrocarbon

branch chain hydrocarbon

2-methylbutane 72 18

7.3

7.3 Hydrogen BondingHydrogen Bonding

Hydrogen bond is a Hydrogen bond is a special dipolespecial dipole––dipole interaction between dipole interaction between

H atom with other

H atom with other atom with high atom with high electronegativityelectronegativity. ( N, O, F ). ( N, O, F )

It is extra stable than normal Van It is extra stable than normal Van derder waalswaals forces and forces and

required a high energy to break the bond. This explained why required a high energy to break the bond. This explained why the boiling point of NH

the boiling point of NH₃₃, H, H₂₂O and HF are higher than other O and HF are higher than other hydrogen compound from each of their particular group. hydrogen compound from each of their particular group.

Decreasing molar massDecreasing molar mass

Hydrogen bond can also be used to explain the different of Hydrogen bond can also be used to explain the different of

boiling point of some organic compound. In the diagram above, boiling point of some organic compound. In the diagram above, the trend of the compound in the same group deviates for N, O the trend of the compound in the same group deviates for N, O and F, as it form hydrogen bond among themselves.

and F, as it form hydrogen bond among themselves.

Hydrogen bond can be compared among NHHydrogen bond can be compared among NH₃₃ , H, H₂₂O and HF. O and HF.

HF has a higher boiling point than NH

HF has a higher boiling point than NH₃₃ due to higher due to higher electronegativity

electronegativity of fluorine compare to nitrogen. So the dipole of fluorine compare to nitrogen. So the dipole moment of H

moment of H––F is greater than NF is greater than N––H, which results greater H, which results greater hydrogen bond. Though, O has a lower

hydrogen bond. Though, O has a lower electronegativityelectronegativity than F, than F, but H

but H₂₂O has a greater boiling point compare to HF because in O has a greater boiling point compare to HF because in between H

between H₂₂O O --- HH₂₂O molecules, they can form 2 hydrogen O molecules, they can form 2 hydrogen bond between the molecule but between HF

bond between the molecule but between HF --- HF can only HF can only form one hydrogen bond. So, the more the hydrogen formed, form one hydrogen bond. So, the more the hydrogen formed, greater the forces, higher the boiling point.

greater the forces, higher the boiling point.

The factors of hydrogen bonding can also use to explain the The factors of hydrogen bonding can also use to explain the

solubility of some organic compound in water, like example, solubility of some organic compound in water, like example, ethane cannot dissolve in water but ethanol can dissolve in ethane cannot dissolve in water but ethanol can dissolve in water, due to the hydrogen bonding.

Some of the molecules gain more stability by forming Some of the molecules gain more stability by forming dimerdimer

with its molecules

with its molecules. E.g. : When . E.g. : When ethanoicethanoic acid is brought to acid is brought to

mass spectrometer for detection and it gives a peak at m/e at mass spectrometer for detection and it gives a peak at m/e at 120. This indicates the shows that

120. This indicates the shows that ethanoicethanoic acid (CHacid (CH₃₃COOH) COOH) has a RMM of 120, as CH

has a RMM of 120, as CH₃₃COOH , RMM = 60.COOH , RMM = 60.

This indicate This indicate ethanoicethanoic acid exist as acid exist as dimerdimer where interaction of where interaction of

hydrogen bonding between end of each functioning group hydrogen bonding between end of each functioning group –– COOH occur.

There is another application of hydrogen bond, which is the There is another application of hydrogen bond, which is the

intermolecular forces and

intermolecular forces and intramolecularintramolecular forces. In 2forces. In 2-- nitrophenol and 4

nitrophenol and 4--nitrophenol, the boiling point of the 2 nitrophenol, the boiling point of the 2 compounds can be explain below :

compounds can be explain below :

Since 2Since 2--nitrophenol form strong hydrogen bond as nitrophenol form strong hydrogen bond as

intramolecular

intramolecular forces, the interaction between 2forces, the interaction between 2--nitrophenol nitrophenol molecules are weaker among each other, compare to 4

molecules are weaker among each other, compare to 4-- nitrophenol, which used hydrogen bond as their

nitrophenol, which used hydrogen bond as their

intermolecular forces. With stronger hydrogen bond which act intermolecular forces. With stronger hydrogen bond which act as the intermolecular forces, the boiling point of 4

as the intermolecular forces, the boiling point of 4--nitrophenol nitrophenol is expected to be higher than 2