Intermolecular Forces In the liquid and sold states, molecules are held together

Question

Intermolecular Forces In the liquid and sold states, molecules are held together by attractions called intermolecular forces. There are several types of intermolecular forces London dispersion forces, found in all substances, result from the motion of electr These work to attract both polar and nonpolar molecules to one another via instantaneous dipole moments Dipole dipole forces aise from molecular moments lon dipole forces result from the interaction of an ion and a molecular dipole. Hydrogen bond forces result from the attraction of a atom bonded to a small highly electronegative atom (N, o and F and the unshared electron pairs of another electronegative atom Physical properties such as boiling point, mehing point, vapor pressure, and surface tension are al affected by the strengh of the intermolecular forces within a substance Part A What happens to these physical properties as the strength of intermolecular forces increases? Drag each item to the appropriate bin. Reset Help

Explanation / Answer

Solution:- (a) Stronger are the forces, more energy is required to melt the compound. So, melting point increases as the intermolecular forces increases.

Vapor pressure decreases as the intermolecular forces increases since less molecules are able to escape the liquid phase to the vapor phase.

Boiling point increases as the intermolecular forces are increases because more energy is required to over comes these forces and bring the molecule into vapor phase.

Viscosity is the resistance of a liquid to flow. Stronger are the intermolecular forces, less would be the flow and higher will be the viscosity. So, the viscosity increases as the intermolecular forces increases.

Surface tension increases as the intermolecular forces increases since surface tension is caused by the attraction of the particles in the surface layer by the bulk of the liquid.

(b) Strongest to weakest order of intermolecular forces is...

ion-dipole > hydrogen bonding > dipole-dipole > London dispersion

(c) CF4 is tetrahedral and symmetrical molecule and so it is non polar as the dipole moment of one C-F bond is canceled out by it's opposite C-F bond. bening non polar it has London dispersion forces.

HF is a polar molecule and also it has hydrogen bonding since H is bonded to the most electron negative atom.

CHF3 and OF2 would be polar since there is electron negativity difference. CHF3 is polar as it has net dipole moment. OF2 would also have net dipole moment since the molecule is bent. So, both of these would have dipole-dipole forces.

(d) Weakest are the forces, lower would be the boiling points. Hydrocarbons have London dispersion forces that increases as the molar mass increases.

Highest boiling point would be for the parafin C36H74 since it's molar mass is highest in the given ones. Next is Octadecane since it is the one with second highest molar mass. 2,2-dimethylpropane is branched alkane where as pentane is straight chain alkane. forces are somehow stronger for the branched chain alkanes as they are best fitted and so their boiling points are higher as compared to the straight chain one. Lowest one would be butane. So, highest to lowest biling point order would be..

Parafin > Octadecane > 2,2-dimethylpropane > pentane > butane

(e) Vapor pressure is decreased if the intermolecular forces are stronger since the stronger forces try to keep the molecules into the liquid phase.

Butanol (the one with OH group) has hydrogen boding and so stronger forces, hence it's vapor pressure woudl be lowest.

Highest vapor pressure would be for methane(CH4) since it has weakest london dispersion forces due to the lowest molar mass in the given ones.

Pentane has higher vapor pressure as compared to 2-methylbutane due to the branching in 2-methylbutane.

so, Highest to lowest vapor pressure order would be..

Methane > pentane > 2-methylbutane > Butanol

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