# 5.1.1: Intermolecular Forces (Problems)

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PROBLEM $$\PageIndex{1}$$

In terms of their bulk properties, how do liquids and solids differ? How are they similar?

Liquids and solids are similar in that they are matter composed of atoms, ions, or molecules. They are incompressible and have similar densities that are both much larger than those of gases. They are different in that liquids have no fixed shape, and solids are rigid.

PROBLEM $$\PageIndex{2}$$

In terms of the kinetic molecular theory, in what ways are liquids similar to solids? In what ways are liquids different from solids?

Particles in liquids and solids are close together, but in liquids they have no regular arrangement or fixed positions.

PROBLEM $$\PageIndex{3}$$

In terms of the kinetic molecular theory, in what ways are liquids similar to gases? In what ways are liquids different from gases?

They are similar in that the atoms or molecules are free to move from one position to another. They differ in that the particles of a liquid are confined to the shape of the vessel in which they are placed. In contrast, a gas will expand without limit to fill the space into which it is placed.

PROBLEM $$\PageIndex{4}$$

Explain why liquids assume the shape of any container into which they are poured, whereas solids are rigid and retain their shape.

Particles in a solid vibrate about fixed positions and do not generally move in relation to one another; in a liquid, they move past each other but remain in essentially constant contact

PROBLEM $$\PageIndex{5}$$

What is the evidence that all neutral atoms and molecules exert attractive forces on each other?

All atoms and molecules will condense into a liquid or solid in which the attractive forces exceed the kinetic energy of the molecules, at sufficiently low temperature.

PROBLEM $$\PageIndex{6}$$

The types of intermolecular forces in a substance are identical whether it is a solid, a liquid, or a gas. Why then does a substance change phase from a gas to a liquid or to a solid?

Intermolecular forces serve to hold particles close together, whereas the particles’ kinetic energy provides the energy required to overcome the attractive forces and thus increase the distance between particles. Changes in physical state may be induced by changing the temperature, hence, the average KE, of a given substance

PROBLEM $$\PageIndex{7}$$

Why do the boiling points of the noble gases increase in the order He < Ne < Ar < Kr < Xe?

The London forces typically increase as the number of electrons increase.

PROBLEM $$\PageIndex{8}$$

Neon and HF have approximately the same molecular masses.

1. Explain why the boiling points of Neon and HF differ.
2. Compare the change in the boiling points of Ne, Ar, Kr, and Xe with the change of the boiling points of HF, HCl, HBr, and HI, and explain the difference between the changes with increasing atomic or molecular mass.

Ne has only dispersion forces, whereas HF is polar covalent and has hydrogen bonding, dipole-dipole, and dispersion forces.

 Compound Boiling Point (°C) Compound Boiling Point (°C) Ne -246 HF 19.5 Ar -185.8 HCl -85 Kr -153.4 HBr -66 Xe -108.1 HI -34

The boiling point of the noble gases increases as you increase the molecular weight because of the increasing strength of the dispersion forces.

For the hydrogen halides, HF does not follow this pattern because it has hydrogen bonding while the other three only has dipole-dipole interactions.

PROBLEM $$\PageIndex{9}$$

Arrange each of the following sets of compounds in order of increasing boiling point temperature:

1. HCl, H2O, SiH4
2. F2, Cl2, Br2
3. CH4, C2H6, C3H8
4. O2, NO, N2

SiH4 < HCl < H2O

F2 < Cl2 < Br2

CH4 < C2H6 < C3H8

N2 < O2 < NO

PROBLEM $$\PageIndex{10}$$

The molecular mass of butanol, C4H9OH, is 74.14; that of ethylene glycol, CH2(OH)CH2OH, is 62.08, yet their boiling points are 117.2 °C and 174 °C, respectively. Explain the reason for the difference.

ethylene glycol contains two OH groups which increase the polarity

PROBLEM $$\PageIndex{11}$$

On the basis of intermolecular attractions, explain the differences in the boiling points of n–butane (−1 °C) and chloroethane (12 °C), which have similar molar masses.

Only rather small dipole-dipole interactions from C-H bonds are available to hold n-butane in the liquid state. Chloroethane, however, has rather large dipole interactions because of the Cl-C bond; the interaction is therefore stronger, leading to a higher boiling point.

PROBLEM $$\PageIndex{12}$$

On the basis of dipole moments and/or hydrogen bonding, explain in a qualitative way the differences in the boiling points of acetone (56.2 °C) and 1-propanol (97.4 °C), which have similar molar masses.

1-propanol contains an OH group, which makes it more polar.

PROBLEM $$\PageIndex{13}$$

The melting point of H2O(s) is 0 °C. Would you expect the melting point of H2S(s) to be −85 °C, 0 °C, or 185 °C? Explain your answer.

−85 °C. Water has stronger hydrogen bonds so it melts at a higher temperature.

PROBLEM $$\PageIndex{14}$$

Identify the intermolecular forces present in the following solids:

1. CH3CH2OH
2. CH3CH2CH3
3. CH3CH2Cl

hydrogen bonding and dispersion forces

dispersion forces