Chapter 11: Liquids and Intermolecular Forces

General practice questions covering intermolecular forces, properties of liquids and solids, phase changes, and vapor pressure based on Chapter 11 lecture notes.

What characterizes the molecular arrangement and physical properties of a gas?

Gases are highly compressible, assume the shape and volume of their container, and have molecules that are far apart with minimal interaction.

What is the difference between a liquid and a solid in terms of shape and volume?

Liquids assume the shape of their container but have a definite volume, while solids have both a definite shape and a definite volume.

Which phases of matter are classified as condensed phases?

Solids and liquids.

What describes a solid with a highly ordered structure?

Crystalline.

How do intermolecular forces compare in strength to ionic or covalent bonds?

Intermolecular forces are much weaker, typically measuring less than $15\%$ as strong as a covalent or ionic bond.

What three types of intermolecular attractions exist between neutral molecules?

Dispersion forces, dipole-dipole forces, and hydrogen bonding forces.

Which two types of forces are collectively known as van der Waals forces?

Dispersion forces and dipole-dipole forces.

What is the definition of dispersion forces (London dispersion forces)?

Temporary attractions resulting from the formation of instantaneous dipoles and induced dipoles in adjacent molecules or atoms.

Define polarizability.

The ease with which an electron distribution (electron cloud) can be deformed.

How does molecular size and weight affect dispersion forces?

As molecular weight and the number of electrons increase, polarizability increases, leading to stronger dispersion forces.

How does molecular shape influence the strength of dispersion forces?

The greater the surface area available for contact, the greater the dispersion forces; for example, cylindrically shaped molecules have stronger forces than spherical ones.

Between which types of molecules do dipole-dipole interactions exist?

Neutral polar molecules.

What conditions are required for hydrogen bonding to occur?

Hydrogen must be bonded to a small, highly electronegative element (usually $F$, $O$, or $N$), and there must be an unshared electron pair on a nearby small electronegative ion or atom.

What is the typical range for the bond energies of hydrogen bonds?

From about $4\,kJ/mol$ to $25\,kJ/mol$.

Why does ice float on water?

Ice is less dense than liquid water because its molecules are arranged in an open, regular hexagon structure that optimizes hydrogen bonding.

What is an ion-dipole force?

An interaction between an ion (such as $Na^+$) and the partial charge on the end of a polar molecule (such as water).

What is the relative strength order of intermolecular forces for neutral species from weakest to strongest?

Dispersion forces < dipole-dipole forces < hydrogen bonding.

Define viscosity.

The resistance of a liquid to flow.

How does temperature usually affect viscosity?

Viscosity usually decreases with an increase in temperature.

Define surface tension.

The amount of energy required to increase the surface area of a liquid by a unit amount.

What is the difference between cohesive and adhesive forces?

Cohesive forces bind molecules to one another, while adhesive forces bind molecules to a surface.

Under what condition is a meniscus U-shaped, such as water in glass?

When adhesive forces are greater than cohesive forces.

Identify the phase changes that are endothermic ($\Delta H > 0$).

Melting (fusion), vaporization, and sublimation.

Identify the phase changes that are exothermic ($\Delta H < 0$).

Deposition, condensation, and freezing.

Why does temperature remain constant during a phase change on a heating curve?

The added energy is used to break intermolecular bonds rather than increase the kinetic energy (temperature) of the molecules.

What is the critical temperature?

The highest temperature at which a substance can exist as a liquid.

Define dynamic equilibrium in the context of vapor pressure.

A condition where the rate of molecules escaping the liquid surface into the gas phase equals the rate of gas molecules returning to the liquid phase.

What is the normal boiling point?

The boiling point of a liquid at a pressure of $760\,mm\,Hg$ ($1\,atm$).