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General Chemistry Study Guide

Chapter 12. Intermolecular Forces and Liquids and Solids

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Yu Wang

OpenStax 10 Liquids and Solids. Brown 11 Liquids and Intermolecular Forces. 12 Solids and Modern Materials.

1. Intermolecular Forces

This section requires you to have the background knowledge of chemical bonds, polar covalent bonds, molecular geometry and polar molecules, which you can find in "Ch09. Chemical Bonding I: The Covalent Bond" and "Ch10. Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals"

Intermolecular forces are attractive forces between molecules, also known as Van der Waals forces.
Intramolecular forces hold atoms together in a molecule, such as covalent bonds.

Interactions Between Polar Molecules

Polar covalent bonds are covalent bonds between different atoms.
A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically.
Examples: $\ce{HF}$, $\ce{H2O}$
A non-polar molecule does not have net dipole either becaue there is no polar bond or because the polar bonds cancel each other.
Examples: $\ce{H2}$, $\ce{O2}$ which do not have polar bonds; $\ce{CCl4}$ in which all polar bonds arranged symmetrically and cancel each other.
Dipole-Dipole Forces are attractive forces between polar molecules.

Interactions Involves Non-Polar Molecules

Dispersion Forces are attractive forces that arise as a result of temporary dipoles induced in atoms or molecules.
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons, and more diffuse electron cloud.

The dipole in the atom (or nonpolar molecule) is said to be an induced dipole because the separation of positive and negative charges in the atom (or nonpolar molecule) is due to the proximity of an ion or a polar molecule.
A nonpolar molecule interact with a polar molecule or an ion because of the induced dipole moment.

An atom or a molecule can have all, most or majority of its electrons shifted to one side by random chance of the electron movements. When this happens a temporary dipole is created causing the side with more concentration of electrons to become more electronegative and the opposite relatively electropositive. This is called an instantaneous dipole.
Nonpolar molecules interact with each other due to the fact of the presence of instantaneous dipoles. This kind of interaction is called London dispersion force.

Special Cases

Ion-Dipole Forces are attractive forces between an ion and a polar molecule.
Hydrogen Bonds is a kind of special dipole-dipole interaction. A hydrogen bond is the electrostatic attraction between two polar groups that occurs when a hydrogen ($\ce{H}$) atom covalently bound to a highly electronegative atom such as nitrogen ($\ce{N}$), oxygen ($\ce{O}$), or fluorine ($\ce{F}$) experiences the electrostatic field of another highly electronegative atom nearby.

General Trend:
Ion-Dipole > Hydrogen bond > Dipole-Dipole > Ion-Induced Diploe > Dipole-Induced Dipole > London Dispersion Force

Requirements

1. Given a molecule, draw its 3D geometry and tell if it is polar or nonpolar.
2. Tell what kind of intermolecular force(s) a molecule has.
3. Compare the relative strength of intermolecular forces for given molecules.
4. Distinguish hydrogen bonds based on the molecular structures.

2. Properties of Liquids

Surface Tension

Surface Tension is a measure of a liquid's resistance to increase surface area. Stronger intermolecular forces result in higher surface tension.

Capillary action is an example of surface tension effect. Capillary action (sometimes capillarity, capillary motion, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.

Cohesion is the intermolecular attraction between LIKE molecules.
Adhesion is an attraction between UNLIKE molecules.

Viscosity

Viscosity is a measure of a fluid's resistance to flow.
Stronger intermolecular forces result in higher viscosity. Generally, higher temperature gives lower viscosity.

Water

Each water molecule can form 4 hydrogen bonds.

Thus, water has relatively high melting and boiling temperature (compared to $\ce{H2S}$, $\ce{NH3}$, $\ce{HF}$); and high specific heat.

Water has highest density at 4 $^\circ$C.

Requirements

1. Understand the concepts.
2. Given several molecules, compare there surface tension or viscosity based on their structures.
3. Remember the special properties of water and understand why.

3. Solids

Crystalline solid, which possesses rigid and long-range order; its atoms, molecules, or ions occupy specific positions.
Amorphous solids, such as glass, lack a well-defined arrangement and long-range molecular order.

Types of crystals

Type of Solids	Bonds in Solids	Soft or Hard	Melting Point	Conductivity of Heat and Electricity	Examples
Molecular Solids	Intermolecular Forces	Soft	Low	Poor	$\ce{Ar}$, $\ce{H2O}$, $\ce{CO2}$
Covalent Network Solids	Covalent Bonds	Hard	High	Poor (exception: graphite)	Diamond, Graphite, Quartz
Ionic Solids	Ionic Bonds	Hard and Brittle	High	Poor (if liquid, good conductor of electricity)	$\ce{NaCl}$, $\ce{MgCl2}$
Metallic Solids	Metallic Bonds	Soft to Hard	Low to High	Good	$\ce{Fe}$, $\ce{Cu}$, $\ce{Hg}$

Crystal Structures

Crystal structures are determined via X-ray diffraction.

A unit cell is the basic repeating structural unit of a crystalline solid. There are SEVEN types of unit cells.

There are three types of cubic unit cells.

Simple Cubic Cell (SCC)
Body Centered Cubic Cell (BCC)
Face Centered Cubic Cell (FCC)

The coordination number is defined as the number of atoms (or ions) surrounding an atom (or ion) in a crystal lattice.
The number of atoms is counted as shown in the following figure.

	SCC	BCC	FCC
No. of atoms in one cell	1	2	4
How atoms are touched	Along cell edge	Along cube diagonal	Along side face diagonal
Coordination No.	6	8	12

Close packing is the most efficient arrangement of spheres. There are two kinds of close packing structures.

• Hexagonal close-packed structure (ABAB... structure) results in hexagonal unit cell.
• Cubic close-packed structure (ABCABC... structure) results in FCC unit cell.

Requirements

1. Given a substance, tell what type of crystal it would form (molecular, ionic, covalent network, or metallic).
2. Remember the general properties of each type of crystals.
3. Compare the melting points of given solids.
4. Remember and understand the concepts given in the section "Crystal Structures".
5. Count the number of atoms in a SCC, BCC and FCC unit cell; calculate the coordination numbers.

4.Phase Changes and Diagrams

A Phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.

Phase Changes are transformations from one phase to another, occur when energy (usually in the form of heat) is added or removed from a substance.

Liquid-Vapor Equilibrium

At any given temperature, a certain number of the molecules in a liquid possess sufficient kinetic energy to escape from the surface. This process is called evaporation, or vaporization.

As the concentration of molecules in the vapor phase increases, some molecules return to the liquid phase, a process called condensation.

The rate of evaporation is constant at any given temperature, and the rate of condensation increases with increasing concentration of molecules in the vapor phase. A state of dynamic equilibrium, in which the rate of a forward process is exactly balanced by the rate of the reverse process, is reached when the rates of condensation and evaporation become equal.

The vapor pressure measured under dynamic equilibrium of condensation and evaporation is called the equilibrium vapor pressure.

Molar heat of vaporization ($\Delta H_\text{vap}$), defined as the energy required to vaporize one mole of a liquid, is a measure of how strongly molecules are held in a liquid is.

Clausius-Clapeyron equation:
$$\ln P = -\frac{\Delta H_\text{vap}}{RT} + C$$
$$\ln \frac{P_1}{P_2}=\frac{\Delta H_\text{vap}}{R}\left(\frac{T_1-T_2}{T_1T_2}\right)$$

Critical Temperature is the temperature above which the gas cannot be made liquefy, no matter how great the applied pressure.
Critical Pressure is the minimum pressure that must be applied to bring about liquefaction at the critical temprature.

Example: Knowing the molar heat of vaporization of water (40.66 kJ/mol), calculate the amount of heat absorbed when boiling 10.0 g of water.
Answer
\begin{align*} & 10.0\text{ g }\ce{H2O}\times\frac{1\text{ mol}}{18.0\text{ g}}=0.556\text{ mol }\ce{H2O} \\ & 0.556\text{ mol}\times 40.66\text{ kJ/mol}=22.6\text{ kJ} \end{align*}

Example: Diethyl ether, $\ce{CH3CH2OCH2CH3}$ is a volatile, highly ammable organic liquid that is used mainly as a solvent. The vapor pressure of diethyl ether is 400 mmHg at 27 $^\circ\text{C}$. Calculate its vapor pressure at 77 $^\circ\text{C}$. ($\Delta H_\text{vap} = 26.0\text{ kJ/mol}$ for diethyl ether).
Answer
\begin{align*} & \ln\left(\frac{P_1}{P_2}\right)=\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_2}-\frac{1}{T_1}\right) \\ & \ln\left(\frac{400\ \text{mmHg}}{P_2}\right) \\ & =\frac{26.0\times10^3\ \text{J/mol}}{8.314\ \text{J/K mol}}\left(\frac{1}{(77+273)\ \text{K}}-\frac{1}{(27+273)\ \text{K}}\right) \\ & \ln\left(\frac{400\ \text{mmHg}}{P_2}\right)=-1.489\\ & \frac{400\ \text{mmHg}}{P_2}=e^{-1.489}=0.226\\ & P_2=\frac{400\ \text{mmHg}}{0.226}=1773\ \text{mmHg} \end{align*}

# Knowing vapor pressure P1 at one temperature T1, 
# calculate P2 at another temperature T2. Delta H is given.
from numpy import*
Hvap = 26 # in kJ/mol
T1=29 # in Celsius
T2=77 # in Celsius
P1=593 # in mmHg
P2=P1/exp(Hvap*1000/8.314*(1/(T2+273)-1/(T1+273)))
P2

2453.6432144684991

Liquid-Solid Equilibrium

The transformation of liquid to solid is called freezing, and the reverse process is called melting or fusion. The melting point of a solid (or the freezing point of a liquid) is the temperature at which solid and liquid phases coexist in equilibrium. The normal melting point(or the normal freezing point) of a substance is the melting point (or freezing point) measured at 1 atm pressure. We generally omit the word “normal” in referring to the melting point of a substance at 1 atm.

Molar Heat of Fusion ($\Delta H_\text{fus}$) is the energy required to melt 1 mole of a solid substance at its freezing point.

Solid-Vapor Equilibrium

Molar Heat of Sublimation ($\Delta H_\text{sub}$) is the energy required to sublime 1 mole of a solid.

Hess's Law:
$$\Delta H_\text{sub} = \Delta H_\text{fus} + \Delta H_\text{vap}$$

Phase Diagrams

A phase diagram summarizes the conditions under which a substance exists as a solid, liquid, or gas.

The point at which all three curves meet is called the triple point. This is the only temperature and pressure at which all three phases can be in equilibrium with one another.

Boundary lines denote co-existence of two phases.

Requirements

1. Remember the general property differences of gases, liquids and solids.
2. Understand the concepts.
3. Do calculations with Clausius-Clapeyron equation.
4. Learn to interpret phase diagrams.

Practice:
1. Which of the following substances have polar molecules?
(A) $\ce{PH3}$
(B) $\ce{H2}$
(C) $\ce{H2S}$
(D) $\ce{CH4}$
(E) $\ce{NH3}$

2. Which of the following substances do not have polar interactions (dipole-dipole forces) between molecules?
(A) $\ce{H2O}$
(B) $\ce{HF}$
(C) $\ce{CO2}$
(D) $\ce{SO2}$
(E) $\ce{CH2Cl2}$

3. Which of the following substances should exhibit hydrogen bonding in the liquid state?
(A) $\ce{PH3}$
(B) $\ce{H2}$
(C) $\ce{H2S}$
(D) $\ce{CH4}$
(E) $\ce{NH3}$

4. Three types of interactions are involved in making a solution. What are the three types of interactions?
(A) Break solvent-solvent interactions; break solute-solute interactions; form solvent-solute interactions
(B) Break solvent-solvent interactions; form solute-solute interactions; form solvent-solute interactions
(C) Form solvent-solvent interactions; break solute-solute interactions; form solvent-solute interactions
(D) Form solvent-solvent interactions; form solute-solute interactions; break solvent-solute interactions
(E) Form solvent-solvent interactions; form solute-solute interactions; form solvent-solute interactions

5. Arrange the following compounds in each group in an order of increasing boiling point and explain why.
(1) $\ce{H2O}$, $\ce{CH4}$, $\ce{CH3OH}$, $\ce{CH3Cl}$
(2) $\ce{CF4}$, $\ce{CCl4}$, $\ce{CBr4}$, $\ce{CI4}$
(3) $\ce{HF}$, $\ce{HCl}$, $\ce{HBr}$, $\ce{HI}$

6. The molecular reason of surface tension is that no intermolecular forces pull $\underline{\hspace{3cm}}$ upward and away, and intermolecular forces for $\underline{\hspace{3cm}}$ are from all the directions that cause the net intermolecular forces is equal to zero.
(A) interior molecules; interior molecules
(B) interior molecules; surface molecules
(C) surface molecules; interior molecules
(D) surface molecules; surface molecules
(E) either surface or interior molecules; either surface or interior molecules

7. When a thin glass tube is inserted into water, water level would rise in the glass tube. The reason for this is most likely because $\underline{\hspace{3cm}}$.
(A) adhesion and cohesion are equal
(B) adhesion is stronger than cohesion
(C) cohesion is stronger than adhesion
(D) adhesion and cohesion are not involved
(E) either adhesion or cohesion could be stronger

8. Viscosity is a measure of a liquid's resistance to flow. If temperature decreases, will the viscosity decrease or increase? What is the order of viscosity from low to high for the molecules $\ce{CCl4}$, $\ce{C2H5OC2H5}$ and $\ce{CH3CH2OH}$?

9. Which of the following substance is an example of a colalent network solid?
(A) diamond
(B) potassium
(C) sodium chloride
(D) iodine
(E) none of above

10. What is the general property of ionic crystals?
(A) Usually have very low melting points.
(B) Usually do not conduct electricity in solid state.
(C) Usually do not conduct electricity when melted.
(D) Usually have higher melting points than covalent networks.
(E) None of above.

11. Given a crystal (SCC, BCC or FCC type), determine the coordination number and the number of atoms/molecules in one unit cell.

12. Knowing the molar heat of melting or vaporization of a substance, calculate the amount of heat absorbed/released when melting/freezing/boiling/condensing certain amount of this substance. (See the example above)

13. Do straightforward calculation using Clausius-Clapeyron equation. (See the example above)

14. Given the phase diagram of water, what will happen if the temperature is increased when the pressure is fixed (e.g. at 0.5 kPa)? What will happen if the pressure is increased when the temperature is fixed (e.g. at 100 $^\circ\text{C}$)?

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