Chapter Twelve:  Intermolecular Forces: Liquids, Solids, and Phase Changes


12.1  An Overview of Physical States and Phase Changes


There are two different types of forces that exist in molecular compounds:


                Intramolecular forces – these are forces that exist within a molecule and influence the chemical properties of the substance


                Intermolecular forces -- these are forces that exist between molecules and influence the physical properties of the substance



Which forces are intramolecular and which intermolecular?

a)       Those preventing oil from evaporating at room temperature.

c)       Those allowing silver to tarnish.







If we have the same substance, but in 3 different states of matter – gas, liquid, and solid – the intramolecular forces would be the same, but the intermolecular forces would be different.


For example, let’s take 300 molecules of water; 100 as water vapor, 100 as liquid water and 100 as ice.  These 300 molecules of water would all be identical to each other and thus would have the same intramolecular forces.  What makes 100 molecules of water vapor different from the 100 molecules of ice are the intermolecular forces between them.


In other words, water vapor has the physical properties it does because the intermolecular forces between the water molecules are relatively weak as compared to the intermolecular forces between the water molecules in ice.


Table 12.1

A Macroscopic Comparison of Gases, Liquids, and Solids


State                       Shape and Volume                               Compressibility                   Ability to Flow

Gas                         Conforms to shape and                       high                                        high

volume of container


Liquid                     Conforms to shape of                          very low                                 moderate

container; volume limited

by surface


Solid                       Maintains its own shape                    almost none                          almost none

and volume



When ice changes to liquid water or liquid water changes to water vapor, we say that a phase change had occurred.




Phase changes have specific names:




                liquid                                                      gas





                                    melting or fusion


                solid                                                       liquid







                solid                                                       gas




Heats of Phase Change

•          DHfusion = heat needed to melt a given amount of substance   (solid ΰ liquid)

•          DHvaporization = heat needed to vaporize a given amount of liquid  (liquid ΰ gas)

•          DHsublimation = heat needed to melt a given amount of substance  (solid ΰ gas)


•          Unit Examples: J/g,   kJ/mol,  cal/g,  kcal/g


12.2 Quantitative Aspects of Phase Changes


Quantitative Aspects of Phase Changes

Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average Ek as the most probable speed of the molecules changes.


q = (amount)(molar heat capacity)(DT)

q = nCDT   or   q = mcDT  

Where n=moles or  m=mass. Units Count!


During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in Ep, as the average distance between molecules changes.


q = (amount)(enthalpy of phase change)

q = nDH   or   q = mDH



HOMEWORK PROBLEM 12.19  Calculating Heat of Phase Changes.



From the data below, calculate the total heat in Joules needed to convert 12.00 g of ice at –5.000C to liquid at .5000C.

MP=O0C, DHf = 6.02 kJ.mol, cliquid=4.21 J/g0C, csolid=2.09 J/g0C




Use equations for heat of temperature change of substance q = ncDT and heat of phase change  q = nDH. Use sketch of T vs heat to define appropriate use of equations.

















The pressure exerted by the vapor at equilibrium is called the equilibrium vapor pressure or just the vapor pressure of the liquid at that temperature.


In figure 12.5, notice how a higher temperature results in more molecules traveling faster.


The fact that more molecules have more energy, means that more molecules have enough energy to overcome the forces that are holding them together in the liquid state.


As a result, more molecules are able to vaporize into a gas.  In general, the higher the temperature of a liquid, the higher its vapor pressure.



The Clausius-Clapeyron Equation


Used for relating vapor pressure to pressure and temperature.

                      Single state form


                Dual state form





HOMEWORK PROBLEM 12.28  Using the Clausius-Clapeyron Equation



At 25.00C, the vapor pressure of butane is 2.3 atm. What is the pressure when the temperature is 1500C?  DHv= 24.3 kJ/mol



We are given 4 of the 5 variables in the Clausius-Clapeyron equation.  Substitute and solve for P2.














Phase diagrams are used to describe the phase changes of a certain substance at various temperatures and pressures.


12.3  Types of Intermolecular Forces


There are 3 types of intermolecular forces:

strongest               1)    Hydrogen bonding (FON bond)                                                               

exists in molecules that have F, O, or N atoms directly connected to a H atom

                              2)    Dipole-dipole forces (dip-dip bond)

exists in molecules that are polar (have a permanent, net dipole)

weakest                  3)    London dispersion forces

exists in molecules that are not polar


All substances that have hydrogen bonding, also have dipole-dipole and London dispersion forces.  It is just that hydrogen bonding is so much stronger than the other two that it becomes the predominant force.


All substances that have dipole-dipole forces, also have London dispersion forces.


In other words, all substances have London dispersion forces, but if they have other, stronger forces, those will dominate over the dispersion forces.


HOMEWORK PROBLEM 12.39  Determining Intermolecular Forces and Drawing Hydrogen Bonds



What is the strongest intermolecular force in a sample of each of the following compounds?

(a) CH3Br,      (b) CH3CH3,    (c) NH3



Draw structures and determine molecules are polar, nonpolar. Look for molecules in which H is bonded to N, O or F. 















The effect of intermolecular forces on some properties of a substance

•          Strong intermolecular forces

–         High melting point

–         High boiling point

–         Low vapor pressure

–         Large surface tension

•          Weak intermolecular forces

–         Low melting point

–         Low boiling point

–         High vapor pressure

–         Low surface tension



HOMEWORK PROBLEM 47  Predicting the Type and Relative Strength of Intermolecular Forces and Effect on a Property



For each pair of substances, identify the dominant intermolecular forces in each substance, and select the substance with the higher vapor pressure.  Explain your choice.

(a)  C2H6 or C4H10            (b)  CH3CH2OH or CH3CH2F          (c)  NH3 or PH3



Determine structure and polarity of each and examine each for hydrogen bonding potential. Weaker intermolecular forces indicate higher vapor pressure.














12.5  The Uniqueness of Water


Special Properties of Water

•          Universal Solvent

•          High boiling point for its low molecular weight

•          High specific heat capacity

•          Liquid over most of the biological temperature range and is a requirement for all life

•          Strong cohesive and adhesive forces

•          The density of solid is less than that of liquid


12.6   The Solid State:  Structure, Properties, and Bonding


There are two broad categories of solids:

                crystalline solids whose particles occur in an orderly arrangement

                amorphous solids whose particles occur randomly

We will concentrate on crystalline solids.


HOMEWORK PROBLEM 12.92 (b)  Determining Size of a Unit Cell



Zinc selenide (ZnSe) crystallizes with the Se-2 ions forming a face-centered cubic arrangement and the Zn+2 ions occupying interstitial positions. The density is 5.42 g/cm3. (b) What is the volume of the unit cell?



First conceptualize the the structure. The unit cell MUST be electrically neutral. Since density = mass/volume, determine the mass inside the unit cell and solve for volume.














There are five types of crystalline solids:

1)       Atomic

elements, nonmetals                                                            ex.  He

2)       Molecular

elements or compounds, nonmetals                                 ex.  O2, CO2

3)       Metallic

elements, metals                                                                   ex.  Na, Fe

4)       Ionic

compounds of metals and nonmetals                               ex.  NaCl

5)       Network

elements or compounds, semimetals                                ex.  C, SiC



Of the five major types of crystalline solids, which does each of the following form:

                a)  C27H45OH                          b)  KCl                                    c)  BN













Molecular Orbital Theory and Conductance


Remember from Chapter 11 that when you mix atomic orbitals to produce molecular orbitals, the number of molecular orbitals you form must equal the number of atomic orbitals you started with.  Since atomic orbitals come from atoms, more atoms you have, the more atomic orbitals you have, which in turn, means you have more molecular orbitals.