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
HOMEWORK PROBLEM 12.6
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, lets 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.
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
Solid Maintains its own shape almost none almost none
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:
melting or fusion
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
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:
elements, nonmetals ex. He
elements or compounds, nonmetals ex. O2, CO2
elements, metals ex. Na, Fe
compounds of metals and nonmetals ex. NaCl
elements or compounds, semimetals ex. C, SiC
HOMEWORK PROBLEM 12.87
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.