intermolecular forces and liquids and solids

Intermolecular Forces and Liquids and Solids

Kinetic Molecular Theory of Liquids and Solids (12.1)Intermolecular Forces (12.2)Properties of Liquids (12.3)Crystal Structures (12.4)Bonding in Solids (12.5)Phase Changes (12.6)Phase Diagrams (12.7)

12.1 The Kinetic Molecular Theory of Liquids and Solids

How are solids and liquids modeled on the particle-level?How does this help explain macroscopic properties of solids and liquids?

Intermolecular Forces and Liquids and Solids

Figure 1.2, p. 4

H2O(l)

H2O(g)

H2O(s)

Table 12.1, p. 403

Does the molecular structure of the substance on the particle level affect function or properties on the macroscopic level?

We finally connect between the abstract (the bonding and structure of substances) to the observed (the function and properties of substances)

12.2 Intermolecular ForcesWhat are intramolecular forces:◦ Forces that hold atoms together in a molecule

What are intermolecular forces:◦ attractive forces between molecules

How do we model these?◦ Let’s consider water:

Intermolecular force (between molecules)

Intramolecular force (covalent bonds inmolecules)

12.2 Intermolecular Forces

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O‐H bonds in 1 mole of water (intra)

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force:

boiling pointmelting pointΔHvap ΔHfus ΔHsub

Focus on pure substances first:How do we model pure substances?How are intermolecular forces separated into categories?

We will use structure and size to assist in approximating intermolecular forces and connect this to physical properties

Can all substances be liquefied?Why?Dispersion Forces:◦ Attractive forces that arise as a result of a temporary

dipole induced in atoms or molecules

Figure 12.5, p. 406

12.2 Intermolecular ForcesPolarizability:◦ The ease with which the electron distribution in the atom

or molecule can be distorted

Polarizability increases with greater number of electrons or more diffuse electron cloud

Dispersion forces usually increase with molar mass.As dispersion forces increase, intermolecular forces increase.Therefore, it takes more energy to overcome these and the melting points and boiling points increase

Table 12.2, p. 406

What are the states of the halogens under standard conditions?

Permanent vs. induced dipole momentsDipole Forces:◦ Attractive forces between polar molecules

Figure 12.1, p. 404

What is the trend of boiling points of HI, HBr, HCl and HF?

HF HCl HBr HI

Specific type of dipole forceHydrogen bonding (IMF – not covalent bond):◦ Special type of dipole-dipole interaction between the

hydrogen atom in a polar bond, such as N-H, O-H or F-H and an electronegative O, N, or F atom.

Figure, p. 409

Figure 12.7, p. 405

Figure 12.6, p. 408

Decreasing molar massDecreasing boiling pointNo hydrogen bonding

Hydrogen bondingGreater IMFHigher boiling pt

12.2 Intermolecular ForcesIM

Dispersion forces

Dipole forces

Hydrogen bonding

12.2 Intermolecular ForcesUsing IMF with pure substances:Approximating relative melting or boiling points1. Determine the intermolecular forces of the

substances (usually comparing between two different pure substances)

2. Consider the number of intermolecular forces:If they are different, then the substance with the higher number of intermolecular forces has the higher boiling pointIf they are the same, then the substance with the higher number of electrons has the higher boiling point

12.2 Intermolecular ForcesUsing IMF with pure substances:Which has the higher boiling point?

1. Determine the IMF

2. Consider the number of intermolecular forces:

If they are different, then the substance with the higher number of intermolecular forces has the higher boiling point

If they are the same, then the substance with the higher number of electrons has the higher boiling point

Substance Boiling Point, oC

C2H4 -104

NF3 71

NCl3States at room temperature?

12.2 Intermolecular ForcesUsing IMF with pure substances:Which has the higher boiling point?

2. Consider the number of intermolecular forces:

If they are different, then the substance with the higher number of intermolecular forces has the higher boiling point

If they are the same, then the substance with the higher number of electrons has the higher boiling point

Substance Boiling Point, oC

butanol 157

methanol 65butanol methanol

States at room temperature?

Figure 12.7, p. 408

Now focus on mixtures:How do we model mixtures?How do the intermolecular forces and sizes of molecules affect solubility?

We will use structure and size to assist in approximating intermolecular forces and connect this to physical propertiesThink about this like a “substitution”

Margin Figure, p. 409

12.2 Intermolecular ForcesUsing IMF with mixtures:Approximating solubility1. Determine the intermolecular forces of the substances

(usually considering the solubility of two pure substances)

2. Determine the relative size of the two molecules (same or different)If both (IMF and size) are the same, then we would approximate that the substances are solubleIf both (IMF and size) are different, then we could approximate that the substances are insolubleIf one (IMF or size) are the same, then we would approximate that the substances are partially soluble

12.2 Intermolecular ForcesUsing IMF with mixtures:What is the solubility of the two substances?

2. Determine the relative size of the two molecules (same or different)

If both (IMF and size) are the same, then the substances are solubleIf both (IMF and size) are different, then the substances are insolubleIf one (IMF or size) are the same, then the substances are partially soluble

Solubility (g in 100g)

Infinitely (miscible)

watermethanol What does this look like?

12.2 Intermolecular ForcesUsing IMF with mixtures:What is the solubility of the two substances?

2. Determine the relative size of the two molecules (same or different)

If both (IMF and size) are the same, then the substances are solubleIf both (IMF and size) are different, then the substances are insolubleIf one (IMF or size) are the same, then the substances are partially soluble

Solubility (g in 100g)

water What does this look like?butanol

12.2 Intermolecular ForcesIM

What do these look like?

Alcohol Solubility (g in 100g H2O)

Infinitely (miscible)

12.2 Intermolecular ForcesUsing IMF with mixtures:Determining IMF that exist between the substances:◦ Dispersion forces (or induced dipole-induced dipole)

between all substances in a solution.◦ A nonpolar and a polar substance would have induced

dipole-dipole forces◦ A polar and a polar substance would have dipole-dipole

forces◦ A ionic and a nonpolar substance would have ion-induced

dipole forces◦ A ionic and a polar substance would have ion-dipole

forces

12.2 Intermolecular ForcesUsing IMF with mixtures:What are the IMF between the two substances?

Determine the IMFDispersion forces (or induced dipole-induced dipole) between all substances in a solution.A nonpolar and a polar substance would have induced dipole-dipole forcesA polar and a polar substance would have dipole-dipole forcesA ionic and a nonpolar substance would have ion-induced dipole forcesA ionic and a polar substance would have ion-dipole forces

watermethanol

dipole-dipole forcesdispersion forces

12.2 Intermolecular ForcesUsing IMF with mixtures:What are the IMF between the two substances?

Determine the IMFDispersion forces (or induced dipole-induced dipole) between all substances in a solution.A nonpolar and a polar substance would have induced dipole-dipole forcesA polar and a polar substance would have dipole-dipole forcesA ionic and a nonpolar substance would have ion-induced dipole forcesA ionic and a polar substance would have ion-dipole forces

waterSalt, NaCl

ion-dipole forcesdispersion forces

Which molecule does not have the intermolecular force of hydrogen bonding as a pure substance?

I II III

(A)Only I (B) I and II(C) I and III (D) II and III

12.3 Properties of Liquids

How is structure related to other properties of liquids (surface tension, viscosity)?What is surface tension?◦ The amount of energy required

to stretch or increase the surface of a liquid by a unit area

◦ Would you expect higher intermolecular forces, higher surface tension

Figure 12.8, p. 410

Figure 12.10, p. 411

12.3 Properties of LiquidsWhat is cohesion?◦ The IMF attraction between like

molecules

What is adhesion?◦ An attraction between unlike

molecules

◦ Would expect similar intermolecular forces, better adhesion

How can these work together?

Capillary action

Adhesion and cohesion

capillary action

Figure 12.10, p. 411

12.3 Properties of LiquidsWhat is cohesion?◦ The IMF attraction between like

molecules

What is adhesion?◦ An attraction between unlike

molecules

◦ Would expect similar intermolecular forces, better adhesion

What if these don’t work together?

Only Cohesionno capillary

action

12.3 Properties of Liquids

What is viscosity?◦ A measure of a fluid’s resistance to flow

◦ Would expect higher intermolecular forces, higher viscosity

Table 12.3, p. 411

Figure 12.13, p. 413

12.3 Properties of LiquidsHow is water unique?

Figures 12.131 and 12.13 p. 412-413

12.3 Properties of LiquidsMaximum Density

Which beaker contains water?

12.4 Crystal Structure

How do we model solids?What are two general ways solids form?Can we model both forms?

Figure 12.14, p. 414

Figure 12.15, p. 414

Figure 12.16, p. 415

12.4 Crystal StructureHow do we determine the number of atoms per unit cell?

PositionFraction of atom in that position

Number of atom(s) in that

position

Number of atoms per unit

cell in that position

On a corner 1/8 8 1On a faceIn the center

Figure 12.18, p. 416

position

On a corner 1/8 8 1On a face 1/2 6 3In the center

Figure 12.18, p. 416

position

On a corner 1/8 8 1On a face 1/2 6 3In the center 1 1 1

Figure 12.18, p. 416

For each simple cubic, elemental structure, we will answer:

1. Number of atoms per unit cell2. What is the length of one side of this crystal

(in terms of the radius of the atom)?3. What is the volume of this crystal (assuming

the atoms are spheres)?4. What is the packing efficiency of this crystal?

Figure 12.19, p. 416

Eu crystallized in a body-centered cubic cell. The density of Eu is 5.26 g·cm–3, what is the unit cell edge length in pm?

For this we will need to calculate:

- The number of atoms per unit cell.

- The mass (in g) of these atoms.

- The volume of the unit cell in cm3.

- The length of one side of the unit cell in cm.

Because Eu is body-centered cubic, we can calculate the number of atoms per unit cell as 2.

Using the density, the molar mass, NA, and the number of atoms per unit cell, we can calculate the volume of the unit cell.

Because this is a cubic unit cell, the length of one side is:

− −⎛ ⎞⎛ ⎞⎛ ⎞⎛ ⎞ = ×⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟×⎝ ⎠⎝ ⎠⎝ ⎠⎝ ⎠

cm 152.0 g mol 2 atoms9.60 10 cm

5.26 g mol unit cell6.022 10 atoms

− −= × = ×3 23 3 83length of one side (BCC) = Volume 9.60 10 cm 4.58 10 cm

Metallic copper crystallizes in a cubic structure. The length of the edge of the unit cell is 361 pm. If copper has a density of 8.96 g·cm–3, which type of unit cell is present in this sample?

- The mass of the unit cell (using density).

- The number of atoms per unit cell (using NA).

Because copper has a unit cell with an edge length of 361 pm, we will first convert into cm (to use the density) and then cube for the volume of the unit cell.

The side length is 361 pm = 361x10-12 m = 361x10-10 cm.

The volume of the unit cell is (361x10-10 cm)3 = 4.70x10-23 cm3

The number of atoms per unit cell is:

4 atoms per unit cell indicates a face-centered cubic packing.

−−⎛ ⎞ ⎛ ⎞× ×⎛ ⎞⎛ ⎞ = ⋅⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠⎝ ⎠ ⎝ ⎠

23 3 231

4.70 10 cm 8.96 g mol 6.022 10 atoms4 atoms unit cell

unit cell 63.55 g molcm

When aluminum crystallizes, it forms face-centered cubic cells. The atomic radius of an Al atom is 143 pm. What is the density of metallic aluminum?

- The length of one side of the unit cell in cm.

- The number of atoms per unit cell.

- The mass (in g) of these atoms.

Because aluminum is face-centered cubic, using the radius, we can calculate the length of one side of the cube.

The radius is 143 pm = 143x10-12 m = 143x10-10 cm.

The face-centered cubic unit cell has the length of one side equal to 4.04x10-8 cm.

The volume of the unit cell is 6.59x10-23 cm3

There are 4 atoms per unit cell.

The density is:

323 23 3

26.98 g mol 4 atoms unit cell 2.72 g cmmol unit cell6.022 10 atoms 6.59 10 cm

−−

⎛ ⎞⎛ ⎞⎛ ⎞⎛ ⎞ = ⋅⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟× ×⎝ ⎠⎝ ⎠⎝ ⎠⎝ ⎠

length of one side (FCC) = 8r

12.5 Bonding in SolidsWhat are the main classes of crystalline solids?◦ Ionic◦ Covalent◦ Molecular◦ Metallic

How do the general properties help us differentiate between these classes?◦ Type of bonding in the

solid◦ Melting points◦ Conductivity (heat or

electricity)◦ Solubility (in polar or

nonpolar solvents)

12.5 Bonding in Solids

CsCl ZnS CaF2

Properties Ionic SolidsType of bonding in the solid electrostatic attraction

Melting points 600-2000 oC

Conductivity (heat or electricity) poor conductor

Solubility (in polar or nonpolar solvents) soluble in polar solvents

Figure 12.22, p. 420

Properties Covalent SolidsType of bonding in the solid covalent bonds

Melting points >1000 oC

Solubility (in polar or nonpolar solvents) insoluble in either type

carbonatoms

Figure 12.24, p. 422

Properties Molecular SolidsType of bonding in the solid intermolecular forces

Melting points Low melting points

Solubility (in polar or nonpolar solvents) soluble in nonpolar solvents

Margin Figure p. 421

Properties Metallic SolidsType of bonding in the solid metallic bonding

Melting points Low to high

Conductivity (heat or electricity) good conductor

Solubility (in polar or nonpolar solvents) insoluble

Cross Section of a Metallic Crystalnucleus &inner shell e-

Mobile “sea”of e-

Figure 12.25, p. 422

Table 12.4, p. 420

12.6 Phase Changes

What are the three phases of matter?◦ Phases are a

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

Consider the liquid-vapor equilibrium on a particle level. How is this an equilibrium?

Figure 12.27, p. 423

12.6 Phase Changes

What is vapor pressure?◦ The pressure exerted by gaseous molecules from

when a liquid evaporates

Consider an ideal gas, how are pressure and temperature related?How is vapor pressure and temperature related?

12.6 Phase Changes

How are vapor pressure and enthalpy of vaporization related?

Table 12.5, p. 425

12.6 Phase Changes

What is the boiling point?◦ The boiling point is the temperature at which the

(equilibrium) vapor pressure of a liquid is equal to the external pressure.◦ The normal boiling point is the temperature at which

a liquid boils when the external pressure is 1 atm.

12.7 Phase Diagrams

What is a phase diagram?◦ summarizes the conditions under which a substance

exists as a solid, liquid or gas.

Consider modeling this for water:◦ What happens at 100oC and 1 atm?

What happens when temperature changesWhat happens when pressure changes

◦ What happens at 0oC and 1 atm?What happens when temperature changesWhat happens when pressure changes

12.7 Phase Diagrams

What is the triple point?◦ the only temperature and pressure at which all three

phase can be in equilibrium with one another.

What are critical temperature (Tc) and critical pressure (Pc)?◦ above which its gas form cannot be made to liquefy; no

matter how great the applied pressure. This is also the highest temperature at which a substance can exist as a liquid. The minimum pressure that must be applied to bring about liquefaction at this critical temperature is the critical pressure

12.7 Phase Diagrams

How do we model phases for a pure substance over a range of temperatures and pressures?

Figure 12.32, p. 430

12.7 Phase Diagrams

How do we model phases for a pure substance over a range of temperatures and pressures?

Figure 12.33, p. 430

Bigger PictureWe have finally connected structure (on the particle level) based on:◦ Atomic structure◦ Electronic structure◦ Bonding◦ Shape◦ Polarity

and function – properties onthe macroscopic levelWe have reason and explanation for observations

As the temperature of a liquid increases, the number of particles in the vapor phase _______________ and the vapor pressure _______________.

(A) decreases; decreases(B) decreases; increases(C) increases; decreases(D) increases; increases

A metal crystallizes in a body‐centered cubic (bcc) crystal structure. If the atomic radius of the metal is 124.1 pm and the density is 7.87 g∙cm–3, what is the metal?

(A) Cd (B) F (C) Fe (D) K

43ra =

Using the phase diagram to the right, which letter indicates (only) the liquid region?(A) A (B) B(C) C (D) D

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