Intermolecular forces

CHEMICAL BOND

Highlight:

• Covalent bond

• Ionic bond

• Metallic bond

CONCEPTS Valence electron Lewis dot formula Electronegativity Polar, non polar bond Polarity Dipole, dipole moment

SUMMARY

Highlight:

• Lewis structure

• Octet rule

• Molecular

geometry

PART 2: MOLECULAR SHAPE

CONCEPTS How to draw a LEWIS structure Octet rule VSEPR (Valence-Shell Electron Pair Repulsion) theory Determine the molecular shape by VSEPR

SUMMARY

THIS WEEK

• Intermolecular forces: forces that hold molecules together

• Different types of forces between the molecules

CONCEPTS

Types of intermolecular

forces

Bond length

Van der Waals force

Polarity vs. polarisability

H-bond

HIGHLIGHT:

Physical State And Phase Changes

Physical change

Physical properties : characteristics of matter that can be observed without changing its chemical identity

Chemical properties: characteristics of matter related to its chemical change

Chemical change

Chemical bond= INTRAmolecular forces -chemical properties

INTERmolecular forces – physical properties

• Type of physical phase changes:

- Liquid to solid: freezing- Liquid to gas: vaporisation

- Gas to liquid: condensation- Gas to solid: freezing

- Solid to gas: vaporisation- Solid to liquid: melting- Dissolution of solid

• Matter can change to many different physical states

Physical State And Phase Changes

Let’s watch this video.

Dissolution of compounds.

It is the intermolecular forces that determine the physical properties.

Answer these questions?1. Why can we dissolve NaCl in water (H2O)?

2. Why can we not do that to Gold?

• Intramolecular forces (bonding forces) exist within each molecule and influence the chemical properties of the substance.

Molecular Forces

• Intermolecular forces (nonbonding forces) exist between the molecules and influence the physical properties of the substance.

Example: Cl2 (Chloride)

•Intramolecular forces are relatively strong because they involve larger charges that are closer together.

• Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart.

All forces (attractive and repulsive interaction) between atoms arise from electrostatic forces

Molecular Forces

The separation distance is known either as the:- Bond length (for intramolecular), or the- Van der Waals (VDW) distance (for intermolecular)

VDW distance > bond length: since the forces of attraction are weaker

2 types of atomic radii (=dist/2)- Covalent radius - VDW radius

Bond length and Van der Waals

INTRAmolecular Forces

1) Ionic charge: when the molecule has an excess of negative (anion) or positive (cation) charge

2) Dipole moment

Do you remember?• Ionic and polar covalent bond have an unequal

sharing of electrons between the two atoms

• Polar compounds: dipole moment expresses the polarity

• Molecular shape affect the overall dipole moment

INTRAmolecular Forces: REVIEW

INTERmolecular Forces

INTERmolecular Forces

1. Ion-dipole

2. Dipole-Dipole

3. Ion-induced Dipole

4. Dipole-Induced Dipole

5. Dispersion (London)

ION-DIPOLE FORCES

Examples

When an ion and a nearby polar molecule (dipole) attract each other, an ion-dipole force results.

The most important example takes place when an ionic compound dissolves in water.

1. Ion-Dipole Forces

Example: Dissolution of NaCl in water

Ion-Dipole Forces

Example: Dissolution of NaCl in water

H2O molecules interact with NaCl by ION-DIPOLE

• The positive end of H2O molecules attract Cl-

ions• The negative end of H2O molecules attract Na+

ions

The interactions cause the salt (NaCl) to break apart/or dissolve in water

Example: Dissolution of NaCl in water

When each ions are separated

More water molecules stay around the ions

This forms hydration shells

DIPOE-DIPOLE FORCES

When polar molecules lie near one another:

• their partial charges act as tiny electric fields

• orient them and give rise to dipole-dipole forces

• the positive pole of one molecule attracts the negative pole of another

2. Dipole-Dipole

Solid Liquid

Dipole-Dipole

Dipole-dipole Forces: decide the boiling points of some compounds

• with similar molar mass:

bp of polar molecules > bp of nonpolar molecules

Why?

The greater the dipole moment the greater the dipole-dipole forces between the molecules the more energy it takes to separate them.

Dipole-Dipole Force: decide the boiling points

• decide the boiling points of some compounds

HYDROGEN BOND: a special type of dipole-dipole forces

A dipole-dipole force arises between molecules that have an H atom bonded to a small, highly electronegative atom with lone pairs.

The most important atoms are N, O, and F.So hydrogen bonding occur when a compound has one of these atoms.

Hydrogen Bond

How to draw a hydrogen bond

Hydrogen Bond

Hydrogen Bond: Practice

Draw the H-bond of these compounds?.

Hydrogen Bond:

Draw the H-bond of these compounds?.

Hydrogen Bond: THE SPECIAL CASE OF WATER

Hydrogen Bond

DIPOE-DIPOLE FORCESSUMMARY of INTERMOLECULAR FORCES

DIPOE-DIPOLE FORCESSUMMARY of INTERMOLECULAR FORCES

WHY do you think IM Forces also are as important as Intramolecular forces?

BECAUSE, some physical properties of substances that are affected by the

strength of the intermolecular force? • Boiling point• Vapour pressure• Heat of vaporisation• Surface tension• Viscosity

DIPOE-DIPOLE FORCESSUMMARY of INTERMOLECULAR FORCES

• BOILING POINT

Dispersions forces between the molecules increase with higher molecular weigh higher boiling point

SUMMARY of INTERMOLECULAR FORCESWith the same MW, the strength of Dipole interaction influence the boiling point

DIPOE-DIPOLE FORCESSUMMARY of INTERMOLECULAR FORCES

• BOILING POINT: Effect of H-BONDS?

DIPOE-DIPOLE FORCESSUMMARY of FORCES