Bonding:Intermolecular Forces

Covalent BondsCovalent bonding leads to two types of molecules

Giant StructuresEg. Diamond, sandStrong covalent bonds

between atoms create a large network resulting in a high melting and boiling point

Simple Molecular Structures

Eg. Methane, hydrogen chloride

Few covalent bonds per molecule. Exist as gases or low boiling liquids therefore have low melting and boiling points.

How we know intermolecular forces exist

Gases have large distances between molecules, short range forces between molecules are insignificant

Cool and Compress

Distances between molecules decrease and the forces between become significant. Gases liquefy (G->L)Cool and Compress

Distances are further reduced. Forces become more significant. Liquid solidifies. (L->S)

Types of Intermolecular Forces

Examples to explain London ForcesBoiling points of noble gases•He- 4K•Xe-165K•Higher atomic mass=higher number of electrons

Boiling points of alkanes•Methane- 111K•Hexane: 341K•As molar mass increases, london forces increase

Boiling points of Halogens•As molar mass of halogens increases from F2 to I2 the boiling points also increase

Boiling points of straight chains & branches Alkanes of the same molar massn pentane BP 309 Kneo pentane BP 283 K

Plastics and PolymersThey have very highMolar mass and very high surface area

Examples to explain Dipole-Dipole Forces

Hydrogen Bonding

Hδ+-Fδ---------------Hδ+-Fδ-

• Strongest of the intermolecular forces• Affects the physical properties of the

compounds

Examples of H-Bonding

Hydrogen Bonding in diff. compounds

Examples of H-Bonding contd….

Examples of H-Bonding contd….

Trends in Boiling points of Groups 4,5,6 and 7

Examples of H-Bonding contd….

Anomalous behavior of Water

Structure of ICE

Hydrogen Bonds in Water, Ice

Compare H2O and H2SProperties Water Hydrogen sulphide

Molar enthalpy of fusion 6.0 kJmol-1 2.4 kJmol-1

Molar enthalpy of vaporization 41.0 kJmol-1 19.0 kJmol-1

State at room temperature & Pressure

Liquid Gas

Electro negativity Very high Less

Size O atom is small S is larger than O

Hydrogen Bonding 2 H bonds per molecule of water

No H Bonding

Effect of H bonding on Organic compounds- case 1

Dimer of Ethanoic acid

Effect of H bonding on Organic compounds- case 2

• Propanone is miscible in water but it has no H bonding between propanone molecules

• This is because its O atoms can bond to water molecules.

Effect of H bonding on Organic compounds- case 3

• p- nitro phenol or 4- nitro phenol is having intermolecular H bonds , stronger forces of interaction between molecules and so its BP is 279°C

• o- nitro phenol or 2- nitro phenol has intramolecular H bonds, weaker forces of interaction between molecules and so has lower BP of 216°C

Biological importance of H bonding

• DNA pairing occurs due to H bonds.• Secondary structure of proteins (α helix and β pleated proteins)

• α helix-is an example of INTRA MOLECULAR H Bonding

Hydrogen bonds in DNA Base Pairs2 H bonds between Adenine and Thymine3 H bonds between Cytosine and Guanine