Chem Bonding [IMF]

Where do id-id interactions arise?

Between all particles

What are id-id interactions?

Instantaneous dipole-induced dipole interactions

How do id-id interactions arise?

• Electron density of a particle being asymmetrical at any given moment, resulting in an instantaneous dipole

• Instantaneous dipole induces a dipole in a neighboring particle

• Short-lived → Weak

What factors affect the strength of id-id interactions

• Increases with the number of electrons in a molecule

  • Larger electron clouds more easily polarised → Greater ease of formation of dipoles

• Increases with the amount of surface area for molecular interactions

  • Straight-chained hydrocarbons have greater surface area for interactions as compared to branched isomers

  • Branching increases → Surface area decreases

What are pd-pd interactions?

Permanent dipole-permanent dipole interactions

Where do pd-pd interactions occur?

Between polar molecules (EFA between delta positive and delta negative ends)

What determines the strength of pd-pd interactions?

Strength increases with increasing dipole moments

What are hydrogen bonds and where do they occur?

• Special type of pd-pd interaction

• Hydrogen atom covalently bonded to a very small, highly electronegative atom with lone electron pairs (F, O, N)

  • Highly electronegative F, O, N atoms cause the H atom to have a high delta positive charge

  • Small sizes of F, O, N allow the lone pair on the other F, O, N atom to come close to the protonic atom

What are the requirements for hydrogen bonding?

• Hydrogen atom covalently bonded to F, O, N

• Lone pair of electrons on F, O, N In a neighboring molecule. bearing a delta- which can attract the delta+ charge on the H atom

What is the general order of strength for the types of covalent bonding between molecules?

H bonds > pd-pd > id-id

Energy (kJmol-1) to overcome

• H bonds → 10 - 40

• pd-pd → 5 - 25

• id-id → 0.05 - 40

How do H bonds determine the boiling and melting point?

• Extensiveness of H bonding

• Strength of bonding

Why is ice less dense than water?

• Highly ordered 3d structure in ice arising from H bonding

• Each oxygen atom is tetrahedrally bonded to 4 H atoms

  • 2 by covalent bonds

  • 2 by hydrogen bonds

• Very open structure preventing molecules from getting too close to one another → Occupies a larger volume for the same mass of water

Why is the molar mass of some carboxylic acids in the vapor phase twice of that calculated from the molecular formula?

• Carboxylic acids exist as dimers in the vapor phase and some non-polar(org) solvents

• 2 carboxylic acids bonded to each other by 2 hydrogen bonds

*H bond with water molecules in aq solutions

Why are the isomers 2-nitrophenol and 4-nitrophenol of significantly different boiling and melting points?

• Intramolecular hydrogen bonding in 2-nitrophenol → Less sites available for H bonding with other 2-nitrophenol molecules → Less extensive intermolecular hydrogen bonding in 2-nitrophenol

• 2-nitrophenol has a lower boiling point and forms fewer H bonds with H2O