Physical Properties of organic compounds; melting point, boiling point, solubility

Differentiate between intermolecular and intramolecular forces (2 points)

1) Location of forces: Intermolecular forces describe attractive forces between molecules whereas intramolecular forces describe attractive forces between atoms within molecules

2) Strength: Intramolecular forces are much stronger than intermolecular forces

Name the 3 intramolecular forces we study

covalent bond, ionic bond, metallic bond

Name the 4 intermolecular forces we study

1) Ion Dipole forces
2) Van der waals forces: london dispersion, dipole-dipole, hydrogen bonding

what is bond length determined by

size of atoms

bond order (the number of bonding electron pairs shared by atoms in a molecule) - eg c-o would bave BO of 1, c=o would have BO of 2, etc.

van der waals radius definition

This is defined as half the distance between the nuclei

of two identical atoms which are not bonded together.

what is the trend with the ionic radii of cations

the radius of a cation is always smaller

than that of the parent atom

what is the trend with the ionic radii of anions

the radius of an anion is always larger

than that of the parent atom

ion dipole forces

Forces of attraction between an ion and the

partial charge on the end of a polar molecule.

The magnitude of the attraction for ion dipole forces increases as:

• The charge of the ion increases (a +2 charge pulls harder than a +1 charge).

• The magnitude of the dipole moment increases (a more polar molecule with stronger partial charges pulls harder than a weakly polar one).

Trend in London Dispersion Forces (Size & Polarizability)

Higher polarizability means the electrons can create bigger temporary dipoles, and bigger temporary dipoles mean a stronger sticky attraction between the molecules!

Larger Molecules → more electrons → Larger electron cloud → Electrons further from the nuclei so less tightly bound to the “distant” nuclei → Easier to distort electron cloud → ↑ POLARIZABILITY → Stronger dispersion forces

what increases the strength of london forces

• greater molecular size

• greater atomic/molecular mass

• increased surface area of the molecule

factors influencing the magnitude of dipole dipole forces

• polarity: greater polarity = greater strength of dipole dipole force

unusual properties of water due to hydrogen bonding

• High melting point

• High boiling point

• Density of ice lower than for water

what is chemical change and what kind of forces are involved

chemical change is a process by which one or more substances is/are changed into one or more different substances

intramolecular forces are involved

factors influencing the magnitude of van der waals forces

• London forces: the size of the molecule, the molecular mass, the surface area

• dipole dipole: the polarity of the molecules (electronegativity difference)

• hydrogen bonding: the amount of hydrogen bond per molecule, the molecular geometry (ie water is bend while HF is linear), polarity between H-F/O/N

what is physical change and what kind of forces are involved

is any change in which the amounts (number of moles) and nature of the substances present do not change. This involves the breaking and forming of INTERmolecular forces

melting/fusion and energy change associated

solid to liquid - endothemic: energy is needed

boiling/vapourisation and energy change associated

liquid to gas - endothemic: energy is needed

condensation and energy change associated

gas to liquid - exothermic: energy is released

freezing and energy change associated

liquid to solid exothemic: energy released

sublimation and energy change associated

solid to gas endothermic: energy needed

deposition and energy change associated

gas to solid exothermic- energy is released