Unit 6 Superdoc

Intramolecular forces are stronger than intermolecular forces
Strength of intermolecular forces can be measured by boiling/melting points
- vapor pressure
- and heats of vaporization, fusion, and sublimation
REMEMBER THAT YOU CAN’T SAY THAT YOU “BREAK IMFs”
- say overcoming them instead

Vapor Pressure

As temperature increases, vapor pressure increases
The stronger the IMFs → the lower the vapor pressure
- less gas (energetic) molecules due to IMFs pulling them together → less collisions → decreased vapor pressure by definition

Ionic Bonding

Strength of attraction = Lattice Energy
- as charge of ions increases → lattice energy increases
- as size of ions decreases → lattice energy increases
  - think Coulomb’s Law

Dipole-Dipole Forces

Dipole-dipole forces are intermolecular forces that occur between polar molecules. Dipole-dipole interactions arise as the positive end of one polar molecule is attracted to the negative end of another, leading to an alignment of the molecules that stabilize the substance.
dipole moment - separation of positive and negative partial charges within a molecule (uneven distribution of electron density in polar molecule)
As dipole moment increases → attraction increases
1% as strong as covalent or ionic bonds

Ion-Dipole Forces

attraction btwn ion and a partial charge on a dipole
- ion’s charge attracts opposite partial charge on polar molecule

London Dispersion Forces (LDFs)

arise due to temporary fluctuations in electron distribution within molecules creating temporary dipoles
temporary dipoles can induce dipoles in neighboring molecules → attraction btwn them
present in all molecules (polar and nonpolar)
- DOES NOT INCLUDE IONIC COMPOUNDS
LDFs increase in strength as size of molecule increases
responsible for dissociation of ionic compounds in polar solvents

Polarizability

the ease with which the electron distribution in the atom or molecule can be distorted
- atom/molecule size and number of electrons increase → polarizability increases
  - b/c larger atoms/molecules have more diffuse electron clouds that are easier to distort

DIpole-Induced Dipole

Dipole-induced dipoles occur when a polar molecule with a permanent dipole induces a temporary dipole in a neighboring nonpolar molecule.
The positive end of the polar molecule attracts the electron cloud of the nonpolar molecule, causing a distortion in its electron distribution and creating a temporary dipole.
This temporary dipole can then interact with the permanent dipole of the polar molecule, resulting in an attractive force between them.
D-i D explains the solubility of nonpolar molecules in polar solvents
Strength of dipole-induced dipole increases with:
- the magnitude of the dipole moment of the polar molecule
- polarizability of the nonpolar molecule

Hydrogen Bonding

Hydrogen is bound to a highly electronegative atom (FON)
large electronegativity difference
small size allows for close approach → strong IMF
occurs btwn hbond donor and hbond acceptor
- donor is hydrogen attached to FON
- acceptor is one of the FON elements
  - acceptor may or may NOT be bonded w hydrogen
  - needs lone pair
strength of hbonding increases as the total number of potential donors and acceptors increases btwn species involved
lone pair causes dipole moment → partial charges → attraction (bond)

Effect of Hydrocarbon Groups on Solubility

a species behaves more like a nonpolar alkane as the size of its alkyl group increases
- An alkane is a type of hydrocarbon that consists solely of carbon (C) and hydrogen (H) atoms, with all carbon-carbon bonds being single bonds.
  - C_nH_2n+2
- Alkyl group (derived from an alkane by removing one hydrogen atom)

When are organic compounds w/ polar groups water soluble?

One polar functional group capable of hbonding w the solvent is needed for every 5 C atoms it contains

FUNCTIONAL GROUPS

Alcohol:

Ether

Aldehyde

Ketone

Carboxylic Acid

Amine

Ester

Amide

Benzene

If two compounds have similar intermolecular forces (IMFs) but one is significantly heavier (higher molar mass) than the other, the heavier compound will typically have a higher boiling point. This is because the strength of intermolecular forces increases with the size and polarizability of the molecules. The larger molar mass corresponds to more electrons and a larger electron cloud, which can lead to stronger London dispersion forces (a type of IMF) in heavier molecules. As a result, the molecules of the heavier compound will have a stronger attraction to each other, requiring more energy (higher temperature) to break those interactions and thus resulting in a higher boiling point.

Higher IMFS → higher surface tension + higher bp