4.4: Intermolecular forces Flashcards
Intermolecular forces meaning
Forces that exist between molecules
3 types of intermolecular forces
London Dispersion forces, dipole-dipole forces, hydrogen bonding
London (dispersion) forces
At some instant, more of the electron cloud happens to be at one end of the molecule; the molecule has an instantaneous dipole (eg: Cl2-Cl2)
Dipole-dipole forces
Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. (HCl)
Hydrogen bond
An electrostatic force of attraction between a hydrogen atom which must be bonded to a highly electronegative element that is nitrogen (N), fluorine (F) and oxygen (O).(eg: H2O)
Strength of intermolecular bonds (weakest to strongest)
LDF->Dipole-dipole forces->Hydrogen bonding
Why is ice less dense than water at atmospheric pressure?
Water is at its densest at 4ºC and becomes less dense as that water molecules begin into the open, hexagonal structure present in ice crystals at its freezing point.
Meaning of intramolecular forces
The bond within the molecule
Van der Waal’s forces
LDF and dipole-dipole forces
What do the intermolecular forces between molecules tell us?
The physical properties of the molecules (ie: boiling point, melting point, and solubility of a substance)
Temporary dipole
Caused by changes of electron density within an atom or molecule, giving it a slight partial positive or negative charge
Induced dipole
When a molecule with a temporary dipole induces a neighbouring dipole
What do the strength of LDF depend on?
- polarisability of the molecule (increases with increasing molar mass)
- Surface area of the molecule
Put these molecules in terms of lowest boiling point to highest
Br2, I2, Cl2, F2
Which molecules and atoms have LDF?
All
How do you know if a molecule is dipole-dipole?
Only exists between polar molecules that have a permanent dipole due difference in electronegativity, giving it a partial negative/positive charge (eg: HCl)
Hydrogen bonding occurs between
Molecules that have an electronegative nitrogen, oxygen, or fluorine atom directly bonded to a hydrogen atom
The effects of a hydrogen bond on boiling points (give examples)
Higher boiling points (Eg: H2O, HF and NH3 all have higher boiling points than other molecules with similar molar masses)
Why is there a gradual increase from H2S to H2Te?
The molar mass increases, creating a stronger LDF
Why do group 14 hydrides have a lower boiling point? (eg: CH4)
They are non-polar, so they only have LDF forces between them
What does ‘like dissolves like’ mean? Give examples
Polar substances dissolve in polar solvents (eg: ethanol and water), non-polar substances dissolve in non-polar solvents (eg: carbon dioxide and hexane)
boiling points of the halogens down a group?
The boiling points increase down a group because of the increasing London dispersion forces between the molecules
why is the boiling point of PH3 is lower than that of NH3?
PH3 is not hydrogen bonded whereas NH3 is hydrogen bonded
