intermolecular forces Flashcards
What are intermolecular forces and what do they determine in a compound?
Weak interactions between the dipoles of different molecules.
They can determine the melting and boiling points of a compound.
What are the three main types of intermolecular forces?
Induced dipole dipole interactions , or london forces.
Permanent dipole dipole interactions
Hydrogen bonding
What are the strengths of the three intermolecular forces starting from the weakest to strongest?
London forces, permanent dipole-dipole interactions and then hydrogen bonding
Where do london forces occur?
London forces occur in all molecules/compounds whether polar or non-polar.
How do london forces form?
London forces form when the movement of electrons produce a changing dipole in a molecule , creating an instantaneous dipole which can induce a dipole on a neighbouring molecule.
These are temporary
What do the strengths of london forces depend on?
The more electrons a compound has , the larger the instantaneous dipoles and therefore the stronger the london forces, therefore higher the bp.
Example question: Why does argon have a higher bp than helium?
Argon has a higher number of electrons than helium meaning that it has more london forces and therefore will require a larger amount of energy to overcome the intermolecular forces.
What do permanent dipole-dipole interactions occur between?
Act between permanent dipoles between different polar molecules.
Why does hydrogen chloride have a higher boiling point than a fluorine molecular even though they have the same number of electrons?
HCL and F2 both have london forces of the same strength, however HCL additionally has permanent dipole-dipole interactions which requires additional energy to overcome.
What is a simple molecular substance, what structure does it have and what type of forces does it have?
Simple molecular substances are made up of simple molecules with a definite molecular formulae, e.g N2, O2, or CO2.
Has a simple molecular lattice held together by weak intermolecular forces.
Why do simple molecular substances have low bp and mp?
They have weak intermolecular forces and can be broken by energy present at low temperatures.
What are the solubilities of simple molecular substances?
When the simple molecular substance is non-polar, it is soluble in a non-polar solvent but not soluble in a polar solvent due to the low amount of interactions.
When the simple molecular substance is polar, it is soluble in a polar solvent but not soluble in a non-polar solvent
Can simple molecular substances conduct electricity?
No because there are no mobile charged particles
What is a hydrogen bond and what does it form between?
A hydrogen bond is a type of permanent dipole-dipole interaction which occurs between an electronegative atom (F, O,N) attached to a hydrogen.
What does the hydrogen bond act between? +
Draw a hydrogen bond.
Hydrogen bond acts between a lone pair of electrons on the electronegative atom and a hydrogen atom on a different molecule.
