Intermolecular forces Flashcards
(22 cards)
How do you decide if something is a polar molecule?
Do the bonds have dipoles? Has to be yes.
Draw shape. Has to be non-symmetrical so polar bonds don’t cancel.
How do you know if a molecule is symmetrical?
If the centre of electronegativity is where the delta positive is.
What is the strongest intermolecular force?
Hydrogen bond
What is the weakest intermolecular force?
London dispersion forces
How do you know if there are hydrogen bonds?
Look for H bonded to nitrogen, oxygen or fluorine AND a lone pair on the nitrogen, oxygen or fluorine.
How do you know if there are london dispersion forces?
If there are not hydrogen bonds then decide if it’s a polar molecule. If it isn’t then there’s only LDF
How do you know if there are permanent dipole - permanent dipole forces.
If there are no hydrogen bonds but it is a polar molecules then there are PD-PD forces.
What has LDF?
Everything but only talk about it when it’s the only force.
How are LDF made?
Electrons in atoms are constantly moving.
Sets up an uneven distribution called an instantaneous dipole which induces dipoles on other molecules.
These dipoles then attract each other.
How do you increase the strength of LDF?
More electrons, larger instantaneous/ induced dipoles, this means there is more energy so stronger LDF
How do you increase the strength of PD-PD forces?
The more electronegative the larger the permanent dipole so the stronger the force.
What do you use to compare boiling points of molecules with hydrogen bonds?
Electronegativity
Ratio between H bonds and the number of lone pairs
What are simple molecules?
Anything you can draw a shape for
Can simple molecules conduct electricity?
No
Why can simple molecules not conduct electricity?
No delocalised electrons (Electrons are in bonds)
No mobile ions (covalently bonded)
What structure is formed by intermolecular forces?
A lattice
What shape is made by water?
Tetrahedral as hydrogen bonds can be formed in 4 places
What causes ice to be less dense than water?
Hydrogen bonds hold water molecules in an open lattice making it less dense.
Why does water have a relatively high boiling point?
They are the strongest intermolecular force but weaker than other bonds (covalent/ionic/metallic). They are relatively hard to break so need a relatively high amount of energy to break them
What will non polar substances dissolve well in?
Non polar solvents as they will form LDF with each other and mix.
What will polar substances dissolve well in?
Polar solvents as they will form PD-PD forces with each other and mix
What is it called when 2 substances don’t mix so form layers?
They are immiscible
