OZ - Intermolecular bonds Flashcards
How strong are intermolecular bonds?
Very weak
What are intermolecular bonds?
The forces between molecules
How strong are intermolecular forces compared to covalent/ionic/metallic bonds?
They are much weaker
What are the 3 types of intermolecular bonds?
Instantaneous dipole-induced dipole bonds.
Permanent dipole-permanent dipole bonds.
Hydrogen bonding
What is the strongest type of intermolecular bond?
Hydrogen bonding
How may types of intermolecular bonds are there?
3
What atoms and molecules form Instantaneous dipole-induced dipole bonds?
All
What intermolecular bond do all atoms and molecules form?
Instantaneous dipole-induced dipole bonds.
What do Instantaneous dipole-induced dipole bonds cause?
All atoms and molecules to be attracted to each other.
How do Instantaneous dipole-induced dipole bonds work?
Electrons in charge clouds are always moving really fast, At any particular moment, the electrons in an atom are likely to be more to one side than the other. So at this moment, the atom would have a temporary (or instantaneous) dipole.
This dipole can induce another temporary dipole in the opposite direction on a neighbouring atom - the 2 dipoles are then attracted to each other.
The second dipole can induce yet another dipole in a third atom - like a domino effect.
Because the electrons are constantly moving, the dipoles are being created and destroyed all of the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
What does instantaneous mean?
Temporary
Why do all atoms have Instantaneous dipole-induced dipole bonds.
Electrons in charge clouds are always moving really fast, At any particular moment, the electrons in an atom are likely to be more to one side than the other. So at this moment, the atom would have a temporary (or instantaneous) dipole.
What do intermolecular bonds in organic molecules depend on?
Their shape.
What about the intermolecular bonds does the shape of an organic compound’s molecules affect?
The strength of them.
What intermolecular bonds are there in alkanes?
There are covalent bonds inside the molecules and between the molecules there are instantaneous dipole-induced dipole bonds.
How does the length of the carbon chain in alkanes affect the strength of the instantaneous dipole-induced dipole bonds between molecules?
The longer the carbon chain, the stronger the instantaneous dipole-induced dipole bonds because there is more molecular surface contact and more electrons to interact.
So, as the molecules get longer, it gets harder to separate them because it takes more energy to overcome the instantaneous dipole-induced dipole bonds.
How does branching in alkanes affect the strength of the instantaneous dipole-induced dipole bonds between molecules?
Branched-alkanes can’t pack as closely together and their molecular surface contact is small compared to straight chain alkanes of similar molecular mass. So fewer instantaneous dipole-induced dipole bonds can form.
How does the weight of the atom or molecule affect the strength of the intermolecular bonds?
The heavier the atom or molecule, the stronger the intermolecular bonds.
Why do heavier atoms/molecules have stronger intermolecular bonds?
Because they have a larger electron cloud, so stronger instantaneous dipole-induced dipole bonds.
How does surface area of a molecule/atom affect the strength of the intermolecular bonds?
The greater the SA, the stronger the instantaneous dipole-induced dipole forces because they have a bigger exposed electron cloud.
What needs to happen for a liquid to boil?
The intermolecular bonds need to be overcome.
What is the relationship between the strength of the intermolecular bonds and boiling point?
Liquids with stronger instantaneous dipole-induced dipole bonds will have higher boiling points as you need more energy to overcome stronger intermolecular bonds.
How does the strength of the instantaneous dipole-induced dipole bonds (and the boiling points) change as you go down group 7?
Increases.
Why does the strength of the instantaneous dipole-induced dipole bonds and therefore the boiling point increase down group 7?
Because as the molecular weight (Mr) increases, the number of shells of electrons increases, and so the atomic/molecular size increases.
What type of molecules have permanent dipole-permanent dipole bonds?
Polar molecules
What are permanent dipole-permanent dipole bonds?
The small pos and neg charges on polar molecules cause weak electrostatic forces of attraction between molecules - these are known as permanent dipole-permanent dipole bonds.
Do permanent dipole-permanent dipole bonds happen as well as or instead of instantaneous dipole-induced dipole bonds?
AS WELL AS!!!
What happens if you put an electrostatically charged rod next to a jet of polar liquid like water, and why?
The liquid will move towards the rod because polar liquids contain molecules with permanent dipoles.
It doesn’t matter if the rod is pos or neg charged as the polar molecules in the liquid can turn around so the oppositely charged end is attracted towards the rod.
What experiment can you use to find out if the molecules in a liquid are polar or non-polar and so work out if they’re likely to form permanent dipole-permanent dipole bonds?
Put an electrostatically charged rod next to a jet of the liquid, if it is a polar liquid, then the jet will move towards the rod and so therefore, it has a permanent dipole.
