Hydrogen Bonding Flashcards
Why is hydrogen bonding a special type of inter-molecular force?
it has characteristics of dipole-dipole attraction and some of a covalent bond they are stronger than dipole-dipole attractions but weaker than covalent bonds they have higher boiling points, we would expect the boiling points to be lower if only van der Waals were operating
What does a hydrogen bond consist of?
a hydrogen atom ‘sandwiched’ between two very electronegative atoms
What are the conditions needed for a hydrogen bond to occur?
a very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom e.g. water
Why is the intermolecular bonding stronger in water than hydrogen chloride?
you would expect to find weak dipole-dipole attraction like in hydrogen chloride however: 1) oxygen atoms in water have lone pairs of electrons 2) in water, hydrogen atoms are highly electron deficient, because water is highly electronegative and attracts the shared electrons in the bond towards it the hydrogen atoms are positively charged and very small, these exposed protons have a very strong electric field because of their small size (large charge density)
What happens to the lone pair of electrons on the oxygen atom of another water molecule?
the lone pair is strongly attracted to the electron deficient hydrogen atom
What atoms form hydrogen bonds and what forms hydrogen bonds with water molecules?
O N and F NH3 (ammonia) form hydrogen bonds with water molecules
What happens to the hydorgen bonds when water is liquid?
they break and reform easily as the molecules are moving about
What happens to the molecules when water freezes?
the molecules are no longer able to move and the hydrogen bonds hold the molecules in fixed positions this results in a 3d structure that resembles a diamond
Why is ice less dense than liquid water and what benefit does this have for life?
in order to fit in the diamond structure, the molecules are slightly less closely packed on top of the liquid this means water forms on the top of ponds rather than the bottom this insulates the pond and enables fish to survive through the winter this must have helped life continue in the relative warmth under the ice during the last ice age molecules are held less closely together
Out of:
- Propanal - CH3CH2CHO
- Prop-2-en-1-ol - CH2CHCH2OH
- Butane - C4H10 (Straight chain)
which has the largest boiling point and why?
Prop-2-en-1-ol > Propanal > Butane
Prop-2-en-1-ol has H-bonds
Propanal has permanent dipole-dipole attraction
Butane has van der waals
How are hydrogen bonds shown?
dotted line from the lone pair of the negative atom to the hydrogen atom
why does hydrogen fluoride requires more hgeat for melting than does hydrogen chloride?
hydrogen bonding in HF
dipole in HCl
H-bonds are stronger than dipole attraction
