6.4 Hydrogen bonding Flashcards
Define hydrogen bond
A strong dipole-dipole attraction between an electron-deficient hydrogen atom of -NH, -OH or HF on molecule and a lone pair of electrons on a highly electronegative atom containing N, O, F on a different molecule
Where can a hydrogen bond be found?
-Between molecules containing an electronegative atom with a lone pair of electrons
eg. N, O, F
-Between molecules containing a hydorgen atom attracted to an electronegative atom
eg. H—O H—N H—F
What does a hydrogen bond do?
It acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule
What is the strongest type of intermolecular attraction
Hydrogen bonding
How is a hydorgen bond represent when drawn out?
As a dashed line
According to hydrogen bonding hey is solid water (ice) less dense than liquid (water)?
-Hydrogen bonds hold the water molecules apart in a open lattice structure
-The water molecules in ice are further apart than in water
-Solid ice is less dense than liquid water and floats
-Water contains oxygen which has two lone pairs and two hydrogen atoms so each water molecule can form 4 hydrogen bonds which extend outwards, holding the water molecules slightly apart in a open tetrahedral shape with the bond angle close to 180°
-The holes in the open lattice structure decrease the density of water on freezing
-When ice melts, the ice lattice collapses and the molecules move closer together
What is the melting and boiling point of water?
-Relatively high melting and boiling point as hydrogen bonds are extra forces over and above London forces
-A lot of energy is required to break hydrogen bonds so water has a high melting and boiling point above those expected from Londons forces
What happens to the hydrogen bonds when water boils?
They break
What other properties do hydrogen bonds contribute to in water?
-Relatively high surface tension and viscosity
