2.2.12 Hydrogen Bonding Flashcards
What is Hydrogen bonding? How is it drawn?
A TYPE OF IM BOND WHICH IS SLIGHTLY LONGER THAN OTHER BONDS, A strong permanent dipole-permanent dipole interaction between:
- An electron-deficient atom: O-Hdelta+, N-Hdelta+ or F-Hdelta+ and
- A lone pair of electrons on a highly electronegative atoms (N,O or F) on a different molecule.
- It is drawn as a dotted line starting at H+ and going to a lone pair on O, N or F delta -
What happens in Hydrogen bonding? This explains why you draw Hydrogen bonding the way you do.
The electron deficient Hdelta+ (because it has permanent dipole with highly electronegative N, F or O) attracts a lone pair of electrons on a O, N or F delta - bonded with a different molecule.
What happens in H bonding in ammonia and water?
H bonds form between O’s lone pair and ammonia’s H as H is very electron-deficient. Is electron deficient as is attached to delta - N and has lone pairs - so is more +ive.
Name two effects Hydrogen bonding has on the properties of water.
- Ice is less dense than water
2. Water has higher than expected melting and boiling points
Why is ice less dense than water?
- H bonds don’t occur often in liq water - molecules slide past each other. In ice, H bonds can form
- Ice has an open lattice made up of 6 rings of O atoms (snowflakes based arnd 6 sided shapes) - H bonds hold water molecules appt, so take up more space - less dense than water, ice floats
- Ice melts, H bonds collapse, so H2O molecules move closer together
Why does water have higher than expected melting and boiling points?
- H bonds are stronger than other IM forces
- This extra strength has to be overcome
- So water has much higher boil pt than group 16 hydrides
What are the other properties of water and why?
Extra IM bonding from H bonds leads to:
- Higher surface tension
- Higher viscosity - so pond skater can skate on H2O
