Hydrogen bondingChapter 12

Pages 64 - 65

Why do snow flakes form 6 sided shapes?

Hydrogen bondingLearning outcomes

• Describe hydrogen bonding between molecules containing –OH and –NH groups.

• Describe and explain the anomalous properties of water resulting from hydrogen bonding.

https://www.youtube.com/watch?v=bOchZanTWHE&feature=plcp

A hydrogen bondA special case of dipole-dipole interaction

Hydrogen bonding affects molecules containing O-H and N-H bonds (and F-H but you don’t need to know this for your exam)

These bonds are polar with permanent dipoles.

It is a particularly strong attractions (the strongest of all of the intermolecular forces you need to know for your exam)

Why?Hydrogen atoms are very small and often

carry a small positive charge

It can approach very closely to a small negative atoms (often a lone pair or a very electronegative atom on a different molecule)

Key features for hydrogen bonding

Hydrogen bondsSo, as in all other

dipole-dipole interactions, there is attraction between the lone pair on the negative atom and the small positive hydrogen atom

Why do snowflakes form six sided shapesBecause ice is an open

network of water molecules

Each molecule has four bonds – two covalent and two hydrogen

Hydrogen bonds are slightly longer

The open structure is made up of rings of 6 oxygen atoms – hence snowflakes shape!

The weirdness of waterCatalyst article

Read the article on water

What are some of its unique properties?

Exceptional properties of waterThe hydrogen bond in water is 5% of the

strength of the O-H covalent bonds but this is still enough to have an effect on the physical properties

Those properties affected are;1. Ice is less dense than water WEIRD!2. Water has high mp and bp WEIRD!

Can you explain points 1 and 2 in your own words?

Ice is less dense than waterIce has an open lattice with the hydrogen

bonds holding the water molecules apart

Ice melts the hydrogen bonds collapse allowing the water molecules closer to each other

High m.p. and b.p.Compare water with a similar molecule

hydrogen sulfide.

Water has a boiling point of 373K whereas hydrogen sulfide has a boiling point of 212K. Much lower even though its relative molecular mass is much greater!

These hydrogen bonds are an extra force over and above van der Waals and these forces have to be overcome in order to melt or boil water.

Other propertiesInsects walking on water are actually using a

raft of hydrogen bonds!

Clever little bugs! I love

bonding me

Hydrogen bonding in biological moleculesReally important in organic compounds

containing O-H and N-H bonds (we’ll meet lots of these later in the year, oh yeah!)

Hydrogen bonds play essential roles in many molecules e.g.Shape determination (e.g. Protein molecules)Holding together the double helix shape of DNA

In DNA, three hydrogen bonds link C and G

In DNA, two hydrogen bonds link A and T

Which is all pretty important for all of us...