Hydrogen bonding

2. Hydrogen bonding Chapter 12 Pages 64 - 65

3. Why do snow flakes form 6 sided shapes?

4. Hydrogen bonding Learning outcomes • Describe hydrogen bonding between molecules containing –OH and –NH groups. • Describe and explain the anomalous properties of water resulting from hydrogen bonding.

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

6. A hydrogen bond • A 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)

7. 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

8. Hydrogen bonds • So, as in all other dipole-dipole interactions, there is attraction between the lone pair on the negative atom and the small positive hydrogen atom

9. Why do snowflakes form six sided shapes • Because 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!

10. The weirdness of water • Catalyst article • Read the article on water • What are some of its unique properties?

11. Exceptional properties of water • The 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; • Ice is less dense than water WEIRD! • Water has high mp and bp WEIRD! Can you explain points 1 and 2 in your own words?

12. Ice is less dense than water • Ice 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

13. 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.

14. Other properties • Insects walking on water are actually using a raft of hydrogen bonds! • Clever little bugs! I love bonding me

15. Hydrogen bonding in biological molecules • Really 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

16. In DNA, three hydrogen bonds link C and G • In DNA, two hydrogen bonds link A and T

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