Intermolecular Forces

1. Intermolecular Forces

2. Intermolecular vs Intramolecular • Intramolecular forces are forces within a molecule such as covalent bonds

3. Intermolecular vs Intramolecular • Intramolecular forces are forces within a molecule such as covalent bonds • Intermolecular forces are forces between molecules

4. Intermolecular vs Intramolecular • Intramolecular forces are forces within a molecule such as covalent bonds • Intermolecular forces are forces between molecules From: http://webclass.lakeland.cc.il.us/iali/CHM151Folder/BLB-Chapter11.htm

5. Intermolecular Forces • We will consider three types of intermolecular forces • Dipole-dipole bonding • Hydrogen bonding • Van der Waals forces

6. Dipole-dipole bonding • This occurs between polar molecules • The positive end of one polar molecule is attracted to the negative end of another From: http://commons.wikimedia.org/wiki/Image:Dipole-dipole-interaction-in-HCl-2D.png

7. Dipole-dipole bondingin general From: http://stezlab1.unl.edu/reu1999/dputn226/ChemHelp/RET_Web_Pages/im_forces/Intmole_Forces.htm

8. Hydrogen Bonding • This is a special case of dipole-dipole bonding

9. Hydrogen Bonding • This is a special case of dipole-dipole bonding • It occurs when hydrogen is bonded to nitrogen, oxygen or fluorine

10. Hydrogen Bonding • This is a special case of dipole-dipole bonding • It occurs when hydrogen is bonded to nitrogen, oxygen or fluorine • These are the strongest cases of dipole-dipole bonding because of the high electronegativity of F, N and O

11. Hydrogen Bonding • This is a special case of dipole-dipole bonding • It occurs when hydrogen is bonded to nitrogen, oxygen or fluorine • These are the strongest cases of dipole-dipole bonding because of the large difference in electronegativity • Very important in the properties of water

12. Hydrogen Bonding • Hydrogen bonding movie From: www.ualr.edu/botany/botimages.html

13. Van der Waal’s forces • These are the weakest intermolecular force

14. Van der Waal’s forces • These are the weakest intermolecular force • They occur between all molecules but are ignored when either dipole-dipole or hydrogen bonding is present as they are much stronger

15. Van der Waal’s forces • These are the weakest intermolecular force • They occur between all molecules but are ignored when either dipole-dipole or hydrogen bonding is present as they are much stronger • These are due to temporary instantaneous dipoles formed in atoms and molecules as the electrons move around the nucleus/nuclei

16. Van der Waal’s forces • These are also sometimes called dispersion forces or London dispersion forces or London forces • Van der Waal’s forces increase with increasing mass From: www.chem.purdue.edu/gchelp/liquids/disperse.html

17. Van der Waal’s animation • Animation

18. Intermolecular Forces strength • Strongest: Hydrogen Bonding • . Dipole-dipole bonding • Weakest: van der Waal’s forces

19. Boiling temperatures • The presence of hydrogen bonding explains why water, hydrogen fluoride and ammonia have a higher boiling point than the other hydrides down the same groups, even though their mass is less • E.g. water compared to hydrogen sulfide

20. Boiling Temperatures From: www.cem.msu.edu/~reusch/VirtualText/physprop.htm

21. Question • What type of bonding would occur between the following molecules? • N2 • HF • CO2 • H2S • CH4 • NH3

22. Question - Answers • What type of bonding would occur between the following molecules • N2 -Van der Waals as a non-polar molecule • HF - hydrogen bonding as polar with H bonded to F, N or O • CO2 - Van der Waals • H2S - dipole-dipole bonding as a polar molecule • CH4 - Van der Waals as a non-polar molecule • NH3 - hydrogen bonding