Energy and the States Of Matter

1. Energy and the States Of Matter Forces between particles States of matter Changes in state

2. Learning objectives • Describe types of motion in molecules • Distinguish among states of matter based on properties • Describe four types of intermolecular forces • Identify main forces operating in simple substances • Perform calculations of heat involved in changes of state

3. Atom motion and temperature • Atoms in molecules have three types of motion • Rotation – moving about the centre of mass • Vibration – vibrating about the centre of mass • Translation – movement of the centre of mass • As temperature increases, the energies of all types of motion increase

4. Three States of Matter Solid: strong interactions • Fixed shape • Not compressible • Rigid • Dense

5. Liquid: medium interactions • Liquid • Not rigid • Assumes shape of container • Not compressible • Dense

6. Gas: no interactions • Not rigid • Completely fills container • Compressible • Low density

7. Plasma: the fourth state • At very high energies (temperatures) all the electrons are removed from the atoms • Not an important state for chemistry

9. May the force be with you • Covalent and ionic bonds are the intramolecular forces that hold the atoms in molecules together • Intermolecular forces hold the molecules together • Collectively, the intermolecular forces are called van der Waals forces • All arise from electrostatic interactions

10. Intermolecular forces • Polar molecules experience strong intermolecular interactions due to existence of positive and negative ends of dipoles • Weaker forces exist between nonpolar molecules. These are due to fluctuations in the electron distributions in the molecules which create momentary (weak) dipoles – London forces • Intermolecular interactions are known collectively as van der Waals forces

11. The Four Forces of the Apocalypse

12. Hydrogen bonding: something about water • High boiling point compared with similar compounds • Liquid at earth temperature • Solid less dense than liquid • Essential for life on earth • High heat capacity • Modifying influence on climate • Universal solvent

13. Hydrogen bonding • The ultimate expression of polarity • Small positive H atom exerts strong attraction on O atom • Other H-bonding molecules: HF, NH3 • H2O is the supreme example: two H atoms and two lone pairs per molecule

14. H2O has optimum combination of lone pairs and H atoms

15. H bonding generates three-dimensional network

16. Ice floats! • Something so familiar we might believe all solids float on their liquids. Not so. Water is the exception.

17. Hydrogen bonding and life • hold the two strands of the DNA double helix together • hold polypeptides together in such secondary structures as the alpha helix and the beta conformation • help enzymes bind to their substrate • help antibodies bind to their antigen • help transcription factors bind to each other • help transcription factors bind to DNA

18. Implications for life on earth • Without H-bonds molecules like DNA would not exist • H-bonds hold the two strands together • Comparative weakness of bonding allows for DNA replication dna

19. Intermolecular forces determine physical properties • Strong ionic bonds mean high melting point • Dipole-dipole interactions – much lower melting points • Dispersion forces only – very low melting points