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

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

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

Three States of Matter Solid: strong interactions  Fixed shape  Not compressible  Rigid  Dense

Liquid: medium interactions  Liquid  Not rigid  Assumes shape of container  Not compressible  Dense

Gas: no interactions  Not rigid  Completely fills container  Compressible  Low density

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

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

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

Name of force OriginStrength Ion-dipole Between ions and molecules Quite strong (10 – 50 kJ/mol) Dipole-dipole Between permanent dipoles Weak (3 – 4 kJ/mol) Hydrogen bonds Polar bonds with H and (O,N) Quite strong (10 – 40 kJ/mol) London dispersion forces Fluctuating dipoles in non-polar bonds Weak (1 – 10 kJ/mol) The Four Forces of the Apocalypse

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

Hydrogen bonding  The ultimate expression of polarity  Small positive H atom exerts strong attraction on O atom  Other H-bonding molecules: HF, NH 3  H 2 O is the supreme example: two H atoms and two lone pairs per molecule

H 2 O has optimum combination of lone pairs and H atoms Compound Number of lone pairs Number of H atoms HF31 H2OH2OH2OH2O22 NH 3 13

H bonding generates three- dimensional network

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

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

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

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