Kinetic Molecular Theory of Gases

2. Kinetic Molecular Theory of Gases • Composed of small, hard spheres of insignificant volume • No forces of attraction or repulsion between particles • Particles are in continuous, rapid, random straight-line motion • Collisions are perfectly elastic • Describes ideal behavior of most gases at high temps and low pressures.

3. Liquids • More dense than gas • Virtually incompressible • Takes shape of container • Definite volume Gases Solids Greater densities Density not affected by temp. Incompressible Completely rigid shape and volume • Low density • Density affected by temp. • High compressibility • Particles spread out to fill container

4. Liquids Gases Solids Particles are very far apart Particles are close together

5. Inter-particle Attractions • Attractions/forces that hold one particle to another in a sample.

6. Inter-particle Attractions • Attractions/forces that hold one particle to another in a sample. • Ionic bonding • Covalent bonding • Intermolecular Forces • These are what hold the particles close together in the condensed states.

7. Inter-particle Attractions • Type and strength affect the physical properties of a substance • For example, the STRONGER the inter-particle attractions • the ____________ the melting point • the ____________ the boiling point • the ____________ the viscosity

8. Intermolecular forces • Inside molecules (intramolecular) the atoms are covalently bonded to each other. • Intermolecular refers to the forces between the molecules.

9. Intermolecular forces • Weaker than intramolecular forces • Easier to vaporize water than separate to hydrogen and oxygen atoms.

10. Intermolecular forces • Weaker than intramolecular forces • Easier to vaporize water than separate to hydrogen and oxygen atoms. • During phase changes • the molecules stay intact. • Energy used to separate molecules from one another = overcome IMFs / inter-particle attractions.

11. Intermolecular forces • Three main types: • Dipole dipole • Hydrogen bonding • London dispersion forces

12. 1. Dipole - Dipole • The positive and negative ends of polar molecules interact with each other, resulting in a net force of attraction. • 1% as strong as covalent bonds • Weaker with greater distance.

13. + + - - - + + - + - - + + - - + - + + -

14. 2. Hydrogen Bonding • Especially strong dipole-dipole forces • Occurs between molecules that contain H attached to F, O, or N • These three because- • They have high electronegativity. • They are very small. • About 10 % as strong as an ordinary covalent bond.

16. Water d+ d- d+

17. The Double Helix of DNA is held together by hydrogen bonding adenine thymine white = hydrogenblue = nitrogenblack = carbonred = oxygen

18. The polymer Nylon is also held together by hydrogen bonding

19. 3. London Dispersion Forces • Non-polar molecules (and atoms) also exert forces on each other. Otherwise, these substances could never be liquid or solid state. • carbon dioxide • iodine

20. 3. London Dispersion Forces • Non-polar molecules (and atoms) also exert forces on each other. Otherwise, these substances could never be liquid or solid state. • Electrons are not evenly distributed at every instant in time. • Have an “instantaneous dipole”. • Induces a dipole in the atom next to it.

21. d- d- d+ d+ d- d+ H H H H H H H H H H H H Example

22. London Dispersion Forces • Weak, short lived. • Last longer at low temperatures. • Eventually long enough to make liquids. • Much weaker than other forces.

23. London Dispersion Forces • Strength based on polarizabilityof the molecule • Polarizability - a measure of the extent to which the electron cloud of an atom or molecule can be distorted by an external electric charge. • More electrons = more polarizable • More polarizable = stronger dispersion forces

24. Example Which one has stronger dispersion forces: H2 or I2? I – I H – H WHY?

25. Summary of Intermolecular Forces • Weakest? When does it occur? • ? When does it occur? • Strongest? Conditions required for it to occur?

27. IMFs affect . . . . • melting and boiling points  the physical state of the substance at a given temperature • conductivity • chemical reactivity • hardness or softness

28. Summary Types of Interparticle Attractions • . • . • . • . • . • . • . Covalent bonding Ionic bonding Metallic bonding Ion-Dipole Forces Hydrogen bonding Dipole-Dipole forces Dispersion forces