Intramolecular and Intermolecular Forces We have just been discussing the covalent bond - the force that holds atoms together making molecules. We have also talked about the ionic bond. These are intramolecular forces. There are also forces that cause molecules to attract each other. These are called intermolecular forces.
Intermolecular Forces Salt (NaCl) is a solid because of the strong electrostatic attraction of the Na + and the Cl – ions: the ionic bond Q: Why is Cl 2 a gas, Br 2 a liquid and I 2 a solid? A: Intermolecular forces Q: Why is molasses “thick” while water has low viscosity? Same answer. Q: Why is it possible to float a needle on water?
Intermolecular Forces (cont’d) Q: Why is water a liquid but H 2 S is a gas at 25ºC? Q: Why are real gases not ideal? A: van der Waals forces (same thing, different name)
States of Matter Gas: No defined shape or volume, compressible, rapid diffusion, flows readily Liquid: Takes shape of container, virtually incompressible, low diffusion, flows readily Solid: Has its own shape and volume, virtually incompressible, extremely slow diffusion, does not flow Liquids and solids are called condensed phases because particles are close together
Intermolecular Forces Much weaker than chemical bonds. Covalent bonds 200 kJ/mol and more. Intermolecular forces less than 50 kJ/mol When a liquid vaporizes, the intermolecular forces must be overcome. But no covalent bonds are broken.
Ion - dipole forces Dissolve an ionic compound in water Charged ions interact with the dipole of water molecules
Dipole-dipole Forces Interactions between the dipoles in a polar liquid leads to a net attraction
Dipole - Induced dipole force In a mixture of two liquids were one is polar and the other is not, the dipole of the polar molecule can induce a dipole in the other. The energy of this force depends on the polarizabilty of the non-polar molecule. Larger molecules are more polarizable
London Dispersion Forces How do we account for the fact that non-polar gases can be liquefied and solidified? Fritz London proposed instant dipoles. These are dipoles that result from the random movement of the electron cloud.
Dispersion forces work over very short distances
Polarizability The ease with which a dipole can be induced depends on the polarizability of the molecule Large molecules are more polarizable - easier to distort the electron cloud
Polarizability and Boiling Points Boiling points (K) of halogens F 2 Cl 2 Br 2 I 2 85.1238.6332.0457.6 Boiling points of Noble gases HeNeArKr Xe 4.6 27.387.5120.9 166.1
Dipole or Dispersion? Dispersion forces operate between all molecules - polar and non-polar Molecules with comparable molecular weights and shapes have approximately equal dispersion forces. Any difference is due to dipole-dipole attractions When molecules differ widely in molecular weight, dispersion forces tend to be the decisive ones
Water And Ammonia Have Unusual Boiling Points!!
The Hydrogen Bomb (err – Bond) A special dipole-dipole intermolecular attraction H in a polar bond (H-F, H-O or H-N) an unshared electron pair on a nearby electronegative ion or atom (generally F, O or N) Hydrogen bonds (4 to 25 kJ/mol, or larger) are weaker than covalent bonds but stronger than most dipole-dipole or dispersion forces.
Molecules with H bonding HF (can behave as if it were H 2 F 2 ) NH 3 H 2 O (ice is less dense than liquid at 0ºC) alcohols (e.g. CH 3 OH) amines (e.g CH 3 NH 2 ) carboxylic acids (e.g. CH 3 COOH)
Liquids - viscosity Viscosity (resistance to flow) Molecules that have strong intermolecular forces cannot move very easily - more viscous Viscosity decreases at higher temperatures. The kinetic energy overcomes the intermolecular forces
Liquids - surface tension Surface tension: the energy that must be expended to increase the surface of a liquid
Surface tension Liquids with strong intermolecular bonds have high surface tensions Water: strong H-bonds, so high surface tension, 7.29 10 –2 J/m 2 Mercury: atoms held by metallic bonds, so even higher surface tension, 4.6 10 –1 J/m 2
Wetting & capillary action Water wets clean glass (spreads out) but beads on a waxy surface Water climbs up a capillary tube Mercury does not wet glass Mercury level is depressed in capillary tube
To wet or not to wet? Competition between two tendencies Cohesive forces; intermolecular forces that bind similar molecules together. Keeps liquid as a bead Adhesive forces: Intermolecular forces between molecules of a liquid and those of a surface. Makes liquid spread out
Are we all wet?? Water on clean glass adhesive forces > cohesive forces explains the upward meniscus of water Water on polished table cohesive forces win because H 2 O molecules are not attracted to wax Mercury on glass - cohesive forces win because Hg molecules are not attracted to glass Explains depression in capillary tube and downward meniscus
Enthalpy of fusion or heat of fusion for water H fus = 6.01 kJ/mol. Enthalpy of vaporization or heat of vaporization for water H vap = 40.67 kJ/mol. cooling effect of evaporation refrigeration (Not Freon-12) steam burn generally severe
Supercooling, superheating A liquid cooled below its freezing point is said to be supercooled requires very clean conditions the molecules are moving slowly but have not organized themselves into the solid form A liquid heated above its boiling point is said to be superheated a danger with heating water in microwave oven
Critical T and P A gas can be liquefied by cooling A gas can be liquefied by increasing pressure but only if the temperature is below the compound’s critical temperature substancecritical temp K and C ammonia405.6 (133) carbon dioxide304 (31) argon150.9 (-122) Critical pressure: pressure needed to liquefy gas at critical temperature Substance at T c and P c - supercritical fluid
Vapour Pressure Every liquid in a closed container gives off vapour until a certain pressure is reached - the liquid’s vapour pressure. The vapour pressure of a liquid increases with increase in temperature. We can explain these facts using the kinetic theory
Volatile A liquid in an open container will evaporate As vapour moves away, the liquid releases more molecules into the vapour phase to try to build up to the correct vapour pressure Liquids with high vapour pressure evaporate more quickly - they are volatile
Boiling point: the temperature at which vp = 760 torr
Boiling A liquid boils when the vp equals the atmospheric pressure Normal boiling point temp -> when vp is 760 torr. We list normal bp values in textbooks Actual boiling point -> then the liquid has a vp equal to the external atmospheric pressure Water boils at temperature lower than 100ºC atop mountains - it never reaches 100ºC Water boils at higher temp in pressure cooker
Phase diagrams A graphical way to show the equilibria between different phases of a substance Thing to look for: critical point triple point ( three phases) how bp (and mp) varies with pressure Triple point is not pressure dependent - the vapour has to be at the critical pressure! useful for thermometer calibration
Structure of Solids Crystalline: atoms, ions, or molecules are well ordered. Have a well-defined melting point. Often the solid has regular shapes. Amorphous: no order to the particles. Examples are glass and rubber. Have no defined mp; they soften over a range of temperatures (important for glass blowing)
Unit cell: crystal lattice In a brick wall there is a repeating pattern, as there is with most wallpaper In a crystalline solid there is a repeating pattern - the unit cell. The unit cell repeats to make the crystal lattice
Some ionic solids- lattice decided by size & charge
From sea to shining... array of metal ions in a sea of electrons A sea of valence electrons Electrons not tightly held but can move Explains electrical conduction Also explains optical properties - most metals “shine”