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Intermolecular Forces

Chemistry Project. Intermolecular Forces. Introduction. What are intermolecular force? They are forces of attraction or repulsion between molecules. . Two Types of Intermolecular forces. 1. van der Waals ’ Force a. Dipole-Dipole Interaction b. Dipole-induced Interaction

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Intermolecular Forces

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  1. Chemistry Project Intermolecular Forces

  2. Introduction • What are intermolecular force? They are forces of attraction or repulsion between molecules.

  3. Two Types of Intermolecular forces • 1. van der Waals’ Force a. Dipole-Dipole Interaction b. Dipole-induced Interaction c. London Forces (instantaneous Dipole-induced Dipole Interaction) • 2. Hydrogen Bonding

  4. Dipoles • Dipole is a pair of electric charges or magnetic poles of equal magnitude but opposite polarity (opposite electronic charges), separated by some (usually small) distance.

  5. Before discuss intermolecular forces, we should know about what is DIPOLE. This can help us easier to understand the intermolecular forces.

  6. Types of Dipole • 1. Permanent Dipole • 2. Instantaneous Dipole • 3. Induced Dipole

  7. Permanent Dipole • Exists in all polar molecules(eg. HCl) • Bond electrons are unsymmetrically distributed • Formed when the distribution of the charge is not uniform

  8. Instantaneous Dipole • Formed due to the fluation of electron clouds • Postive charge and negative charge exist temporarily (Electron cloud is displaced) • In non-polar molecules (e.g. H2)

  9. Induced Dipole • When a dipole molecule comes close to a non-polar molecules, the non-polar molecule will be induced to form a dipole temporarily

  10. Van der Waals’ Forces—Dipole-Dipole Interaction • Attraction between the negative end of one polar molecule and the positive end of another.

  11. Exist between molecules that are polar. • Requires the presence of polar bonds and unsymmetric molecule • weak when compare to a covalent bond but is one of the stronist intermolecular attractions • Attractive force between molecules are maximized; Repulsive force are minimized

  12. Van der Waals’Force—Dipole-inducedDipole Interaction • An original dipole and an induced dipole attract each other • The induced dipole will be in an orientation opposite to that of the polar molecule • Generally is weaker than the Dipole-Dipole interactions

  13. van der Waals’Forces—LondonForces • Induced a neighbouring atom to form a dipole, the induced dipole will be in an orientation opposite to that of the instantaneous dipole

  14. Exists between non-polar molecules • Weak attraction • Also named—Dispersion force • Type of weak attraction that exists between non-polar molecules

  15. Strength of van der Waals’ Forces • Factors affect the strength of the forces 1.Sizes of Electron cloud 2.Shapes (surface area) of Molecule

  16. The size of electron cloud increase and increase the radius of the atom. More electrons , and more distance over which they can move Bigger the possible temporary dipoles and therefore the bigger the London forces. Boiling point of elements helium -269°C neon -246°C argon -186°C krypton -152°C xenon -108°C radon -62°C (Boiling points increase as down the group) 1. Size of Electron Cloud

  17. 2. Shapes (surface area) of Molecule • Long thin molecules can develop bigger temporary dipoles (due to electron movement) than short fat ones containing the same numbers of electrons. • Long thin molecules(A larger surface area) can also lie closer together

  18. Example • Hydrocarbon molecules butane and 2-methylpropane • Both have a molecular formula C4H10, but the atoms arearranged differently. Butane --carbon atoms are arranged ina single chain, 2-methylpropane --shorter chain with a branch. • Butane has ahigher boiling pointbecause the London forces are greater. The molecules arelonger(and so set up bigger temporary dipoles) and canlie closer togetherthan the shorter, fatter 2-methylpropane molecules.

  19. Hydrogen Bonding the electrostatic attraction between thelone pairof electrons on thehighly electronegativeatom and the slightly positivelyhydrogen atom.

  20. Strength of Hydrogen Bonds • Depends on itslength, angle, orientation, electromagnetic, magnetic effects and positions of the other bonded and non-bonded atoms and 'lone pair' electrons • Length increase, the hydrogen bond strength increase. • If the hydrogen bond is substantially bent then it follows that the bond strength is weaker

  21. Comparing the strength of van der Waals’ force and Hydrogen Bond

  22. As hydrogen bonds are much stronger then van der Waals’ force, more energy is needed to break the hydrogen bonds in HF, NH3 and H2O. As the result, the boiling points of these hydrides are relatively higher.

  23. Importance of … • 1. van der Waals’ Forces —Structure of graphite

  24. Graphite is made up of hexagonal arrays of carbon atoms arranged in layers. Weak van der Waals’ forces hold the layers together. The weak van der Waals’ forces of attracion between the layers allow one layer of bonded atoms to slide easily over another layer. This explains why graphite is soft and slippery and can be used a lubicant. Application--- • Pencil • Lubricant in motor oil.

  25. 2. Van der Waals’ force--Dry ice • Covalent compounds usually crystallize as molecular solids, in which the lattice points are occupied by individual molecules, such as the structure of dry ice. • The weaker intermolecular bonds between these molecules result from the relatively weak van der Waals forces • The van der Waals forces holding CO2 molecules together in dry ice • For example, they are so weak that dry ice sublimes it passes directly from the solid to the gas phase at –78.5C.

  26. More, the individual molecules are held together by relatively strong intramolecular bonds between the atoms.

  27. 3. Hydrogen Bond--Water • Structure of water--

  28. Liquid water's high boiling point is due to the high number of hydrogen bonds each molecule can have relative to its low molecular mass. • Water has oxygen atom that has two lone pairs and two hydrogen atoms--- the total number of bonds of a water molecule is four.

  29. Structure of Ice

  30. In ice, the crystalline lattice is dominated by a regular array of hydrogen bonds which space the water molecules farther apart than they are in liquid water. • This accounts for water is decrease in density upon freezing(lowing than 4oC ). • the presence of hydrogen bonds enables ice to float, because this spacing causes ice to be less dense than liquid water. • In application---In winter or cold places, the ice will form float on the water surface, but under the layer, fish can keep alive.

  31. Protein—Secondary structure • Hydrogen bond plays a crucial role in the structure of protein • It forms between the N-H and C=O groups. • It cause the polypeptide chains to become twisted into tightly coiled helics. • Example: Hair, wool, enzymes and nails.

  32. Structure of DNA • DNA molecules consists of… • A sugar • A phosphoric acid unit • A nitrogen-containing heterocyclic base (Adenine, Cytosine, Guanine, Thymine)

  33. Hydrogen bond hold the TWO nucleic acid chain together. They are formed between specific pairs of bases on the chain. Like the figures below. In DNA replication, the hydrogen bonds are broken down. When a complementary chain is formed adjacent to each of the original chain by the formation of NEW hydrogen bonds.

  34. Solubility Solubility Rule and Summary “LIKES DISSOLVE LIKES” • Ionic or Polar solutes dissolve in Polar solvents. • Non-polar solutes dissolve in Non-polar solvents • Polar and Ionic solutes DO NOT dissolve in Non-polar solvents and vice versa.

  35. The dissolving process involves a consideration of the relative strength of intermolecular attractive forces. • The type of forces between solute-solute molecules and solvent-solvent molecules must be considered. • These intermolecular attractions must be broken before new solute-solvent attractive forces can become effective. Perhaps the bond breaking and bond forming processes take place simultaneously.

  36. A solute will dissolve in a solvent if --- • solute-solvent forces of attraction are great than the solute-solute andsolvent-solvent forces of attraction. • Generally, if ALL three of the intermolecular forces of attraction are roughly equal, the substances will be soluble in each other.

  37. Ionic Solute with Polar Solvent • Ionic Solute with Non-polar Solvent

  38. Non-polar Solute with Non-polar Solvent • Non-polar Solute withPolar Solvent

  39. Example for each situation • POLAR SOLUTE & POLAR SOLVENT---Ammonia Dissolves in Water • NON-POLAR SOLUTE & NON-POLAR SOLVENT---Iodine dissolves in Bromine

  40. Theory of intermolecular force to explain the properties of the substance • Evaporation the magnitude of the temperature decrease during evaporation is an indication of the strength of the intermolecular forces present. A small drop in temperature i.e. slow rate of evaporation is an indication of strong intermolecular forces. • Surface tension Surface tension is a result of intermolecular forces. Molecules at the surface of a liquid are attracted to the molecules beneath, leading to an inward force on the liquid and a kind of skin on thesurface.

  41. Viscosity • Intermolecular forces is importance in viscosity. stronger attractions between molecules cause them to resist flow more strongly. • For example, water has low viscosity and runs easily. Cold maple syrup flows slowly, so it has high viscosity

  42. Team member 1. Chan Wai Yee (6B, 4) 2. Cho Wing Yan (6B, 7) Date :18-2-05

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