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Intermolecular Forces. Chapter 11.2 Pages 385-392. Set-up the following chart in your notes. Forces of Attraction. Intramolecular forces – forces of attraction that hold particles together. chemical bonds – ionic, covalent and metallic are intramolecular forces.
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Intermolecular Forces Chapter 11.2 Pages 385-392
Forces of Attraction Intramolecular forces – forces of attraction that hold particles together. chemical bonds – ionic, covalent and metallic are intramolecular forces. covalent bonds hold the hydrogen and oxygen atoms together – intramolecular Intermolecular forces – forces of attraction between neighboring molecules. there is an attraction between one water molecule and another water molecule, which causes water to be a liquid at room temperature. Intermolecular forces are weaker than intramolecular forces
INTRAMOLECULAR FORCES • Are within a molecule itself such as: • Covalent bonds • Ionic bonds • Metallic bonds
Chemical Bonds Metallic bonds - attractive forces between fixed positive ions and the moving valence electrons in a metal. moving valence electrons + + + + + + + + + fixed positive ions Metallic bonds are strong bonds which is why most metals are solids at room temperature. Ionic bonds are solids at room temperature - very strong bonds. Covalent bonds form molecules. Molecules can be solids, liquids or gases at room temperature.
ORDER OF STRENGTH OF INTRAMOLECULAR FORCES • Metallic – strongest • Ionic • Covalent Network (difference in electronegativity is “0”) • Molecular • Polar • Non-Polar
Bonding in Solids Solid Particles Attraction Properties System Forces Molecular Atoms or (Covalent) Molecules Dispersion Dipole-Dipole Hydrogen Bond Soft, low m.p., poor electrical conductors ice Covalent Atoms Network Covalent Bonds Very hard, high m.p., poor conductors diamond Ionic Ions Ionic Bonds Hard & brittle, high m.p., poor conductors, nonvolatile NaCl Metallic Metal Atoms Metallic Bonding Shiny, malleable, ductile, good conductors Al
What are the intramolecular forces? • Choices to consider are metallic, ionic, covalent, or molecular crystals. • C2H6 __________ • Na2O ____________ • SiO2 ______________ • CO2 ______________ • N2O5 ____________ • NaNO3 ______________ • Al ________________ • C(diamond) _______ • SO2 ________________
INTERMOLECULAR FORCES • Are the forces between one molecule and another • Are weak forces compared to “bonding forces” • Involves the attraction of the electrons of one atom for the protons of another atom • Are called van der Waals forces
KINDS OF van der WAALS FORCES • dipole – ion (dissolving) • dipole – dipole • London dispersion forces (temporary dipole forces) • hydrogen bonding
Dipole – ion forces • Ionic compounds dissociate into cations and anions because water is a polar molecule and the negative end of the water molecule is attracted to the cation and the positive end of the water molecule is attracted to the anion.
δ - δ + δ - δ + Dipole-Dipole Forces • Electrostatic attraction between polar molecules
Dipole-dipole Forces The dipole-dipole force is the attraction that the δ+ end of one molecule has for the δ- end of a neighboring molecule. δ+ δ- Cl H δ+ H δ- Cl δ+ Cl δ- H H H Cl Cl δ- δ- Polar molecules form a dipole (one end of the molecule has a partial positive charge and the other end of the molecule has a partial negative charge) because of difference in electronegativity.
δ - δ + Dipole-Induced Dipole • A polar molecule forces a dipole in a nonpolar molecule. • “Likes dissolve likes” - +
Dispersion Distribution of electrons is not always even. When electrons become unevenly distributed the one end of the atom becomes -(the end with more electrons) and the other end becomes +. Causing one atom to be attracted to another. Attractive force between the + of one atom and the - of a neighboring atom. + - + - This is a very weak intermolecular force.
Dispersion (London) Forces • The random movement of electrons creates temporary dipoles in nonpolar molecules. • These movements occur in all molecules to some extent. + - + -
London Dispersion Forces All of the noble gases are gases at room temperature, however, at extremely low temperatures and/or high pressure, they can be condensed. This indicates that there is some attraction between the particles. Dispersion forces - intermolecular force of attraction between induced or temporary dipoles. atom of neon with uneven distribution of electrons. atom of neon with electron evenly distributed in the electron cloud.
F + - + - Hydrogen Bonding • Dipole-Dipole attraction between hydrogen and a highly electronegative element such as F, O, or N. H
Hydrogen Bonding Hydrogen bond is a dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to N, O or F. The difference in electronegativity results in particularly strong dipole-dipole attraction. Water exhibits hydrogen bonding. A hydrogen bond is the strongest of all of the intermolecular forces, which is why water is a liquid at room temperature.
ORDER OF STRENGTH OF INTERMOLECULAR FORCES • hydrogen bonding – strongest • dipole – ion • dipole – dipole • London dispersion forces
Comparing Intermolecular Forces Interacting molecules or ions
Identify the Intermolecular Attraction Forces • NF3 • Dipole-Dipole • C6H6 • London • HCl • Dipole-Dipole • H2 Dispersion (London) • Ne London • C2H4Cl2 Dipole-Dipole • HF Hydrogen (Dipole-Dipole)
What intermolecular forces? • HCl • I2 • CH3F • CH3OH • H2O • H2S • SiO2 • SO2 • Cu • Fe • Kr • CH3OH • CuO • NH3 • CH4 • HCl(g) • NaCl • SiC
Properties affected by intermolecular forces • Boiling point • Retention of volume and shape • Surface tension • Evaporation • Vapor pressure • Viscosity
Properties are affected by forces • Physical properties depend on these forces. The stronger the forces between the particles, • (a) The higher the melting point. • (b) The higher the boiling point. • (c) The lower the vapor pressure (partial pressure of vapor in equilibrium with liquid or solid in a closed container at a fixed temperature). • (d) The higher the viscosity (resistance to flow). • (e) The greater the surface tension (resistance to an increase in surface area).