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Liquids

  

 

The Nature of Intermolecular Forces

  Dispersion Forces

  • The strength of intermolecular interactions in a liquid determines its normal boiling point, which is the temperature at which liquid converts to vapor at a pressure of 1.00 atm
      Low boiling point ­ small intermolecular forces
      High boiling point ­ large intermolecular forces
  • Dispersion forces (or London forces) are the net attractive forces generated by temporary charge imbalances (induced dipoles) among molecules
  • The molecular polarizability is a measure of how easy it is to distort the electron cloud of a molecule. Size affects polarizability-Less polarizability means weaker intermolecular forces

Dipolar Forces

  • Dispersion forces exist between all molecules, but some substances remain liquid at much higher temperatures than can be accounted for by dispersion forces alone.
  • Dipolar forces are attractive interactions between molecules with permanent dipoles

Hydrogen Bonding

  • A hydrogen bond occurs when electrons from a highly electronegative atom are partially shared with a positively polarized hydrogen atom.
  • Hydrogen bonds are only 5 to 10% as strong as covalent bonds, but the are comparable to and sometimes stronger than dipolar and dispersion interactions

    Requirements for hydrogen bond formation

    1. The covalent bond to hydrogen must be highly polar
    2. There must be nonbonding electrons on a highly electronegative atom

  • These requirements restrict hydrogen bond formation to molecules that have hydrogen atoms bonded to fluorine, oxygen, and nitrogen
  • Hydrogen bonding can occur between different molecules and identical molecules and within a molecule

 

Properties of Liquids

  Physical Properties of the States of Matter
  Property Gas Liquid Solid
  Volume Variable Fixed Fixed
  Shape Variable Variable Fixed
  Compressibility Large Almost Zero Almost Zero
  Fluidity Very High High Very Low
  Diffusion Rate High Moderate Very Low
 
  • Surface tension is the resistance of a liquid to an increase in its surface area
      A molecule in the interior of a liquid is completely surrounded by other molecules. A molecule at a liquid surface has other molecules beside it and beneath it but none above it. As a result, the net intermolecular force on molecules at the surface pulls them towards the interior of the liquid.
  • Cohesive forces ­ the intermolecular forces between liquid molecules
  • Adhesive forces ­ the intermolecular forces between the molecules in the wall of a container and the molecules of a liquid
  • Capillary action ­ the upward movement of water against the force of gravity
  • A liquid's resistance to flow is called its viscosity. Viscosity can be determined by measuring the time it takes for a specific amount of liquid to flow through a tube of known diameter under the force of gravity. Viscosity measures how easily molecules slide by one another and this depends on molecular shapes and intermolecular forces. Viscosity descreases as temperature increases

 

The Nature of Solutions

 
  • A saturated solution is when the upper limit to the amount of solute that will dissolve in a given amount of solvent has been reached.
  • The solubility of a substance in a particular solvent is the concentration of the saturated solution. Solubilities vary widely because they depend on the intermolecular forces in the solute and solvent.
  • Substances which form solutions in all proportions are said to be completely miscible
  • Whether or not a given substance dissolves in a liquid depends primarily on three types of intermolecular coulombic forces of attraction:

    1. Those between ions or molecules of the pure solute
    2. Those between solvent molecules
    3. Those between solvent and solute in the solution

  • A substance dissolves if the forces of attraction between the solvent molecules and the solute molecules are comparable or greater than the solute-solute and solvent-solvent interactions.
  • Substances that dissolve in each other usually have similar types of intermolecular interactions

Solubility of Solids
  • Covalent solids are insoluble in all solvents but may be chemically attacked by some liquids or vapors
  • Metals are the next most difficult solids to dissolve
      Remember, reactions with solvents are not equivalent to being dissolved in a solvent
      Metals are insoluble in common liquid solvents but can dissolve in each other to make an alloy
      Mercury is special because it is a liquid at room temperature and a solution of another metal in mercury is called an amalgam
  • Ionic solids do not dissolve unless considerable solvent-ion interactions exist to counterbalance the energy cost of breaking the ions free from the lattice
      Some ionic solids dissolve in water because it is a highly polar liquid in which strong ion-dipole interactions exist between the water molecules and ions in solution
      We use the solubility guidelines to categorize the ionic salts
  • Molecular solids readily dissolve in solvents with similar types of intermolecular forces

Solubility of Gases in Liquids
  • A few gases interact strongly with water to form concentrated aqueous solutions
  • Gas solubility increases with the partial pressure of the gas in contact with the solution (Henry's Law):
    Ci = KH * p
    where Ci is the concentration of gas in the solution, p is the partial pressure of the same gas in the vapor phase above the solution, and KH is the Henry's law constant
 

Properties of Aqueous Solutions

 
  • The normal freezing point (fp) of a substance is the temperature at which solid and liquid coexist at equilibrium under a pressure of 1 atm.
  • The normal boiling point (bp) of a liquid is the temperature at which liquid and vapor coexist at equilibrium under a pressure of 1 atm
  • The addition of solutes decreases the freezing point of a solution because collisions between solvent molecules and crystals of solid solvent occur less frequently than in the pure solvent
  • The change in freezing point of a solution, DTf, obeys the equation:
    DTf = Kf * Xsolute
    where Xsolute is the total mole fraction of solutes and Kf is the freezing point depression constant. The constant is different for different solvents but does not depend on the identity of the solutes.
  • A boiling point is increased by adding a nonvolatile solute.
  • The change in boiling point of a solution, DTb, obeys the equation:
    DTb = Kb * Xsolute
    where Xsolute is the total mole fraction of solutes and Kb is the boiling point elevation constant. The constant is different for different solvents but does not depend on the identity of the solutes.
  • For ionic substances, the total mole fraction of solutes is always greater than the mole fraction of the ionic substance itself.
  • We modify the previous equations to account for this:
    DTf =i * Kf * Xsolute and DTb =i * Kb * Xsolute
    by adding the factor i which is a dimensionless number that gives the number of ions generated in solution by one formula unit of solute.
  • A semipermeable membrane allows only selected molecules or ions to pass through it.
  • If water passes selectively through a semipermeable membrane, the process is called osmosis.
  • Osmotic pressure (P) is the pressure difference necessary to equalize the transfer rates through a semipermeable membrane:
    P = MRT

    where M is the total molarity of all solutes, T is the temperature in kelvins, and R is the gas constant.
  • You can also use osmotic pressure to determine the molar mass of large compounds:

    MM = mRT/PV
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Last Revised : Tuesday, September 9, 1997

Copyright © 1997
Louisiana State University, Department of Chemistry.
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