Chem 116
Fall 1998

Lecture Notes
September 9, 1998

Topics

Mixtures

Separations

Polarity

Mixtures

• Homogeneous - also called a solution. A solution contains two more substances but is completely uniform at the macroscopic level. In other words, one region of a solution differs from another only on a molecular or atomic scale. Solutions may occur in the gas, liquid, or solid phases. Stainless steel is an example of a solid solution containing iron, chromium, nickel, and carbon. When describing solutions, we often refer to the
  • Solvent - This is the major component of a solution. For example, when sugar is dissolved in water, water is referred to as the solvent.
  • Solute - All minor components are referred to as solutes. In the example given above, sugar would be the solute.
• Heterogeneous - also contains two or more substances, but differs from a solution in that distinct phases may be observed at the macroscopic level; you can visually tell that two distinct substances are present. A mixture of water and sand constitutes a heterogeneous mixture. Oil and water would also be a heterogeneous mixture. Actually, spam with anything is heterogeneous mixture.

Separations

• Interesting fact - over 12 million pure substances have been extracted from natural mixtures or generated in the laboratory (from The Chemical World: Concepts and Applications, Moore, Stanitski, Wood, and Kotz; 2nd Edition, Saunders, 1998).
  
• Separations based on physical properties
  • Centrifugation - based on density. This is used for heterogeneous mixtues.
    
  • Zone Refining - based on melting point. A movable heater melts an impure solid. Only the pure substance resolidifies (or freezes) after the heater has passed by. Impurities are left behind in the molten region.
    
  • Distillation - based on boiling point. An impure solution is heated to boiling. The resulting vapor contains more of the substance with the lower boiling point than did the original solution (if it has a lower boiling point, it evaporates more easily). When the vapor is condensed, the resulting liquid will also contain a higher percentage of the more volatile substance (more volatile means lower boiling point).

    In fractional distillation, the solution goes through the vaporization / condensation cycle repeatedly; this is done in a long condenser column. Each time condensation occurs, the resulting liquid is richer in the more volatile component. Petroleum fractions are separated this way.

    Click here for distillation diagrams.

  • Extractions - based on solubility. To extract a substance from a mixture, we use a solvent that has a greater affinity for the substance to be extracted. Under these circumstances, the substance will dissolve preferentially in the added solvent. Ths means that the substance has been extracted from th emixture. For example, we note that  carbon extract chloride (CCl₄) may be used to iodine (I₂) from water. Extractions may be based on chemical properties (like acid/base reactivity) or on spam, oops, I mean physical properties (like polarity).
    
    • Polarity - this is indication of the charge distribution in the molecule. Nonpolar molecules have a completely symmetrical charge distribution; there is no excess charge anywhere in the molecule. You can identify nonpolar molecules by their symmetry - they are completely symmetrical. Polar molecules are less symmetrical. (We will learn how to assess polarity in more detail later in the semester.) Hydrocarbons tend to be relatively nonpolar.

      For purposes of solubility, we will note that nonpolar molecules dissolve in nonpolar solvents and polar molecules dissolve in polar solvents ("like dissolves like"). Consequently, a nonpolar solvent can extract a nonpolar compound from a polar solvent. The in-class demonstration illustrated this; we extracted I₂ (nonpolar) from water (polar) with CCl₄ (nonpolar).

      A quantitative measure of a molecule's polarity is its dipole moment. Molecules that are completely nonpolar have a dipole moment of zero. A more polar molecule has a larger dipole moment. The table below lists the dipole moments of a few common molecules. (The units of dipole moment are Debye.)

      Name	Dipole Moment (Debye)
      carbon tetrachloride (CCl4)	0.0
      benzene (C6H6)	0.0
      chloroform (CHCl3)	1.01
      ethanol (C2H5OH)	1.69
      methanol (CH3OH)	1.70
      water (H2O)	1.85
      acetone (C3H6O)	2.88

    • Acid/Base Reactivity - We can extract an acid from a solvent by reacting it with a basic solvent.

Questions? Send email to pholt@bellarmine.edu