Aqueous Solutions and Solution Chemistry

Document Sample
Aqueous Solutions and Solution Chemistry Powered By Docstoc
					Aqueous Solutions and Solution
         Chemistry
       Aqueous Solutions
          Electrolytes
        Reaction Types
          Aqueous Solutions
• Solution: homogeneous mixture or more than
  one compound
• Aqueous solution: Water is the solvent and
  the other species (present in small amounts)
  are the solutes.
• The symbol aq means the compound is
  dissolved in water. E.g. KCl(aq), MgCl2(aq)
    Why is Water a Good Solvent?
• Because water is polar (there is a charge separation
  between the O and H atoms), it has a very high
  capacity to dissolve ionic cmps.
• The negative end of water (O) attracts cations (+) in
  the solid compound. The positive end (H) attracts
  the anions (-) in the solid.
• When ionic cmps (or electrolytes) dissolve in water, a
  solution of ions (cations and anions) forms.
Figure 4.1
The Water
Molecule
is POLAR
(note net
partial
charges on
O and H)
 Fig 4.2 Polar H2O Molecules Interact
with the + and - Ions of a Salt Assisting
 in the Dissolving Process (hydration)
           Strong Electrolytes
• Three classes of strong electrolytes (cmps that
  dissolve @100% in water producing ions;
  these solns conduct electricity)
  – soluble salts (ionic cmps)
  – strong acids: cmps that produce H+(aq) and anion
    in water: HBr(aq)  H+(aq) + Br-(aq)
  – strong bases: cmps that produce OH-(aq) and
    cation in water:
    KOH(aq)  K+(aq) + OH-(aq)
            Weak Electrolytes
• Compounds that do not dissolve in water or
  only a small fraction dissolves in water thus
  producing a small amount of ions.
• These cmps include insoluble salts, weak acids
  and weak bases.
• When a solid insoluble salt is added to water,
  most of it sinks to the bottom of the beaker as
  a solid.
         Aqueous Solutions (2)
• Water can also dissolve nonionic cmps,
  especially those that are polar. (ethanol)
• Finally, many nonpolar molecules dissolve in
  water but do not produce ions in water; these
  are nonelectrolytes (sucrose, glycerol).
     Types of Chemical Reactions in
           Aqueous Solutions
• Precipitation is the formation of solid or
  insoluble salt from two aqueous solutions.
• Acid-Base neutralization is the formation of
  water from Acid + Base
• Oxidation-Reduction or redox is the transfer of
  electrons from one reactant atom to another
  reactant atom.
 Fig 4.13 Precipitation: K2CrO4(aq) +
Ba(NO3)2(aq)  2KNO3(aq) + BaCrO4(s)
            ACID + BASE RXNS
• Acids donate protons, i.e. provide H+(aq) or
  H3O+ (aq, hydronium) ions in water
• Bases accept protons.
• The acid + base rxn (neutralization) is
  H+(aq) + OH-(aq)  H2O(ℓ)
  OXIDATION-REDUCTION REACTIONS
• A redox reaction involves the transfer of electrons
  between atoms in the reactants.
• Electrons gained by one atom must equal electrons
  lost by another. (conservation of electrons)
• Oxidation states or numbers are assigned to atoms
  and they change in a redox rxn.
• Both oxidation and reduction must occur
  simultaneously. (or electrons would not be
  conserved)
 CHEMICAL REACTIONS

Acid-Base Reaction: A Vital Class of
       Chemical Reactions
            Chapter 6
                      ACIDS
• Acids donate protons, i.e. provide H+(aq) or H3O+
  (aq, hydronium) ions in water (Arrhenius).
• Examples: acetic acid [CH3COOH] in vinegar,
  citrus acid [C5H8O5COOH]
• in oranges, malic acid [C3H5O3COOH] is a
  sweetener, lactic acid [C2H5OCOOH] in milk,
  phosphoric acid in Coca-Cola, HCl in gastric juice.
• Polyprotic acids: sulfuric (H2SO4), phosphoric
  (H3PO4).
                    BASES
• Bases accept protons; we will recognize them
  by the presence of the hydroxide ion (OH-)
  such as in NaOH = lye.
• Examples: ammonia (NH3) in cleaners, NaOH
  in drain cleaners, milk of magnesia (Mg(OH)2),
  bicarbonate in Alka Seltzer.
   ACID-BASE NEUTRALIZATION RXN
• Neutralization Rxn: Acid + Base → Salt + Water
• Hydrochloric acid + sodium hydroxide react to
  form sodium chloride and water
• HCl (aq) + NaOH(aq) → NaCl(aq) + H2O(ℓ) or
• H+ (aq) + OH-(aq)  H2O(ℓ)
• The key in a neutralization reaction is that one
  H+ ion reacts with one OH- ion to form one
  molecule of water.
   [H+] = Concentration of Protons
• The control of [H+] is critical to countless
  chemical reactions:
   – Metabolism, respiration, muscle function
   – Food, medicine and chemical industries
• and familiar consumer goods
   – Personal care products
   – Household products
   – Batteries
                  pH (p. 72)
• pH is a scale which tells us the concentration
  of H+ ions in acids and bases (“power of
  hydrogen”).
• The pH scale was first proposed in 1909 by
  Danish chemist, Soren Sorenson.
                    pH (2)
• The pH scale is a logarithmic scale.
• pH = -log [H+] or [H+] = 10-pH .
• If the concentration of protons = [H+] is 1E-4
  M, then the pH = 4. If [H+] = 1E-11, then the
  pH = 11.
• The pH scale generally goes from 1 to 14.
• pH 7 means neutral or [H+] = 1E-7 M = [OH-].
                    pH (3)
• If pH < 7, solution is acidic; the lower the pH,
  the more acidic. If [H+] = 0.001 or 1E-3, then
  pH = 3. If [H+] = 0.00001 M or 1E-5, then pH =
  5. Which solution has the lower [H+] ? is more
  acidic?
• The pH = 5 soln is _____ times [more or less]
  acidic than the pH = 3 soln.
• If pH > 7, solution is basic; the higher the pH,
  the more basic.
http://upload.wikimedia.org/wikipedia
    /commons/4/46/PH_scale.png
      Neutralizing Stomach Acid
• The stomach is very acidic in order to digest
  food and kill bacteria. The acid denatures the
  proteins in food and then enzymes secreted
  by the stomach break the peptide bonds
  between amino acids.
• Antacids are commonly taken to neutralize
  excess gastric acid (HCl) in the stomach.
• Antacid + acid  neutralization reaction
                 Antacids
• aluminum hydroxide [Al(OH)3] or Maalox,
  magnesium carbonate [MgCO3], calcium
  carbonate [MgCO3] or TUMS, magnesium
  hydroxide [Mg(OH)2] or milk of magnesia,
  sodium bicarbonate [NaHCO3] or Alka-Seltzer,
  bismuth subsalicylate [C7H5BiO4] or Pepto-
  |Bismol or Kaopectate.
                pH Websites

• FDA site of “pH values of various foods”
http://vm.cfsan.fda.gov/~mow/app3a.html
• Interactive pH scale.
http://www.johnkyrk.com/pH.html
              CHEMISTRY OF GLASS
• Go to the Corning Museum of Glass website; click on
  Glass Resources; then on the Glass Chemistry Game.
  (play and take notes). http://www.cmog.org/default.aspx
• From the Home Page, go to the Glass of the Alchemists.
• Also under Glass Resources, take a look at some Glass
  Making Videos.
• Under the same link, click on Resource on Glass and then
  Science and Technology.
   – Chemistry of Glass
   – Properties of Glass: Chemical
     FROSTED LIGHT BULBS (p. 75)
• Developed in 1925 by K. Fuwa of Toshiba, Inc.
• Gives diffuse light vs glare.
• SiO2(s) + 6HF(aq)  H2SiF2 (aq) + 2H2O(ℓ)
  Silica or glass + hydrofluoric acid       fluorosilicic acid +
  water
• H2SiF2 (aq) + NaOH(aq)  NaHSiF2 (aq) + 2H2O(ℓ)
• CHEMICAL DANGERS of H2SiF2 (aq):
  The substance decomposes on heating producing toxic
  fumes including hydrogen fluoride. The solution in water is
  a strong acid, it reacts violently with bases and is corrosive.
  Reacts with water or steam to produce toxic and corrosive
  fumes. Attacks glass and stoneware.

  http://www.inchem.org/documents/icsc/icsc/eics1233.htm