Transcript CHEMICAL REACTIONS

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