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					            Chapter 20
Bonding and Chemical Reactions
          20-1
How Atoms Combine
      A Chemical Bond is...
(p. 542)
the force that holds the atoms in a compound
together
What’s a compound? Two or more atoms
chemically bonded together to form a
molecule.
What’s a molecule? Two or more atoms
bonded together to share electrons--the
properties of the individual atoms are gone.
    Bonds: Ionic vs. Covalent

Ionic bond: the bond resulting from
the attraction between a positive and
a negative ion (metal + nonmetal)
Covalent bond: bonds made by
sharing the electrons in atoms’
outermost shells (nonmetal +
nonmetal)
          20-2
Chemical Formulas
      Chemical Formulas

H2O: two atoms of Hydrogen
chemically combined with one atom of
oxygen to form a single molecule. The
subscript 2 goes to the H, but not to the
O.
SO4: one atom of Sulfur chemically
combined with four atoms of oxygen.
The 4 goes with the O, but not the S.
               Sulfates

In this room, there might be 500 SO4 molecules
floating around in the air. How could I
designate this?
500SO4
The 500 goes to the WHOLE MOLECULE, but
the 4 only goes to the O.
How many TOTAL Sulfur and TOTAL Oxygen
atoms would be present?
           Sulfates

If 500 SO4 molecules were present,
how many total sulfur and total
oxygen atoms would be part of them?
500 x 4 = 2000 Oxygen atoms
500 x ? = ? Sulfur atoms
500 x 1 = 500 Sulfur atoms
Pretend it’s S1O4
           Charge (p. 548)

“In any compound, the net charge of the
compound must equal zero. That is, the number
of positive charges must equal the number of
negative charges”.
Example 1: Mg+2 + F-1 in a compound would
require how many of each to balance the
charges? Cross multiply: Mg1F2 = MgF2
By the way: How is OS different than Os?
               OS vs Os

Os is one element, Osmium. OS is a
compound of two elements: Oxygen and
Sulfur. How can I, Mr. Martin, tell the
difference?
A new element starts with each capital letter.
The symbol for each element on the periodic
table is either one capital letter, or a capital
letter followed by a small leter. (H, He, etc.)
  Polyatomic Ions (p. 548)

“All for one: Some ionic
compounds contain ions made from
several nonmetals that are bonded
together covalently. These ions
form a unit, and they have a single
charge.”
Examples: NH4 = Ammonium, etc.
          20-3
Chemical Reactions
   Section Three: Chemical
Reactions (i.e. the IMPORTANT
               part)
  p. 553: Chemical equation: lists
  the compounds or elements
  involved in a reaction, and the
  new compounds or elements that
  form as a result.
          Chemical Equations
Look like this (p. 554):

2Al + Fe2O3 --> 2Fe + Al2O3
Notice:
1. There are four types of molecules in this
reaction: two REACTANTS (before the arrow)
and two PRODUCTS (after the arrow).
2. The equation is BALANCED. There are two
Fe, two Al, and three O on each side of arrow.
    Coefficients vs. Subscripts
4Al2O3
Notice the two different types of numbers:
The 4 is a COEFFICIENT, which tells you how
many of that molecule are present. It stretches
across the WHOLE molecule.
The 2 and 3 are subscripts, which tell you how
many of each element are in a molecule. They
don’t stretch--they just go to the element before
the number.
 Coefficients vs. Subscripts

By the way, if there is no coefficient, or
no subscript, you can
pretend/imagine/write in a ONE.

  4H2O + 2O2 = 4H2O2
= 4H2O1 + 2O2 = 4H2O2
   Parentheses in Reactions

Pb(NO3)2
The 3 goes to only the O,
The 2 goes to the NO3
So, the count for this molecule is?
1 Pb, 2 N, 6 O
    Synthesis Reaction


Two or more substances
COMBINE to form one new
substance
2H2 + O2 --> 2H2O
 Decomposition Reaction



Involves BREAKING a compound
into two or more substances.
2HgO --> 2Hg + O2
  Replacement Reactions


One atom or a group of atoms in a
compound is REPLACED with
another atom or group of atoms
Cu + 2AgNO3 --> Cu(NO3)2 +2Ag
Double Replacement Reaction


Two different types of atoms or group
of atoms EXCHANGE places in a
reaction
2KCl + Pb(NO3)2 --> PbCl2 + 2KNO3
        Precipitate



A solid that comes out of a
reaction (because it is not
soluble with the mixture)
       Tips for Balancing Chemical
                 Equations
1. Treat standard groups (SO4) as a single
item
2. Balance the elements (H2) LAST
3. Remember, you can only change
coefficients; you can’t change subscripts.
4. It’s okay to end up with big numbers as
your coefficients, but remember to divide by
GCF.
http://richardbowles.tripod.com/chemistry/balance.htm

				
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