Balancing Chemical Equations

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Balancing Chemical Equations Powered By Docstoc
					Unit 2 – Lecture 8
Chemical Equations
 A shorthand way to describe a chemical reaction using
 chemical symbols and formulae

 Three Parts to a Chemical Equation
   Reactants
     substances present before the reaction
   Yield Arrow
     indicates the direction of a reaction
     some rxns are reversible [double-sided arrow]
   Products
     substances present after the reaction
Chem. Equations – cont’d
 Chemical symbols give a “before-and-after” picture of
  a chemical reaction
      Reactants                       Products
   MgO     +      C                 CO    + Mg

 magnesium oxide to form       carbon monoxide
 reacts with carbon            and magnesium

      or… magnesium oxide reacting with carbon
       yields carbon monoxide and magnesium
Chemical Equations – cont’d
 Coefficient – (large) # in front of an element
   applies to all elements in the group to the right of it
   ex: 4C
     4 carbon atoms
   ex: 5KI
     5 potassium atoms & 5 iodine atoms

   ex: 4SO4
     4 Sulfur atoms & 16 Oxygen atoms (4x4)
Chemical Equations – cont’d
 Subscript – (small) # to the lower right of an element
   applies only to the element it is attached to
   if outside of parentheses [(XY)3]
    applies to everything inside of parentheses
   ex: H2
     2 hydrogen atoms

   ex: CO2
     1 carbon atom, 2 oxygen atoms

   ex: (SO4)3
     3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)
Chemical Equations - cont’d
Try this:
 Al2(SO4)3
    Aluminum?
      2 atoms

    Sulfur?
      3 atoms

    Oxygen?
      12 atoms (4x3)
Balancing Chem Equations
 Law of Conservation of Matter
   matter is neither created nor destroyed in a
    chemical reaction, it is conserved
   biology: energy is lost, matter is RECYCLED
     (changed from one thing to another)

   ie, if you begin a reaction with 5 atoms of Hydrogen,
    you must end up with 5 atoms of Hydrogen
Balancing – cont’d
 Balanced = same # of each element on each side




 Al    + S           Al2S3       Unbalanced


 2Al   +   3S          Al2S3     Balanced
Balanced or Unbalanced?
 C3H8 + 5O2           4H2O + 3CO2

   Carbon?
                                     Is it balanced?
     = 3 vs 3
                                         Yes.
   Hydrogen?
     = 8 vs 8

   Oxygen
     = 10 vs 10 (4 + 6)
Balanced or Unbalanced?
 Fe3O4 + H2     Fe + H2O

   Iron               Is it balanced?
     = 3 vs 1             No.
   Oxygen
                       So, now what?
     = 4 vs 1

   Hydrogen
     = 2 vs 2
Balancing a Chemical Reaction
 Set up two columns: labeled “reactants” &“products.”
 Identify & record reacting elements on the left.
 Record elements in the SAME ORDER on the right.
 Count & Record # of each reacting element.
 Count & Record # of each element produced.
 Unequal? Balance Coefficients. [can’t change subscripts]
    you can treat Polyatomic Ions as one if they appear
     on both sides of the equation like (SO4) in
     parentheses
    change your greatest inequality first
    leave solitary elements for last
Practice
   Fe3O4 +   H2   Fe +   H2 O
Practice
 On your worksheet, complete the two problems on the
 front of your sheet.

   2                2
 ____H2 + ____O2  ____H2O
            1

   1         2          1        2
 ____Cl2 + ____NaBr  ____Br2 + ____NaCl
Homework
 Page 6 –
    Balancing Chemical Equations w/s
    1-9 on-level
    all for honors

				
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