Download Chemical Equations

Unit 6
Chemical Reactions and
Equations
What They Represent &
How to Balance Them
Chemical Reactions
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Reactions are chemical changes in
which new substances form.
How do you know???? If the material
looks different, smells different,
behaves in a new way, guess what???
It IS different!!!
A chemical reaction is simply a
REARRANGEMENT of atoms.
ACTIVITY
IN THIS ACTIVITY, YOU WILL EXAMINE A
VARIETY OF CHEMICAL AND PHYSICAL
CHANGES IN A ZIPLOCK BAG.
POST ACTIVITY DISCUSSION
What happens at various points in the minilab???
When water is added to calcium chloride?
What kind of change is this?????
When baking soda is added to calcium chloride?
What kind of change is this?????
5 Indications of Chemical Rxn
1.
2.
3.
4.
5.
Color Change
Heat / Light Released or Absorbed
Formation of a Solid (precipitate)
Formation of a Gas
Formation of an Odor
5 Types of Chemical Reactions
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Synthesis (Combination)
A, B, C, D are elements.
A + B  AB
AB, BC, AC, CD, CB, and AD
Decomposition
are all compounds of
AB  A + B
elements A, B, C, and D.
Single Displacement
A + BC  B + AC
CnHm is a specific
Double Displacement
hydrocarbon.
AB + CD  AD + CB
Combustion
CnHm + O2  CO2 + H2O
Classify the following reactions:
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Fe2(SO4)3 + Ba(OH)2  BaSO4 + Fe(OH)3
Double displacement
Al + CuSO4  Al2(SO4)3 + Cu
Single displacement
KClO3  KCl + O2
Decomposition
Mg + N2  Mg3N2
Synthesis
C6H14 + O2  CO2 + H2O
Combustion
The Chemical Equation
2 Fe(s)
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+ 3 Cl2(g)
 2 FeCl3(s)
The chemical equation is a shorthand representation
of the substances involved in a chemical reaction.
Reactants are on the left. Products are on the right.
The arrow means “produces” or “changes into”.
The letters in parentheses are the physical states of
the substances:
(s) -- solid
(l) -- liquid
(g) -- gas
(aq) -- aqueous
The Chemical Equation
2 Fe(s)
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+ 3 Cl2(g)
 2 FeCl3(s)
The smaller numbers within a formula are called
subscripts, indicating the number of each atom in the
formula for a substance.
The larger numbers are called coefficients, indicating
the number of atoms or molecules of each substance
reacting or being produced in the chemical change.
The Balanced Chemical
Equation

What is it?
A balanced chemical equation has the same number
of each type of atom on BOTH sides of the equation.

Why?
All (non-nuclear) chemical reactions must obey the
law of conservation of mass. The mass of the
reactants equals the mass of the products in ordinary
chemical reactions.
The Balanced Chemical
Equation

How?
Examine the chemical equation to see if there are the same
number of each type of atom on both sides of the arrow.
If not, place coefficients in front of each formula to make
them equal. Re-inventory each time a coefficient is added.
When counting the atoms of each element, the coefficient
gets distributed onto each atom in the formula it precedes.
Balancing Equations
Two Helpful Hints
1. Balance H & O last. (usually)
2. If there is an even # on one side and an odd # on
the other side, always change odd to even by
making a common multiple.
Fe
4 Fe
+
+
O2 
3 O2 
Fe2O3
2 Fe2O3
Balance The Following
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BF3 +
H2O  HF
1,3,3,1
+ H3BO3
SO2+ CaCO3+ O2  CaSO4 +
2,2,1,2,2
C3H8 + O2  H2O +
1,5,4,3
CO2
CO2
ACTIVITY
YOU WILL CLOSELY EXAMINE THE
CHEMICAL REACTION BETWEEN
SILVER NITRATE AND COPPER WIRE.
PREDICTING PRODUCTS IN
CHEMICAL REACTIONS
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How is it done?
One must know the type of reaction.
Then, elements must be combined,
separated, or switched and recombined
according to the type of reaction that
occurs.
Formulas for Products
The formulas for the products are determined
by:
 Switching charges for compounds
 Determining if a subscript is needed for the
stand alone elements. (Diatomic elements
are H2, N2, O2, F2, Cl2, Br2, and I2) Most
others have no subscript like Fe, Al, Mn, Si,
and Xe.
SYNTHESIS
A + B  AB
This is the reaction where elements combine to form a
compound:
H2 + Cl2  what is the product?
When hydrogen and chlorine combine, what are the
subscripts?
Look up hydrogen
H+1
Look up chlorine
Cl-1
The resulting formula is HCl, the result of the criss-cross and
drop the sign method from the formula chapter.
SYNTHESIS
The resulting unbalanced equation is:
H2 + Cl2  HCl
The balanced equation is:
H2 + Cl2  2 HCl
SYNTHESIS
This is the reaction where elements combine to form a
compound:
Ca + N2  what is the product?
When calcium and nitrogen combine, what are the subscripts?
Look up calcium
Ca+2
Look up nitrogen
N-3
The resulting formula is Ca3N2.
SYNTHESIS
The resulting unbalanced equation is:
Ca + N2  Ca3N2
The balanced equation is:
3 Ca + N2  Ca3N2
SYNTHESIS
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Na + P 
Na+1 P-3 = Na3P
Na + P  Na3P
Al + F2 
Al+3 F-1 = AlF3
Al + F2  AlF3
Zn + O2 
Zn+2O-2 = ZnO
Zn + O2  ZnO
Must go back and balance all of these!! 311, 232, and 212
DECOMPOSITION
AB  A + B
This is when a compound breaks up into simpler
substances.
When H2O decomposes, what are the products?
The products are hydrogen and oxygen in their
elemental forms.
Check to see if either or both are diatomic
elements, written with a 2 as the subscript.
H2O  H2 + O2 unbalanced
2 H2O  2 H2 + O2 balanced
DECOMPOSITION
NH3 
The products are nitrogen and hydrogen
Are they diatomic or not?
Yes, therefore products are N2 and H2
NH3  N2 + H2 unbalanced
2 NH3  N2 + 3 H2 balanced
DECOMPOSITION
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CuBr 
Copper is not diatomic, bromine is.
CuBr  Cu + Br2
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GaF3 
Gallium is not diatomic, fluorine is.
GaF3  Ga + F2
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N 2O 
Nitrogen and oxygen are both diatomic.
N 2O  N 2 + O2
Must go back and balance these: 221, 223, 221
SINGLE DISPLACEMENT
A + BC  B + AC
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If A is a metal, it will replace B,
the metal or H, in BC, releasing B
as an element.
For the reaction to occur, A must
be higher on the activity series
than B. Otherwise, the reaction
will not occur.
SINGLE DISPLACEMENT
Mg + HCl 
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Mg is a metal that will replace H in HCl. They switch
places, releasing H as an element.
Check to see if H is diatomic.
Yes, so H2.
Check the charges of Mg and Cl.
Mg+2 and Cl-1 , so MgCl2 is the formula.
Then balance the final equation.
Mg + HCl  H2 + MgCl2
1,2,1,1
SINGLE DISPLACEMENT
A + BC  C + BA
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If A is a non-metal, it will replace C, the
nonmetal, in BC, releasing C as an
element. BA will form as a compound in
a ratio that depends on the charges.
For the reaction to occur, A must be
higher on the activity series than C.
Otherwise, the reaction will not occur.
SINGLE DISPLACEMENT
F2 + KBr 
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F is a non-metal that will replace Br in KBr. They
switch places, releasing Br as an element.
Check to see if Br is diatomic.
Yes, so Br2.
Check the charges of K and F.
K+1 and F-1 , so KF is the formula.
Then balance the final equation.
F2 + KBr  Br2 + KF
1,2,1,2
SINGLE DISPLACEMENT
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Zn + Cu(NO3)2 
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Na + MgCl2 
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Cl2 + AgI 
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I2 + AgCl 
DOUBLE DISPLACEMENT
AB + CD  CB + AD
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A and C switch places.
C combines with B.
A combines with D.
The ratio of the atoms depends on the
charge.
Look up charges of each ion, criss-cross
and drop the sign in the formula.
Balance the equation.
DOUBLE DISPLACEMENT
KI + Pb(NO3)2 
K and Pb switch places.
 Pb combines with I. K combines with
NO3.
 Pb+2 I-1 = PbI2 and K+1 NO3-1 = KNO3
 So, KNO3 and PbI2 are the products.
 Balance the equation.
KI + Pb(NO3)2  KNO3 + PbI2
2,1,2,1
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DOUBLE DISPLACEMENT
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MgCl2 + AgNO3 
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Na2CO3 + CaCl2 
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NaOH + H3PO4 
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K2CrO4 + PbCl2 
CnHm
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COMBUSTION
+ O2  CO2 + H2O
Simple combustion reactions involve the
reaction of hydrocarbons with oxygen to
form carbon dioxide and water.
Balance the C, H and O atoms in that order.
If, in the end, there is an odd number of O
atoms on the right, you may need to double
the hydrocarbon by simply placing a 2
coefficient in front of the CnHm compound.
COMBUSTION
C5H12 + O2 
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Complete the reaction with CO2 and
H2O.
Balance C, then H then O.
C5H12 + O2  CO2 + H2O
1,8,5,6
COMBUSTION
C4H10 + O2 
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Complete the reaction with CO2 and
H2O.
Balance C, then H then O.
C4H10 + O2  CO2 + H2O
2,13,8,10