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CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
Introduction
– Chemical reactions occur when bonds
between atoms are formed or broken
– Chemical reactions involve changes in
matter, the making of new materials with
new properties, and energy changes.
– Chemical reactions are written in
symbolic form
– How can you tell a chemical reaction is
happening? – Color, heat/light change,
odor, bubbles
Energy is in the bonds!
• Endothermic rxns –
bonds made and
energy is stored
• Exothermic rxns –
bonds broken and
energy is released
Parts of a Reaction Equation
– reactants (molecules on left of arrow)
– products (molecules on right of arrow).
– “+” sign separates molecules on same side
– “” is read as “yields”
– Ex: carbon plus oxygen react to yield carbon
dioxide
C + O2  CO2
Other symbols
• Coefficients - The numbers in front
– Show relative amounts of reactants &
products
– Shows the “recipe”
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
• Letters (s), (g), and (l) are the
physical states of compounds.
Chemical Equations
Because of the principle of the
conservation of matter,
an equation
must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
Balancing Equations
– When balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction, but
you may
not
change the subscripts.
• Changing the subscripts changes the
compound. Subscripts are determined
by the valence electrons (charges for
ionic or sharing for covalent)
Subscripts vs. Coefficients
• The subscript tell you how many atoms of a
particular element are in a compound.
– CO carbon monoxide VS. CO2 carbon dioxide
• The coefficient tells you about the quantity, or
number, of molecules of the compound.
– 3 CO2 = 3 molecules of carbon dioxide
Steps to Balancing Equations
1. DO NOT CHANGE THE FORMULAS!
2. Always start on the left.
3. Compare the number of atoms for each element on
either side of the reaction
4. Add or change COEFFICIENTS to balance.
5. Every time you make a change, go back to the
beginning.
6. If polyatomic ions are present on both sides, treat
them as one unit.
7. Check your answer to see if:
– The numbers of atoms on both sides of the
equation are now balanced.
– The coefficients are in the lowest possible whole
number ratios. (reduced)
Balancing Equations
2 H2(g) + ___ O2(g) ---> ___
2 H2O(l)
___
What Happened to the Other Oxygen Atom?????
This equation is not balanced!
What coefficients need to be added to balance this
equation?
2 Al(s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
___
Balancing
Equations
5
____C3H8(g) + _____
O2(g) ---->
3
4
_____CO
2(g) + _____ H2O(g)
2
11
5.5
____B
H
(g)
+
_____
O2(g) ---->
4 10
4
10
2 B2O3(g) + _____
5
___
H2O(g)
Balancing Equations
Sodium phosphate + iron (III) oxide 
sodium oxide + iron (III) phosphate
2 Na3PO4 +
3 Na2O +
Fe2O3 ---->
2 FePO4
Which Is Balanced?
Mg + O2  MgO
2Mg + O2  2MgO
Mg + ½O2

MgO
Mg2 + O2  2MgO
4Mg + 2 O2  4MgO
Now Try These:
a) P4 + 5 O2  P4O10
b) 2 Li + 2 H2O  H2 + 2 LiOH
c) 2 Bi(NO3)3 + 3 K2S  Bi2S3 + 6 KNO3
d) C2H6 +3.5 O2  2 CO2 + 3 H2O
2 C2H6 + 7 O2  4 CO2 + 6 H2O
Balance these skeleton equations:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Mg + 2HCl  MgCl2 + H2
3Ca + N2  Ca3N2
NH4NO3  N2O + 2H2O
2BiCl3 + 3H2S  Bi2S3 + 6HCl
2C4H10 + 13O2  8CO2 + 10H2O
6O2 + C6H12O6  6CO2 + 6H2O
3NO2 + H2O  2HNO3 + NO
Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
Al4C3 + 12H2O  3CH4 + 4Al(OH)3
Here are some more to balance:
a)
b)
c)
d)
e)
f)
g)
h)
i)
2KNO3  2KNO2 + O2
2Pb(NO3)2  2PbO + 4NO2 + O2
P4 + 6I2  4PI3
3MgO + 2H3PO4  Mg3(PO4)2 + 3H2O
Br2 + 2KI  I2 + 2KBr
Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O
Bi2O3 + 3H2  2Bi + 3H2O
3Fe + 2O2  Fe3O4
2CaO + 5C  2CaC2 + CO2
a)
b)
c)
d)
e)
f)
g)
h)
2Li + 2H2O  H2 + 2LiOH
P4 + 5O2  P4O10
2C2H6 + 7O2  4CO2 + 6H2O
CS2 + 3O2  CO2 + 2SO2
2AsCl3 + 3H2S  As2S3 + 6HCl
3AgNO3 + FeCl3  3AgCl + Fe(NO3)3
2KClO3  2KCl + 3O2
2SO2 + O2  2SO3