What is a chemical reaction?
Chemical
= a process in which elements and/or
compounds interact to form
new substances
Reactions:
= a process in which atoms are
rearranged to form
new substances
Equations
and
Classification
2Na + Cl2  2NaCl
Law of the Conservation of Mass
Matter cannot be created or destroyed… so it
follows that atoms can not be created or destroyed
in a chemical reaction.
All chemical reactions can be written as:
chemical equations:
word equation:
Methanol + Oxygen 
Carbon Dioxide + Water
REACTANTS
PRODUCTS
2Na + Cl2  2NaCl
# of atoms of X
in reactants
# of atoms of X
=
in products
New substances are formed
by the re-arrangement of atoms.
formula equation:
CH3OH
+

REACTANTS
Anatomy of a Chemical Equation
coefficients
O2
CO2 +
H2O
PRODUCTS
State Symbols
H2O (s)
2Na + Cl2  2NaCl
(reactants)
H2O (l)
(products)
subscript
“yields”
yields”
H2O (g)
• are subscripts at the end of a formula
• tell you what state (phase) the chemical is in.
1
State Symbols
Symbol
Meaning
(cr) or (s)
Crystalline (Solid)
(l)
Liquid
(g)
Gas
(aq)
Aqueous (dissolved in water)
Example:
NaHCO3 (s) + HCl (aq)  NaCl(aq) + CO2(g) + H2O(l)
aqueous
solid
(dissolved
in water)
aqueous
(dissolved
in water)
gas
liquid
5 Signs (Evidence)
of a Chemical Reaction
1. Color change
2. Energy transfer (heat)
3. Energy transfer (light)
4. Formation of a gas
(bubbles, fizzing)
5. Formation of a solid
5 Types of Chemical Reactions
1.
2.
3.
4.
5.
Synthesis
Decomposition
Single Displacement
Double Displacement
Combustion
( = “precipitate”)
*(these signs are not proof of a chem. rxn!)
Synthesis
A + B  AB
• Two or more elements combine to
form a new compound.
Synthesis Examples:
Mg(s) + O2(g)  MgO(s)
2Mg(s) + O2(g) 2MgO(s)
AB + AC  ABC + A
• Two or more simple compounds form
a new more complex compound.
2
Predicting Products:
Synthesis Reactions
A + B  AB
• We will only consider the synthesis of simple
ionic compounds:
Example:
2 Mg(s)
+
O2(g) 
CO(g) + 2H2(g)  CH3OH(g)
2 MgO(s)
Use the preferred charges of the ions to
predict the chemical formula of the product.
After predicting the products,
then balance the equation.
Decomposition
(Photosynthesis)
H20 (l)  H2 (g) + O2 (g)
• the opposite of synthesis;
• a compound breaks down into 2 or more
elements.
• a complex compound breaks into 2 or more
simper compounds.
Predicting Products:
Decomposition Reactions
AB  A + B
• Again, we’ll only consider simple ionic compounds:
Example:
+
6CO2(g) + 6H2O(l)  C6H12O6(s) + 6O2(g)
Decomposition Examples
AB  A + B
2 NaCl (s)  2 Na (s)
More Synthesis Examples:
Cl2(g)
Remember:
•These elements are always diatomic:
2H20 (l)  2H2 (g) + O2 (g)
Al2(SO4)3 (s) 
Al2O3 (s) + 3SO3 (g)
Single Displacement
AB + X  AX + B
• One element replaces a similar element in a
compound.
• “One element steals another’s date”
H2, N2, O2, F2, Cl2, Br2, I2
• Metals do not form molecules.
After predicting the products, then balance the equation!
3
Examples:
Cl2(g) + 2 KBr(aq)  2KCl(aq) + Br2(g)
(Cl displaces Br)
2Al(s) + 3Pb(NO3)2 (aq)  3Pb(s) + 2Al(NO3)3 (aq)
(Al displaces Pb)
Examples:
FeS(s) + 2HCl(aq)  H2S(g) + FeCl2(aq)
Fe replaces H
… or S replaces Cl
Combustion Reactions
Fuel + O2  CO2 + H2O
• Are always exothermic.
• O2 is always a reactant
• Hydrocarbon compounds – (fuel) – are
always another reactant.
• CO2 and H2O are always products.
Double Displacement
AB + XY  AY + XB
• Two elements or ions in different
compounds switch places.
• Often polyatomic ions are involved.
• “double date swap”
• All reactants and products are compounds.
Examples:
2KI(aq) + Pb(NO3)2(aq)  2 KNO3(aq) + PbI2 (s)
K replaces Pb
…or I replaces NO3
Examples:
• CH4 (g) + O2 (g)  CO2 (g) + H2O (g)
• CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)
• C3H8 (g) + O2 (g)  CO2 (g) + H2O (g)
• C3H8 (g) + 5 O2 (g)  3CO2 (g) + 4H2O (g)
4
Sample Problems
Practice…
S

Al2S3
+ Fe2O3

Al2O3
+

K
N2

PtS2
Al +
• 8Ba (s) + S8 (s)  8 BaS(s)
Al
• 2 Na(s) + Cl2 (g)  2NaCl
K3N
• 2HgO (s)  2Hg (g) + O2 (g)
→
CH3OH +
O2
Na
→
+
Cl
CO2 + H2O
NaOH
+
KCl 
KOH +
NaCl
2Mg
+
O2
→
2MgO
Zn
+
2HCl 
ZnCl2+
H2
+
HBr
+
S8
Al2O3  AlBr3
+
H2O
A Strategy for
Recognizing Reaction Types:
More Practice:
NaCl
Pt
+
Fe
Look at the reactants…
(“E” = element,
“C” = compound)
•
•
•
•
•
E+E
C
E+C
C+C
C + O2
Synthesis
Decomposition
Single Displacement
Double Displacement
Combustion
5