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Chapter 8
Chemical Equations and Reactions
8.1 Describing Chemical Equations
Indicators of Chemical
Reactions
• Evolution of Heat & Light- Exothermic
• Production of a Gas: CO2, H2, H2S
• Formation of a Precipitate
BaCl2 (aq) + Na2SO4 (aq) → 2NaCl (aq) + BaSO4 (s)
Chemical Equations
• A formula equation uses symbols &
formulas to represent the identities &
relative amounts of reactants & products.
• There are also word equations which use
chemical names instead of formulas.
• See p. 246 Table
Reactants (s) → Products (g)
Original
Substances
State of Matter
Resulting
Substances
Chemical Equations
• Equations must:
– Represent known facts.
– Contain correct formulas for all
compounds.
– Satisfy the law of conservation of mass.**
** This is called balancing the equation.
Balancing Equations
• RULE: The number and type of atoms on the
left side of the equation must equal the
number and type of atoms on the right
• Example:
H2O → H2 + O2
*not OK
• missing 1 oxygen on the left side of the
equation
Balancing Equations
Sodium + Chlorine → Sodium Chloride
Na (s) + Cl2 (g) → NaCl (s)
1
Na
2
Cl
1
Na
1
Cl
Balancing Equations
Sodium + Chlorine → Sodium Chloride
2Na (s) + Cl2 (g) → 2NaCl (s)
2
Na
Add
coefficients
to the front of
a formula!
2
Cl
2
Na
2 Na : 1 Cl2 : 2 NaCl
2
Cl
Must be
the
smallest
Ratio!
Coefficients vs. Subscripts
CH4 + 2O2 → CO2 + 2H2O
Subscript
Coefficient
• NEVER change subscript, only the
coefficient when balancing!!!
Steps for Balancing Equations
• Identify the names of reactants and products,
and write a word equation.
• Write a formula equation by substituting
correct formulas for the names of the
reactants and the products.
• Balance the formula equation according to
the law of conservation of mass.
• Count atoms on both sides to be sure that
the equation is balanced!
Balancing Equations
• The coefficients indicate relative amounts
of substances, a ratio.
H2 (g) + Cl2 (g) → 2HCl (g)
1 mole H2 : 1 mole Cl2 : 2 mole HCl
Now, convert the moles to mass values to
indicate the relative masses!
Practice
• Write the Word Equation:
FeS (aq) + H2SO4 (aq) → FeSO4 (aq) + H2S (g)
Iron (II) + Sulfuric → Iron (II) + Hydrosulfuric
Sulfide
Acid
Sulfate
Acid
Practice
• Write the formula equation.
Sodium + Carbon + Water (g) → Sodium Hydrogen
Oxide (s) Dioxide (g)
Carbonate (s)
Na2O (s) + CO2 (g) + H2O (g) → NaHCO3 (s)
• Now balance the equation!
Na2O (s) + 2CO2 (g) + H2O (g) → 2NaHCO3 (s)
• Now…. Translate into a sentence!
Chapter 8
Chemical Equations and Reactions
8.2 Types of Chemical Reactions
Types of Reactions
• Most chemical reactions can be
classified into 5 major categories:
Synthesis
Decomposition
Single Replacement (Displacement)
Double Replacement (Displacement)
Combustion
Synthesis (Composition)
Reactions
Two or more
substances
combine to form a
new compound.
A + X  AX
 Reaction of elements with oxygen to form oxides
 Reactions of elements with sulfur to form sulfides
 Nonmetals react with oxygen to form oxides
 Reactions of halogens to form salts
 Active metal oxides react with water to form metallic
hydroxides
Nonmetal oxides react with water to form oxyacids
(acid rain)
Synthesis
Reactions
2 or more
substances
combine to form a
more complex
substance.
• Reaction of elements with oxygen to
form oxides:
4Al (s) + 3O2 (g) → 2Al???
2O3 (s)
• Reactions of elements with sulfur to
form sulfides:
8Ba (s) + S8 (s) → 8BaS
??? (s)
Synthesis Reactions
• Nonmetals react with oxygen to form
oxides:
C (s) + O2 (g) → CO???
2 (g)
• Reactions of halogens to form salts:
??? (s)
2Na (s) + Cl2 (g) → 2NaCl
Multiple Reactants
One Product
Synthesis Reactions
• Active metal oxides react with water to
form metallic hydroxides:
MgO (s) + H2O (l) → Mg(OH)
??? 2 (s)
• Nonmetal oxides react with water to
form oxyacids (acid rain):
???3 (aq)
SO2 (g) + H2O (l) → H2SO
Decomposition Reactions
A single compound
undergoes a reaction that
produces two or more
simpler substances
AX  A + X
Decomposition of:
1. Binary compounds
2.
3.
4.
5.
(electric current)
Metal carbonates
Metal hydroxides
Metal chlorates
Oxyacids
Decomposition
AX → A + X
One complex substance
breaks down into
simpler substances.
1. 2H2O (l)  2H2 (g) + O2 (g)
2. CaCO3 (s)  CaO (s)???
+ CO2 (g)
3. Ca(OH)2 (s)  CaO (s) + H2O (g)
4. 2KClO3 (s)  2KCl (s)???+ 3O2 (g)
5. H2CO3 (aq)  CO2 (g) + H2O (l)
Single Replacement
(Displacement) Reactions
A + BX  AX + B
Replacement of:
1. Metals by a more active metal
BX + Y  BY + X
2. Hydrogen in water by an active metal
3. Hydrogen in an acid by a metal
4. Halogens by more active halogens
Single Replacement
A + BX → B + AX
BX + Y → BY + X
One element takes the
place of another in a
compound.
1. Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
2. Ca (s) + 2H2O (l) → Ca(OH)2???
(aq) + H2 (g)
3. Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
4. Cl2 (g) + 2KBr (aq) → 2KCl (aq)
???+ Br2 (g)
Double Replacement
(Displacement) Reactions
The ions of two compounds exchange places in an
aqueous solution to form two new compounds.
AX + BY  AY + BX
One of the compounds formed is usually a
precipitate, an insoluble gas that bubbles out of
solution, or a molecular compound, usually water.
Double Replacement
Ions of 2
compounds
exchange
places to
form 2 new
compounds
AX + BY → AY + BX
BaCl2 (aq) + Na2SO4 (aq) → 2NaCl (aq) + BaSO4 (s)
Precipitate
FeS (aq) + H2SO4 (aq) → FeSO4 (aq) + H2S (g)
Gas
NaOH (aq) + HCl (aq) → NaCl (aq)
+ H2O (l)
???
Water
Combustion
Reactions
A substance combines with
oxygen, releasing a large
amount of energy in the form of
light and heat.
Reactive elements combine with oxygen
P4 (s) + 5O2 (g)  P4O10 (s)
(This is also a synthesis reaction)
The burning of natural gas, wood, gasoline
C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (g)
(The products are ALWAYS carbon dioxide & water)
Chapter 8
Chemical Equations and Reactions
8.3 Activity Series of the Elements
Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Lead
Hydrogen
Bismuth
Copper
Mercury
Silver
Platinum
Gold
The Activity Series
Decreasing Activity


















Metals can replace other metals,
provided that they are above the
metal that they are trying to
replace.
Metals above hydrogen can
replace hydrogen in acids.
Metals from sodium upward can
replace hydrogen in water.
Activity Series Guidelines
• All metals above hydrogen displace hydrogen from
hydrochloric acid or dilute sulfuric acid. HCl or H2SO4
• Metals above magnesium vigorously displace
hydrogen from water. Magnesium displaces hydrogen
from steam. Mg(OH)2
• Metals above silver combine directly with oxygen;
those near the top do so rapidly.
• Metals below mercury form oxides only indirectly.
• Oxides of metals below mercury decompose with mild
heating.
Activity Series Guidelines
• Oxides of metals below chromium easily undergo
reduction to metals by heating with hydrogen.
• Oxides of metals above iron resist reduction by
heating with hydrogen.
• Elements near the top of the series are never
found free in nature. (highly reactive)
• Elements near the bottom of the series are often
found free in nature. (mildly reactive)
 Fluorine
 Chlorine
 Bromine
 Iodine
Decreasing Activity
The Activity Series of the
Halogens
Halogens can replace other
halogens in compounds, provided
that they are above the halogen
that they are trying to replace.
2NaCl (s) + F2 (g) 
2NaF (s) + Cl2 (g)
???
MgCl2 (s) + Br2 (g) 
No Reaction
???
Practice:
SR
SR
S
DR
DR
1.
2.
3.
4.
5.
Complete and balance the
reactions, if they will occur…
Cu (s) + AgNO3 (aq) → ?
Zn (s) + CuSO4 (aq) → ?
Na2O (s) + CO2 (g) + H2O (g) → ?
H2SO4 (aq) + BaCl2 (aq) → ?
Al2O3 (s) + H2SO4 (aq) → ?
Practice:
SR
SR
S
DR
DR
1.
2.
3.
4.
5.
Complete and balance the
reactions, if they will occur…
Cu (s) + 2AgNO3 (aq) → ? Cu(NO3)2 (aq) + 2Ag (s)
Zn (s) + CuSO4 (aq) → ? Cu (s) + ZnSO4 (aq)
Na2O (s) + 2CO2 (g) + H2O (g) → ? 2NaHCO3 (s)
H2SO4 (aq) + BaCl2 (aq) → ? 2HCl (aq) + BaSO4 (s)
Al2O3 (s) + 3H2SO4 (aq) → ?Al2(SO4)3 (aq) + 3H2O (l)
Practice:
SR
D
SR
SR
S
SR
Complete and balance the
reactions, if they will occur…
6. Ca (s) + H2O (l) → ?
7. KClO3 (s) → ?
8. MgCl2 (s) + Zn (s) → ?
9. Al (s) + HCl (g) → ?
10. Cd (s) + O2 (g) → ?
11. I2 (s) + KF (g) → ?
Practice:
SR
D
SR
SR
S
SR
Complete and balance the
reactions, if they will occur…
6. Ca (s) + 2H2O (l) → ? Ca(OH)2 (aq) + H2 (g)
7. 2KClO3 (s) → ? 2KCl (s) + 3O2 (g)
NR
8. MgCl2 (s) + Zn (s) → ?
9. 2Al (s) + 6HCl (g) → ? 2AlCl3 (s) + 3H2 (g)
10. 2Cd (s) + O2 (g) → ?
2CdO (s)
NR
11. I2 (s) + KF (g) → ?