CHAPTER 8
CHEMICAL REACTIONS
LEARNING THE PATTERNS OF
CHEMICAL REACTIONS
INTRODUCTION
Section 8.1 Describing Chemical Change
A. What is a CHEMICAL EQUATION?
B. Why must an equation be balanced?
INTRODUCTION
Section 8.1 Describing Chemical Change
A. What is a CHEMICAL EQUATION?
• A chemical sentence that describes
chemical changes.
B. Why must an equation be balanced?
• to obey the law of conservation of mass
• mass of reactants must equal mass of
products
C. 5 TYPES OF CHEMICAL
REACTIONS
1. Combination (synthesis)
A + B  AB
2 reactants

(each just 1 element)
1 Product
(a compound)
(monatomic or diatomic)
Example:
2H2 +
O2

2H2O
C. 5 TYPES OF CHEMICAL
REACTIONS
1. Combination (synthesis)
A + B  AB
2. Decomposition
AB  A + B
1reactant
(a compound)

2 Products
(each just elements)
C. 5 TYPES OF CHEMICAL
REACTIONS
3. Single Replacement
A + BC  AC + B
(A = metal) [metal, A, bonds with anion C]
X + YZ YX + Z
(X = nonmetal) [nonmetal, X, bonds with cation Y]
C. 5 TYPES OF CHEMICAL
REACTIONS
4. Double Replacement
AB + CD  AD + CB
CATION FROM ONE COMPOUND
REPLACES THE CATION FROM THE
OTHERAND THE ANION
FROM ONE WILL
REPLACE THE ANION
FROM THE OTHER.
C. 5 TYPES OF CHEMICAL
REACTIONS
5. COMBUSTION (burning a hydrocarbon)
– CXHY + O2  XCO2 + y/2 H2O
Combustion of Propane:
C3H8 + 5 O2  3 CO2 + 4 H2O
These are all examples of COMPLETE combustion
C. 5 TYPES OF CHEMICAL
REACTIONS
5. COMBUSTION, continued
During COMPLETE combustion, there
is enough oxygen to completely burn
off all the fuel – the product is CO2.
During INCOMPLETE combustion, CO
is produced instead, since there is not
enough oxygen available.
CHEMICAL EQUATIONS
D. Define REACTANTS -
– starting substances
E. Define PRODUCTS -
– ending substances
• Reactants  products
• “Reactants yield products”
Table 8.1
Symbols Used in Equations
Use your textbook, page 206, to write the
symbols for the following words/phrases
•
•
•
•
•
•
yields
precipitate is made
a gas is produced
heat is being applied
a catalyst is used
a reversible reaction
•
•
•
•
•
solids
liquids
gases
aqueous
heat
It would be in your best interest to commit these symbols
and their meanings to memory.
DEFINITIONS
G. What is a skeleton equation?
– not balanced yet
H. What is a catalyst?
– a substance that speeds up a
reaction without being used up.
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
1. Aluminum metal reacts with oxygen in the air to form
aluminum oxide.
2. When solid mercury (II) sulfide is heated with oxygen,
liquid mercury metal and gaseous sulfur dioxide are
produced
3. Oxygen gas can be made by heating potassium chlorate
in the presence of the catalyst manganese dioxide.
Potassium chloride is left as a solid residue..
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
1. Aluminum metal reacts with oxygen in the air to form
aluminum oxide.
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
1. Aluminum metal reacts with oxygen in the air to form
aluminum oxide.
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
2. When solid mercury (II) sulfide is heated with oxygen,
liquid mercury metal and gaseous sulfur dioxide are
produced
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
2. When solid mercury (II) sulfide is heated with oxygen,
liquid mercury metal and gaseous sulfur dioxide are
produced
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
3. Oxygen gas can be made by heating potassium chlorate
in the presence of the catalyst manganese dioxide.
Potassium chloride is left as a solid residue..
Practice Problems
• Write a skeleton equation for each of these
chemical reactions
3. Oxygen gas can be made by heating potassium chlorate
in the presence of the catalyst manganese dioxide.
Potassium chloride is left as a solid residue..
• Write sentences that completely describe
each of the chemical reactions shown in
these skeleton equations.
– KOH(aq) + H2SO4 (aq)  H2O(l) + K2SO4(aq)
– Na(s) + H2O(l)  NaOH(aq) + H2(g)
• Write sentences that completely describe
each of the chemical reactions shown in
these skeleton equations.
– KOH(aq) + H2SO4 (aq)  H2O(l) + K2SO4(aq)
When potassium hydroxide, in aqueous solution combines with
aqueous sulfuric acid, the result is liquid water and a
potassium sulfate solution.
– Na(s) + H2O(l)  NaOH(aq) + H2(g)
• Write sentences that completely describe each
of the chemical reactions shown in these
skeleton equations.
– KOH(aq) + H2SO4 (aq)  H2O(l) + K2SO4(aq)
When potassium hydroxide, in aqueous solution combines with
aqueous sulfuric acid, the result is liquid water and a potassium
sulfate solution.
– Na(s) + H2O(l)  NaOH(aq) + H2(g)
Solid sodium in liquid water yeilds a solution of sodium
hydroxide and hydrogen bubbles.
Balancing equations
• Remember, in a chemical reaction, atoms
are not created or destroyed, but simply
rearranged.
• In every balanced equation, each side of
the equation has the same number of
atoms of each element.
Rules for Balancing Equations
from pg. 208-209
• 1. Determine the correct
formulas for all the
reactants and products in
the reaction.
• 2. Write the formulas for
the reactants on the left
and the formulas for the
products on the right with
an arrow in between. If
two or more reactants or
products, separate them
with a + sign
• 3. Count the number
of atoms of each
element in the
reactants and products.
• 4. If a polyatomic ion
appearing unchanged
on both sides of the
equation is counted as
a single unit.
Rules Con’t
• 5. Balance the
elements one at a
time using
coefficients. NEVER
CHANGE A
SUBSCRIPT TO
BALANCE AN
EQUATION!!!!!
• CHECK EACH
ATOM OR
POLYATOMIC ION
TO BE SURE THAT
THE EQUATION IS
BALANCED
• 6. MAKE SURE
COEFFICIENT
RATIO IS LOWEST
POSSIBLE RATIO.
PRACTICE BALANCING
• PROBLEM 3 PG 209
• PROBLEM 4 PG 209