Lesson 9
Chemical Reactions
Anything in black letters = write it in
your notes (‘knowts’)
Quick Review…
Write the chemical formula for sodium carbonate
+1
Na
-2
CO3
sodium ion
carbonate ion
Na 2CO3
Section 1 – Counting Atoms
Coefficient
2 Na2CO3
Subscripts
Subscripts represent the number of atoms in a
compound.
Compound
CaCl2
Na2CO3
NaCl
CaCO3
Number and type of atom in
compound
Ca
Cl
Na
C
O
Coefficients represent the number of each
element or compound
Compound
3CaCl2
2Na2CO3
2NaCl
4CaCO3
Number and type of atom in
compound
Ca
Cl
Na
C
O
Section 2 – Balancing Chemical Equations
Chemical equations represent chemical reactions
Reactants  Products
Reactant mass = Product mass
# reactant atoms = # product atoms
Chemical equations must be
balanced to obey the LAW!
Word Equations
“Sodium carbonate reacts with calcium chloride to
produce sodium chloride and calcium carbonate”
Chemical Equations
Na2CO3 + CaCl2  NaCl + CaCO3
This is NOT balanced
Na2CO3 + CaCl2  2 NaCl + CaCO3
subscripts
coefficient
When balancing equations, never change subscripts.
Instead, add coefficients
When balancing equations, never change subscripts.
Instead, add coefficients
2 H2
+
O2
→ 2 H2O
2 Al
+ 3 F2
→ 2 AlF3
2 NaClO3
→ 2 NaCl + 3 O2
More on Chemical Equations
Symbols Used in Chemical Equations
Symbol
(s), (l), (g)
(aq)
Explanation
Designates a reactant or product in the solid state,
liquid state, or gaseous state; placed after the formula
Designates an aqueous solution; the substance is
dissolved in water; placed after the formula
Δ
or
Indicates that heat is supplied to the reaction
heat
Pt
A formula written above or below the yields sign indicates
its use as a catalyst (in this example, platinum).
Catalyst – substance that speeds up a chem.
rxn but is not part of the chem. equation
Balancing Equations Help…
1. Balance 1 element at a time, from left to right.
2. Treat any polyatomic ions as single units if
they are on both sides of the reaction.
3 Zn(OH) 2 + 2 H3PO4
→
Zn3(PO4)2
+ 6 H2O
(OH) is NOT on both sides
There is a (PO4) on each side
Balancing Equations Hints…
3. Odd # atoms → even # atoms;
multiply through by 2
2 CH3OH
+ 3 O2
→ 2 CO2
4
+ 2 H2O
4. Balance oxygen last, it just helps sometimes.
5. Remember, N2, O2, F2, Cl2, Br2, I2, H2
Quiz #1
Balance the following chemical equations
1.
FeCl3 +
H2S →
FeCl2 +
2.
MnO2 +
HCl → MnCl2 +
HCl
H2O
+
S
+ Cl2
3. Hydrochloric acid reacts with solid sodium hydrogen
carbonate. The products formed are aqueous sodium
chloride, water, and carbon dioxide gas. Write a
skeleton equation for this chemical reaction.
Section 3 – Types of Chemical Equations
Most chemical rxns will fit into 1 of 5 types.
1. Combination (Synthesis)
2. Decomposition
3. Single Replacement
4. Double Replacement
5. Combustion
1. Combination
A + B → AB
Magnesium metal
and oxygen gas
combine to form
the compound
magnesium oxide.
2Mg(s) + O2 (g) → 2 MgO(s)
2. Decomposition
AB
→
A + B
2HgO(s)
heat
2Hg(l) + O2(g)
3. Single Replacement
A + BC
→ AB + C
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
4. Double Replacement
AB + CD
→ AC + BD
5. Combustion
A substance reacts with oxygen (O2) and releases
energy
If the substance is a hydrocarbon, then CO2 and
H2O are products,
CH4(g)
+
2O2(g)
→ CO2(g) + 2H2O(g)
Which type of reaction?
H2O
electricity
H2(g) + O2(g)
K2CO3(aq) + BaCl2(aq) → 2KCl(aq) + BaCO3(s)
2KI(aq) + Pb(NO3)2(aq) → PbI2 + 2KNO3(aq)
2Fe(s) + 3S(g) → Fe2S3(s)
2NaN3(s) → 2Na(s) + 3N2(g)
2K(s) + Cl2(g) → 2KCl(s)
S(s) + O2(g) → SO2(g)
Zn(s) + Cu(NO3)2(aq) → Cu(s) + Zn(NO3)2(aq)
2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g)
Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)
2Mg(s) + O2(g) → 2MgO(s)
Cu(s) + S(s) → CuS(s)
Can you balance these?
a)
Al
+
b) C3H8
c) FeSO4
d)
F2
+
+
NaClO3
O2
→
AlF3
→
CO2
+
Ba(OH)2 →
Fe(OH)2
+
→
+
NaCl
O2
H2 O
BaSO4
Section 4 – Writing Chemical Equations
Common acids:
H2SO4 – sulfuric acid
HCl – hydrochloric acid
HNO3 – nitric acid
Diatomic Molecules:
H2, N2, O2, F2, Cl2, Br2, I2
If the substance combusting is a hydrocarbon, then
CO2 and H2O are products.
Write a balanced chemical equation for the combustion of
propane (C3H8).
Write a balanced chemical equation for the combination
of hydrogen gas and oxygen gas to form water.
Write a balanced chemical reaction for the decomposition
of copper(II) hydroxide into copper(II) oxide and water.
Write a balanced chemical equation for the double
replacement reaction of sodium nitrate with calcium
chloride.
Write a balanced chemical equation for the single
replacement reaction of sodium metal with water to form
sodium hydroxide and hydrogen gas.
Exploratory info for next chapters…
What does a coefficient mean?
4 Ag
+
O2
→ 2 Ag2O
_____ atoms of Ag will react with
_____ molecule(s) of O2 to form
_____ formula units of Ag2O
A coefficient represents the combining ratio
of reactants & products in a chemical rxn.
Exploratory info for next chapters…
4 Ag
+
O2
→ 2 Ag2O
1. How many molecules of oxygen (O2) would be
needed to react with 4 atoms of silver?
2. How many molecules of oxygen (O2) would be
needed to react with 8 atoms of silver?
3. How many formula units of silver oxide would
be formed in 1? In 2?
Exploratory info for next chapters…
4 Ag
+
O2
→ 2 Ag2O
How many oxygen molecules would
be needed to form 20 formula units of
silver oxide?
How many atoms of silver would be
required as well?
Exploratory info for next chapters…
4 Ag
+
O2
→ 2 Ag2O
If 20 atoms of silver react with 20 molecules of
oxygen, which reactant would be used up
completely? Which reactant would be leftover?
How many formula units of silver oxide would be
formed?
Exploratory info for next chapters…
Limiting Reactant Reactant that is completely used up;
limits the amount of product that
can be produced.
Excess Reactant Reactant that remains un-reacted; is
not completely used up.
Exploratory info for next chapters…
Fe2O3 + 3 CO
→ 2 Fe
+ 3 CO2
1. How many molecules of CO are needed to
produce 4 atoms of Fe?
6 CO molecules
2. How much Fe2O3 is assumed to be present in
the question above?
Assuming that there is at least 2 formula units of Fe2O3