CHAPTER 5
Chemical Reactions
General, Organic, & Biological Chemistry
Janice Gorzynski Smith
CHAPTER 5: Chemical Reactions
Learning Objectives:
 Chemical vs Physical changes
 Chemical equations & balancing them
 Avagadro’s number, Moles, Molar Mass
 Stoichiometry
 Reaction percent yeild
 Limiting Reactant
 Percent concentration
 Redox Reactions:
 Identify oxidation, reduction, oxidizing agent,
reducing agent
 Half reactions
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Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical
Rxns
Chemical vs Physical Changes
• A physical change alters the physical state of a
substance without changing its composition.
• A chemical change (a chemical reaction) converts
one substance into another.
1. Breaking bonds in the reactants (starting materials)
2. Forming new bonds in the products
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Chemical
Rxns
Chemical Reactions
aA (physical state) + bB (state)  cC (state) + dD (state)
A, B = reactants
C, D = reactants
a, b, c, d = coefficients to indicate
molar ratios of reactants and products
CH4 and O2
Smith. General Organic & Biolocial Chemistry 2nd Ed.
CO2 and H2O
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Chemical
Rxns
Balancing Chemical Reactions
Balancing Chemical Equations:
Unbalanced equation:
C4H10 + O2  CO2 + H2O
Balanced equation:
2C4H10 + 13O2  8CO2 + 10H2O
2 molecules of
C4H10
13 molecules of O2
Smith. General Organic & Biolocial Chemistry 2nd Ed.
8 molecules of
CO2
10 molecules of
C4H10
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Chemical
Rxns
Balancing Chemical Reactions
HOW TO Balance a Chemical Equation
Step [1] Write the equation with the correct formulas.
•The subscripts in a formula can never be changed
to balance an equation, because changing a
subscript changes the identity of a compound.
Step [2]
Balance the equation with coefficients one
element at a time.
Step [3]
Check to make sure that the smallest set
of whole numbers is used.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Chemical
Rxns
Avagadro’s Number & The Mole
A mole is a quantity that contains 6.02 x 1023 items.
•1 mole of C atoms = 6.02 x 1023 C atoms
•1 mole of CO2 molecules = 6.02 x 1023 CO2 molecules
•1 mole of H2O molecules = 6.02 x 1023 H2O molecules
The number 6.02 x 1023 is Avogadro’s number.
1 mol
6.02 x 1023 molecules
Smith. General Organic & Biolocial Chemistry 2nd Ed.
1 mol
6.02 x 1023 atoms
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Chemical
Rxns
Molar Mass
•The formula weight is the sum of the atomic weights
of all the atoms in a compound, reported in atomic
mass units (amu).
•The molar mass is the mass of one mole of any
substance, reported in grams.
The value of the molar mass of a compound in grams
equals the value of its formula weight in amu.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Chemical
Rxns
Stoichiometry: Mole Ratios
Mole Ratios Within Molecules:
A xB y
Mole ratio of A:B = x:y
Mole Ratios Between Molecules:
aA + bB  cC + dD
Mole ratio of A:B:C:D = a:b:c:d
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Chemical
Rxns
Stoichiometry
aA +
bB
mass A
x÷
MM
moles A

mass B
+
mass C
x÷
MM
a:b
cC
mass D
x÷
MM
moles B
b:c
moles C
dD
x÷
MM
c:d
moles D
a:c
a:d
Smith. General Organic & Biolocial Chemistry 2nd Ed.
10
Chemical
Rxns
Limiting Reactant
aA +
bB
mass A
x÷
MM
moles A

mass B
x÷
MM
a:b
+
mass C
Limiting Reactant: Compare moles A &
moles B after applying mole ratio.
The reactant with the least number of
moles AFTER mole ratio considered is
the limiting reactant.
Use limiting reactant # moles to
determine moles of products that form
dD
mass D
x÷
MM
moles B
Smith. General Organic & Biolocial Chemistry 2nd Ed.
cC
x÷
MM
moles C
moles D
a:c
a:d
If A is the limiting
reactant then use
mole ratio between
A:C and A:D to
determine # moles of
product
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Chemical
Rxns
Percent Yield
æ actual yield ö
÷÷ ´ 100%
percentage yield = çç
è theoretical yield ø
Actual yield is determined experimentally, it is the mass
of the product that is measured.
Theoretical yield is the calculated mass of the products
based on the initial mass or number of moles of the
reactants.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
12
Chemical
Rxns
Concentration: % Composition by Mass
æ Mass due to specific component ö
percent composition = ç
÷ ´100%
è Total molar mass of compound ø
Solution Stoichiometry:
• Often work with solutions when conducting reactions
• Determine the amounts needed to completely react just like
any other stoichiometry problem.
• Use percent concentration to obtain mass of each
substance.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
13
Chemical
Rxns
Oxidation & Reduction
Oxidation is the loss of electrons from an atom.
Reducing agents are oxidized
Reduction is the gain of electrons by an atom.
Oxidizing agents are reduced.
•Both processes occur together in a single reaction
called an oxidation−reduction or redox reaction.
Thus, a redox reaction always has two components,
one that is oxidized and one that is reduced.
•A redox reaction involves the transfer of electrons
from one element to another.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
14
Chemical
Rxns
Oxidation & Reduction Reactions
Cu2+ gains 2 e−
Zn + Cu2+
Zn2+ + Cu
Zn loses 2 e–
•Zn loses 2 e− to form Zn2+, so Zn is oxidized.
•Cu2+ gains 2 e− to form Cu, so Cu2+ is reduced.
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Chemical
Rxns
Oxidation & Reduction Reactions
Cu2+ gains 2 e−
Zn2+ + Cu
Zn + Cu2+
Zn loses 2 e–
Each of these processes can be written as an
individual half reaction:
Zn2+ + 2 e−
loss of e−
Oxidation half reaction:
Zn
Reduction half reaction:
Cu2+ + 2e−
gain of e−
Smith. General Organic & Biolocial Chemistry 2nd Ed.
Cu
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