1. CHAPTER 5 Chemical Reactions General, Organic, & Biological Chemistry
Janice Gorzynski Smith

2. 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
Smith. General Organic & Biolocial Chemistry 2nd Ed.

3. 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.
Breaking bonds in the reactants (starting materials)
Forming new bonds in the products
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

4. aA (physical state) + bB (state)  cC (state) + dD (state)
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
Use modeling kit!
CH4 and O2
CO2 and H2O
Smith. General Organic & Biolocial Chemistry 2nd Ed.

5. Balancing Chemical Equations:
Chemical Rxns
Balancing Chemical Reactions
Balancing Chemical Equations:
Unbalanced equation:
C4H10 + O2  CO2 + H2O
Balanced equation:
2C4H O2  8CO H2O
8 molecules of CO2
2 molecules of C4H10
13 molecules of O2
10 molecules of C4H10
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

6. Balancing Chemical Reactions
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.
Balance the equation with coefficients one
element at a time.
Step [2]
Use modeling kit!
Check to make sure that the smallest set
of whole numbers is used.
Step [3]
Smith. General Organic & Biolocial Chemistry 2nd Ed.

7. Smith. General Organic & Biolocial Chemistry 2nd Ed.
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
Use modeling kit!
The number 6.02 x 1023 is Avogadro’s number.
1 mol
6.02 x 1023 molecules
1 mol
6.02 x 1023 atoms
Smith. General Organic & Biolocial Chemistry 2nd Ed.

8. Smith. General Organic & Biolocial Chemistry 2nd Ed.
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.
Use modeling kit!
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.

9. Mole Ratios Within Molecules: Mole Ratios Between Molecules:
Chemical Rxns
Stoichiometry: Mole Ratios
Mole Ratios Within Molecules:
AxBy
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
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

10. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Stoichiometry
aA bB  cC dD
mass A
mass B
mass C
mass D
x ÷ MM
x ÷ MM
x ÷ MM
x ÷ MM
moles A
moles B
moles C
moles D
a:b
b:c
c:d
a:c
a:d
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

11. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Limiting Reactant
aA bB  cC dD
mass A
mass B
mass C
mass D
x ÷ MM
x ÷ MM
x ÷ MM
x ÷ MM
moles A
moles B
moles C
moles D
a:b
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
a:c
a:d
Use modeling kit!
If A is the limiting reactant then use mole ratio between A:C and A:D to determine # moles of product
Smith. General Organic & Biolocial Chemistry 2nd Ed.

12. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Percent Yield
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.
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

13. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Concentration: % Composition by Mass
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.
Use modeling kit!
Smith. General Organic & Biolocial Chemistry 2nd Ed.

14. Oxidation is the loss of electrons from an atom.
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.
Use modeling kit!
A redox reaction involves the transfer of electrons
from one element to another.
Smith. General Organic & Biolocial Chemistry 2nd Ed.

15. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Oxidation & Reduction Reactions
Cu2+ gains 2 e−
Zn Cu2+
Zn Cu
Zn loses 2 e–
Zn loses 2 e− to form Zn2+, so Zn is oxidized.
Use modeling kit!
Cu2+ gains 2 e− to form Cu, so Cu2+ is reduced.
Smith. General Organic & Biolocial Chemistry 2nd Ed.

16. Smith. General Organic & Biolocial Chemistry 2nd Ed.
Chemical Rxns
Oxidation & Reduction Reactions
Cu2+ gains 2 e−
Zn Cu2+
Zn Cu
Zn loses 2 e–
Each of these processes can be written as an
individual half reaction:
Use modeling kit!
Oxidation half reaction: Zn
Zn e−
loss of e−
Reduction half reaction:
Cu e− Cu
gain of e− 16
Smith. General Organic & Biolocial Chemistry 2nd Ed.