Chemical Reactions: An
Introduction
Chapter 6
Chemical Equations
Chemical change involves a
reorganization of the atoms in one or
more substances.
4 Evidences a Chemical
Reaction Has Occurred
1.A color change is apparent.
2. A precipitate (solid) forms.
4 Evidences a Chemical
Reaction Has Occurred
3. A gas (bubbles) is formed.
4. Evolution/absorption of energy
--heat, light, electrical energy.
Reaction of Zinc and Iodine
Reactants: Zn + I2
Product: Zn I2
Chemical Equation
A representation of a chemical reaction:
word equation -- qualitative significance only
ethanol + oxygen yields carbon dioxide + water vapor
formula equation -- qualitative & quantitative
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g)
reactants
products
Chemical Equations
Chemical equations give two important pieces
of information:
1. The identities of the reactants and products
-- qualitative information.
2. The relative number of each --quantitative
information.
Physical States
•
•
•
•
solid (s)
liquid (l)
gas (g)
aqueous (aq)
Important Equation Symbols
• yields
------>
• heat

------->
cat.
• catalyst ------->
• light
light
-------->
H2SO4
• catalyst ------>
elect.
• electricity ------>
Chemical Equation
Quantitative Significance
C2H5OH (l) + 3O2(g)  2CO2(g) + 3H2O(g)
The equation is balanced.
1 mole of ethanol reacts with 3 moles of
oxygen
to produce
2 moles of carbon dioxide and 3 moles of
water
Chemical Equations
Quantitative Significance
4 Al(s) + 3 O2(g)
---> 2 Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules
---give--->
2 molecules of Al2O3
4 moles of Al + 3 moles of O2
---give--->
2 moles of Al2O3
Chemical Equations
Because of the principle of
the conservation of
matter, an equation must
be balanced.
It must have the same
number of atoms of the
same kind on both sides.
The total mass for the
reaction cannot change.
Lavoisier, 1788
Balancing Equation Prerequisites
Student must have memorized:
• 44 chemical symbols
• Table 5.2 on page 133 in text
• Table 5.3 on page 137
• Table 5.4 on page 142
• Table 5.5 on page 146
• Table 5.6 on page 146
• Type I, II, III, and acid nomenclature
• Count HOFBrINCl
Key Words
Reactants are indicated by:
reacts with…..
combines with…..
oxidizes……
burns in……
….are required
….are needed
….decomposes
Products are indicated by:
….is formed
….is produced
….is given off
….is precipitated
yields….
….is prepared
…..is synthesized
Coefficients, Subscripts, &
Superscripts
Coefficient
2Hg
2
2
Superscript
Subscript
Four Steps in Balancing Equations
1. Get the facts down.
2. Check for diatomic molecules (subscripts).
3. Balance charges on compounds containing a
metal, ammonium compounds, and acids
(subscripts).
4. Balance the number of atoms (coefficients).
a. Balance most complicated molecule first.
b. Balance other elements.
c. Balance hydrogen next to last.
d. Balance oxygen last.
Balancing Equations Caution
The identities (formulas) of
the compounds must never be
changed in balancing a
chemical equation!
Only coefficients can be used
to balance the equationsubscripts will not change!
Combustion of Methane
methane gas burns to produce carbon dioxide gas
and liquid water
• whenever something burns it combines with O2(g)
CH4(g) + O2(g)  CO2(g) + H2O(l)
O
H
H
C
H
H
+
O
O
C
+
O
H
H
O
1C+4H
+
2O
1C+2O +2H+O
1C+2H+3O
Combustion of Methane
Balanced
The balanced reaction must obey the Law of
Conservation of Mass:
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
H
H
C
H
H
O
+
O
C
+
O
1C + 4H + 4O
O
O
O
O
+
H
H
+
O
H
H
1C + 4H + 4O
Balancing Equations
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)
Chemical Reactions
What evidence did
you observe that a
chemical reaction
occurred?
1. Evolution of
heat/light
2. Color change
Balancing Equations
2 Al(s) + 3 Br2(l) ---> Al2Br6(s)
Balancing Equations
___C3H8(g) + ___ O2(g) ----> ____CO2(g) + ____ H2O(g)
___B4H10(g) + ___ O2(g) ----> ____ B2O3(g) + ____ H2O(g)
Balancing Equations
C3H8(g) + 5 O2(g) ----> 3CO2(g) + 4 H2O(g)
2 B4H10(g) + 11 O2(g) ----> 4 B2O3(g) + 10 H2O(g)
Final Equation Balancing Check
SiO2(s) + 4 HF(aq) -----> SiF4(g) + 2 H2O(l)
Make a grid to do a final check to be sure the Law of
Conservation of Matter has been obeyed:
Reactants
1 Si
2O
4H
4F
Products
1 Si
2O
4H
4F
Figure 6.5(a): The reaction of potassium with water
Figure 6.5(b)&(c): The reaction of potassium with water
What evidence is there for a chemical reaction?
Reaction of Potassium
and Water
Write and balance the word and formula equation for
potassium and water.
potassium(s) + water(l)  potassium hydroxide(aq) + hydrogen(g)
2K(s) + 2HOH(l)  2KOH(aq) + H2(g)