1. 1. What is a Chemical Reaction?
2. How Do We Know a Chemical Reaction
Occurs?
3. Writing Chemical Equations
4. Predicting Chemical Reactions
5. Representing Reactions in Aqueous Solution
5-1
2. Chemical reactions
occur all around us. Figure 5.1
How do we make
sense of these
changes? What
patterns can we
find? Figure 5.29
Figure 5.8F 5-2
3. A chemical reaction is a chemical change.
A chemical reaction occurs when one or more
substances is converted into one or more new
substances.
Reactant – a substance that we start with that
undergoes a change
Product – a new substance that forms during
the reaction
Products differ from reactants only in the
arrangement of their component atoms.
5-3
4. What observations might indicate that a
chemical reaction has taken place?
Figure 5.7
5-4
6. Common observations that may accompany a
chemical reaction are:
change in color
production of light
formation of a solid (such as a precipitate in solution, or
smoke in air, or a metal coating)
formation of a gas (bubbles in solution or fumes in the
gaseous state)
absorption or release of heat (sometimes appearing as a
flame)
5-6
7. A chemical equation is a symbolic
representation of a chemical reaction.
A chemical equation shows:
the formulas for reactants and products
the physical states of each substance (s, l, g, aq)
relative numbers of reactants that combine and
products that form
special conditions required for the reaction, such as
constant heating.
5-7
8. When hydrogen gas is ignited in the presence
of oxygen, an explosive reaction occurs
producing gaseous water molecules.
Is mass conserved?
Figure 5.5 Figure 5.6 5-8
9. Balanced equation
The number of atoms of each element is
the same in the products as in the
reactants.
Conservation of mass is always
maintained.
5-9
10. When a powdered mixture
of aluminum metal and
iron(III) oxide is heated, it
reacts to form liquid iron
metal and aluminum(III)
oxide.
Al(s) + Fe2O3(s) Al2O3(s) +
Fe(l)
Figure 5.9
5-10
11. Al(s) + Fe2O3(s) Al2O3(s) + Fe(l)
Is this equation balanced?
What has to change? Figure from p. 171
5-11
12. Al(s) + Fe2O3(s) Al2O3(s) + Fe(l)
We balance the atoms in equations with
coefficients.
The aluminum atoms are not balanced so we place a
coefficient of 2 in front of the Al reactant:
2Al(s) + Fe2O3(s) Al2O3(s) + Fe(l)
The iron atoms are not balanced so we place a
coefficient of 2 in front of the Fe product:
2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(l)
Figure
from
p. 172
5-12
13. Methane + oxygen carbon dioxide + water
CH4(g) + O2(g) CO2(g) + H2O(g)
The number of atoms of each element in the unbalanced
equation is shown below. These numbers were obtained
by multiplying the subscript to the right of the element’s
symbol by the stoichiometric coefficient.
# of atoms (reactants) # of atoms (products)
1C 1C
4H 2H
2O 3O
5-13
14. CH4(g) + O2(g) CO2(g) + H2O(g)
First, we look at the carbon atoms. Since the number
of carbon atoms on the reactant side is already equal
to the number of carbon atoms on the product side,
we don’t need to add coefficients.
Next, we look at the hydrogen atoms. Currently,
there are four hydrogen atoms on the reactant side
and 2 hydrogen atoms on the product side. Thus, we
need to add a coefficient of 2 in front of water to make
the hydrogen atoms equal.
CH4(g) + O2(g) CO2(g) + 2H2O(g)
5-14
15. CH4(g) + O2(g) CO2(g) + 2H2O(g)
Finally, we look at the oxygen atoms. Currently, there
are 2 oxygen atoms on the reactant side and 4 oxygen
atoms (combined from carbon dioxide and water) on
the product side. Thus, we add a coefficient of 2 in
front of the oxygen gas.
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
Now the equation is balanced!
5-15
16. 1. Identify the reactants and products and write their
correct formulas.
2. Write a skeletal equation including physical states.
3. Change coefficients one at a time until the atoms of
each element are balanced. (Start with the
elements that occur least often in the equation.)
4. Make a final check by counting the atoms of each
element on both sides of the equation.
5-16
17. Balance the following equations:
1. Potassium chlorate potassium chloride +
oxygen
2. Aluminum acetate reacts with potassium
sulfate to form potassium acetate and
aluminum sulfate
3. Hexane (C6H14) reacts with oxygen gas to
form carbon dioxide and water
4. Zinc reacts with hydrochloric acid to form
zinc chloride and hydrogen gas
5-17
18. 1. Potassium chlorate potassium chloride + oxygen
First, translate the chemical names into chemical
formulas:
Potassium chlorate potassium chloride + oxygen
K+ ClO3− K+ Cl− O2
(diatomic)
KClO3(s) KCl(aq) + O2(g)
Next, balance the chemical equation:
2KClO3(s) 2KCl(aq) + 3O2(g)
5-18
20. 3. Hexane (C6H14) reacts with oxygen gas to form carbon
dioxide and water
Hexane was given. Oxygen gas is diatomic.
Carbon dioxide has 1 carbon atom and 2 oxygen
atoms (using the Greek prefixes) and water has 2
hydrogen atoms and 1 oxygen atom.
C6H14(l) + O2(g) CO2(g) + H2O(g)
Balanced:
C6H14(l) + 9.5O2(g) 6CO2(g) + 7H2O(g)
Fractional coefficients are not acceptable, so we
multiply all coefficients by 2:
2C6H14(l) + 19O2(g) 12CO2(g) + 14H2O(g)
5-20
21. 4. Zinc reacts with hydrochloric acid to form
zinc chloride and hydrogen gas
Zinc + Hydrochloric acid Zinc chloride + hydrogen
Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)
Balanced:
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
5-21
