___________________________________ Slide 1 ___________________________________ ___________________________________ Chapter 6
___________________________________ Chemical Reactions: An
Introduction
___________________________________ ___________________________________ ___________________________________ Slide Chapter 6
2 Table of Contents
6.1
6.2
6.3
___________________________________ ___________________________________ Evidence for a Chemical Reaction
Chemical Equations
Balancing Chemical Equations
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Slide Section 6.1
3 Evidence for a Chemical Reaction
2
___________________________________ ___________________________________ What are the clues that a chemical change has taken place?
•
•
Chemical reactions often give a visual
signal.
But reactions are not always visible.
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Slide Section 6.1
4 Evidence for a Chemical Reaction
___________________________________ ___________________________________ Some Clues That a Chemical Reaction Has Occurred
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Slide Section 6.1
5 Evidence for a Chemical Reaction
4
___________________________________ ___________________________________ Exercise
___________________________________ What is a clue that a chemical reaction has
occurred?
___________________________________ ___________________________________ a)
b)
c)
d)
The color changes.
A solid forms.
Bubbles are present.
A flame is produced.
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Slide Section 6.1
6 Evidence for a Chemical Reaction
5
___________________________________ ___________________________________ Exercise
___________________________________ What is a clue that a chemical reaction has
occurred?
___________________________________ “Colorless hydrochloric acid is added to a red solution
of cobalt(II) nitrate, turning the solution blue.”
a)
b)
c)
d)
___________________________________ The color changes.
A solid forms.
Bubbles are present.
A flame is produced.
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Slide Section 6.1
7 Evidence for a Chemical Reaction
___________________________________ ___________________________________ Exercise
___________________________________ What is a clue that a chemical reaction has
occurred?
___________________________________ “A solid forms when a solution of sodium dichromate is
added to a solution of lead nitrate.”
a)
b)
c)
d)
___________________________________ A gas forms.
A solid forms.
Bubbles are present.
A flame is produced.
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Slide Section 6.2
8 Chemical Equations
7
___________________________________ ___________________________________ • Chemical reactions involve a rearrangement of the ways
atoms are grouped together.
___________________________________ • A chemical equation represents a chemical reaction.
 Reactants are shown to the left of the arrow.
 Products are shown to the right of the arrow.
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Slide Section 6.2
9 Chemical Equations
8
___________________________________ ___________________________________ • In a chemical reaction atoms are not created or destroyed.
___________________________________ • All atoms present in the reactants must be accounted for in
the products.
 Same number of each type of atom on both sides of the
arrow.
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Slide Section 6.2
___________________________________ 10 Chemical Equations
Balancing a Chemical Equation
• Unbalanced Equation:
___________________________________ ___________________________________ ___________________________________ •
Balancing the Equation:
___________________________________ ___________________________________ •
The balanced equation:
CH4 + 2O2  CO2 + 2H2O
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Slide Section 6.2
11 Chemical Equations
___________________________________ Return to TOC
10
___________________________________ ___________________________________ Physical States
•
Physical states of compounds are often
given in a chemical equation.
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Slide Section 6.2
12 Chemical Equations
11
___________________________________ ___________________________________ Example
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Slide Section 6.2
13 Chemical Equations
___________________________________ ___________________________________ Exercise
___________________________________ When blue light shines on a mixture of
hydrogen and chlorine gas, the elements react
explosively to form gaseous hydrochloric acid.
What is the unbalanced equation for this
process?
a)
b)
c)
d)
___________________________________ ___________________________________ H2(g) + CH4(g)
HCl(g)
HCl(g)
H(g) + Cl(g)
HCl(g)
H(g) + Cl(g)
H2(g) + Cl2(g)
HCl(g)
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Slide Section 6.3
14 Balancing Chemical Equations
13
___________________________________ ___________________________________ •
•
•
The principle that lies at the heart of the
balancing process is that atoms are
conserved in a chemical reaction.
Atoms are neither created nor destroyed.
The same number of each type of atom is
found among the reactants and among the
products.
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Slide Section 6.3
15 Balancing Chemical Equations
14
___________________________________ ___________________________________ •
•
Chemists determine the identity of the
reactants and products of a reaction by
experimental observation.
The identities (formulas) of the compounds
must never be changed in balancing a
chemical equation.
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Slide Section 6.3
16 Balancing Chemical Equations
___________________________________ ___________________________________ How to Write and Balance Equations
1. Read the description of the chemical
reaction. What are the reactants, the
products, and their states? Write the
appropriate formulas.
___________________________________ ___________________________________ Hydrogen gas (H2) and oxygen gas (O2)
combine to form liquid water (H2O).
___________________________________ 2. Write the unbalanced equation that
summarizes the information from step 1.
___________________________________ H2(g) + O2(g)  H2O(l)
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Slide Section 6.3
17 Balancing Chemical Equations
16
___________________________________ ___________________________________ How to Write and Balance Equations
3. Balance the equation by inspection,
starting with the most complicated
molecule.
___________________________________ ___________________________________ Equation is unbalanced by counting the
atoms on both sides of the arrow.
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Slide Section 6.3
18 Balancing Chemical Equations
17
___________________________________ ___________________________________ How to Write and Balance Equations
3. Balance the equation by inspection,
starting with the most complicated
molecule.
___________________________________ ___________________________________ We must balance the equation by adding
more molecules of reactants and/or products.
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___________________________________ Slide Section 6.3
19 Balancing Chemical Equations
___________________________________ How to Write and Balance Equations
4. Check to see that the coefficients used give
the same number of each type of atom on
both sides of the arrow. Also check to see
that the coefficients used are the smallest
integers that give the balanced equation.
The balanced equation is:
2H2(g) + O2(g)  2H2O(l)
or could be:
4H2(g) + 2O2(g)  4H2O(l)
___________________________________ ___________________________________ ___________________________________ preferred
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___________________________________ Slide Section 6.3
20 Balancing Chemical Equations
___________________________________ Exercise
___________________________________ Balance the following equation in standard
form (lowest multiple integers) and determine
the sum of the coefficients?
___________________________________ ___________________________________ FeO(s) + O2(g)  Fe2O3(s)
a)
b)
c)
d)
___________________________________ 3
4
7
14
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___________________________________ Slide Section 6.3
21 Balancing Chemical Equations
___________________________________ Exercise
___________________________________ Which of the following correctly balances the
chemical equation given below? There may be more
than one correct balanced equation. If a balanced
equation is incorrect, explain what is incorrect about
it.
___________________________________ ___________________________________ CaO + C  CaC2 + CO2
I.
II.
III.
IV.
CaO2 + 3C  CaC2 + CO2
2CaO + 5C  2CaC2 + CO2
CaO + (2.5)C  CaC2 + (0.5)CO2
4CaO + 10C  4CaC2 + 2CO2
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___________________________________ Slide Section 6.3
22 Balancing Chemical Equations
___________________________________ Exercise
___________________________________ Of the three that are correct, which one is preferred
most (the most accepted convention)? Why?
___________________________________ CaO + C  CaC2 + CO2
I.
II.
III.
IV.
___________________________________ CaO2 + 3C  CaC2 + CO2
2CaO + 5C  2CaC2 + CO2
CaO + (2.5)C  CaC2 + (0.5)CO2
4CaO + 10C  4CaC2 + 2CO2
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Slide Section 6.3
23 Balancing Chemical Equations
22
___________________________________ ___________________________________ Concept Check
When balancing a chemical equation, which of the
following statements is false?
a) Subscripts in the reactants must be conserved in the
products.
b) Coefficients are used to balance the atoms on both
sides.
c) When one coefficient is doubled, the rest of the
coefficients in the balanced equation must also be
doubled.
d) Phases are often shown for each compound but are
not critical to balancing an equation.
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Slide Section 6.3
24 Balancing Chemical Equations
___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Return to TOC
23
___________________________________ ___________________________________ Notice
•
•
•
•
The number of atoms of each type of element
must be the same on both sides of a balanced
equation.
Subscripts must not be changed to balance an
equation.
A balanced equation tells us the ratio of the
number of molecules which react and are
produced in a chemical reaction.
Coefficients can be fractions, although they are
usually given as lowest integer multiples.
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Slide 25 ___________________________________ Section 6.3
Balancing
Chemical Equations
Chapter
6 Homework
___________________________________ Homework
___________________________________ • Reading assignment
– Pages 145 through 158
___________________________________ • Homework Problems
– Questions and problems 3, 5, 9, 11, 12, 15, 17, 19,
21, 23, 25, 27, 29, 31, 39, 41, 43.
___________________________________ ___________________________________ • Due on
___________________________________ Return to TOC
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25