Chapter 8
Chemical Reactions
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Chapter 8 Goals
Learn the mechanics of writing an equation.
Learn how to identify four different kinds of
reactions.
Learn how to predict the products of each
kind of reaction and write the formulas of
those products.
Given potential reactants, write the equation
for the probable reaction.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.1
Evidence of a
Chemical Equation
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Goal 1
Describe five types of evidence detectable by
human senses that usually indicate a
chemical change.
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Chemical change occurs when the chemical identity of a
substance is destroyed and a new substance forms.
Five possible indicators of chemical reactions are
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1. Color change
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2. Formation of a solid
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3. Formation of a gas
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4. Absorption or release of heat energy
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5. Emission of light energy
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Section 8.2
Evolution of a
Chemical Equation
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When solid sodium is added to liquid water,
a reaction occurs, producing
hydrogen gas, sodium hydroxide solution, and heat
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Chemists have developed a shorthand method for
expressing such chemical reactions
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Solid sodium
Na(s)
plus
+
liquid water
H2O(l)
yields
hydrogen gas
plus
sodium hydroxide
solution
>
H2(g)
+
NaOH(aq)
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Unbalanced chemical equation
An equation is balanced by placing a coefficient
in front of one or more of the formulas,
indicating that it is used more than once
2 Na(s) +
2 H2O(l)
Increase # of H on left
> H (g) + 2 NaOH(aq)
2
Balance H
Balance Na
The balanced equation is both a qualitative and a
quantitative description of the reaction
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Do’s and Don’ts in Balancing an Equation
Do: Balance the equation entirely by
use of coefficients placed before the
different chemical formulas
Don’t: Change a correct chemical formula in
order to make an element balance
Don’t: Add some real or imaginary chemical
species to either side of the equation just
to make an element balance
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Writing and Balancing a Chemical Equation
1. Write a qualitative description of the reaction.
In this step you write the formulas
of reactants and products.
2. Quantify the description
by balancing the equation.
This is done by adding coefficients.
Do not change the qualitative
description of the reaction.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.3
Balancing Chemical
Equations
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Goal 2
Given an unbalanced chemical equation,
balance it by inspection.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Balancing a Chemical Equation:
A Formal Approach
Step 1: Place a “1” in front of the formula with
the largest number of atoms. If two formulas
have the same number of atoms, select the
one with the greater number of elements. We
will call this formula the starting formula
in the discussion that follows.
Na(s) +
H2O(l)
> H (g) + 1 NaOH(aq)
2
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Balancing a Chemical Equation:
A Formal Approach
Step 2: Insert coefficients that balance the elements that
appear in compounds. Use fractional coefficients, if necessary.
Do not balance element-only formulas, such as Na or O2, at this
time. We call these uncombined elements. Choosing elements
in the following order is usually easiest:
a) Elements in the starting formula that
are in only one other compound
b) All other elements from the starting formula
c) All other elements in compounds
Na(s) +
1 H2O(l)
> H (g) + 1 NaOH(aq)
2
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Balancing a Chemical Equation:
A Formal Approach
Step 3: Place coefficients in front of formulas of
uncombined elements that balance those elements. Use
fractional coefficients, if necessary.
1 Na(s) +
1 H2O(l)
1
> H (g)
2 2
+ 1 NaOH(aq)
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Balancing a Chemical Equation:
A Formal Approach
Step 4: Clear fractions, if any, by multiplying all
coefficients by the lowest common denominator.
Remove any “1” coefficients that remain.
2 Na(s) +
H2O(l)
> 2 H (g) + 2 NaOH(aq)
2
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Balancing a Chemical Equation:
A Formal Approach
Step 5: Check your work.
2 Na, 4 H, 2 O on each side, OK
2 Na(s) +
H2O(l)
> 2 H (g) +
2
2 NaOH(aq)
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
You can treat chemical equations exactly the
same way as you treat algebraic equations
You can multiply or divide an equation
by some number by multiplying or dividing
each term by that number
The order in which reactant or products
is written may be changed
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.4
Interpreting Chemical
Equations
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Goal 3
Given a balanced chemical equation or
information from which it can be written,
describe its meaning on the particulate, molar,
and macroscopic levels.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
The particulate-level interpretation
of a chemical equation
2 H2(g) + O2(g)
> 2 H2O(g)
Two molecules of hydrogen react with one
oxygen molecule to form two water molecules
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The particulate interpretation
of a chemical equation
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The molar-level interpretation
of a chemical equation
2 H2(g) + O2(g)
> 2 H2O(g)
Two moles of hydrogen molecules react with
one mole of oxygen molecules to form
two moles of water molecules
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
mole
mole
mole
The grouping unit (dozen, mole) interpretation
of a chemical equation
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
The macroscopic-level interpretation
of a chemical equation
2 H2(g) + O2(g)
> 2 H2O(g)
No direct information about masses
Molar mass is the link between
the molar-level interpretation
and macroscopic measurements of mass
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.5
Writing Chemical
Equations
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Writing and Balancing a Chemical Equation
1. Classify the reaction type.
2. Write a qualitative description of the reaction. In this
step you write the formulas of the given reactants to the
left of an arrow and the formulas of the given or
predicted products to the right.
3. Quantify the description by balancing the equation.
Do this by adding coefficients. Do not change the
qualitative description of the reaction by adding,
removing, or altering any formula.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.6
Combination Reactions
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Goal 4
Write the equation for the reaction in which a
compound is formed by the combination of
two or more simpler substances.
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Combination Reaction
A + X > AX
Reactants: Elements and/or compounds
Product: A single compound
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Example:
Potassium combines with oxygen
to form potassium oxide.
4 K
+
O2
2 K2O
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Summary:
Combination Reactions
Reactants:
Reaction type:
Equation type:
Any combination of elements
and/or compounds
Combination
A + X
>AX
Products:
One compound
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Section 8.7
Decomposition
Reactions
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Goal 5
Given a compound that is decomposed into
simpler substances, either compounds or
elements, write the equation for the reaction.
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Decomposition Reaction
AX > A + X
Reactant: A compound
Products: Elements and/or compounds
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Example:
Molten sodium iodide is
decomposed into its elements.
2 NaI
2 Na
+
I2
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Example:
Water is decomposed into its elements.
2 H2O
2 H2
+
O2
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Summary:
Decomposition Reactions
Reactants:
One compound
Reaction type:
Decomposition
Equation type:
AX
>A + X
Products:
Any combination of elements
and compounds
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.8
Single-Replacement
Reactions
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Goal 6
Given the reactants of a single-replacement
reaction, write the equation for the reaction.
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Single-Replacement
Oxidation–Reduction (Redox) Reaction
A + BX
AX + B
Reactants: An element (A) and
an ionic compound or an acid (BX)
Products: An ionic compound (AX) and
an element (B)
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Example:
A magnesium wire is placed in
a solution of nickel nitrate.
Mg +
Ni(NO3)2
Mg(NO3)2 +
Ni
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Summary:
Single-Replacement Reactions
Reactants:
Reaction type:
Equation type:
Element (A) plus a solution of either an
acid or an ionic compound (BX)
Single-replacement redox
A + BX
AX + B
Products:
An ionic compound (usually in
solution) (AX) plus an element (B)
Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.
Section 8.9
Double-Replacement
Reactions
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Goal 7
Given the reactants in a double-replacement
precipitation or neutralization reaction, write
the equation.
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Double-Replacement Precipitation Reaction
AX + BY
AY + BX
Reactants: Solutions of ionic compounds
or acids
Products: A solid ionic compound (precipitate)
and an ionic compound or acid
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Example:
Zinc bromide and potassium carbonate
solutions are combined.
ZnBr2 +
K2CO3
ZnCO3
+ 2 KBr
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Summary:
Double-Replacement Reactions
Reactants:
Reaction type:
Equation type:
Solutions of two ionic compounds or a solution of
an ionic compound and an acid solution
Double-replacement precipitation
AX + BY
AY + BX
Products:
Precipitate of ionic compound and
second ionic compound or acid
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Section 8.10
Summary of Reactions
and Equations
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Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.