Download Section 6.3 Balancing Chemical Equations

Chapter 6
Chemical Reactions: An
Introduction
Chapter 6
Table of Contents
6.1
6.2
6.3
Evidence for a Chemical Reaction
Chemical Equations
Balancing Chemical Equations
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Section 6.1
Evidence for a Chemical Reaction
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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Section 6.1
Evidence for a Chemical Reaction
Some Clues That a Chemical Reaction Has Occurred
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Section 6.1
Evidence for a Chemical Reaction
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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Section 6.1
Evidence for a Chemical Reaction
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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Section 6.1
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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Section 6.2
Chemical Equations
• 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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Section 6.2
Chemical Equations
• 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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Section 6.2
Chemical Equations
Balancing a Chemical Equation
• Unbalanced Equation:
•
Balancing the Equation:
•
The balanced equation:
CH4 + 2O2  CO2 + 2H2O
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Section 6.2
Chemical Equations
Physical States
•
Physical states of compounds are often
given in a chemical equation.
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Section 6.2
Chemical Equations
Example
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Section 6.2
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)
H(g) + Cl(g)
HCl(g)
H2(g) + Cl2(g)
HCl(g)
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Section 6.3
Balancing Chemical Equations
•
•
•
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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Section 6.3
Balancing Chemical Equations
•
•
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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Section 6.3
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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Section 6.3
Balancing Chemical Equations
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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Section 6.3
Balancing Chemical Equations
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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Section 6.3
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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Section 6.3
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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Section 6.3
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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Section 6.3
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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Section 6.3
Balancing Chemical Equations
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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Section 6.3
Balancing Chemical Equations
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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Section 6.3
Balancing Chemical Equations
Driving Forces for a Reaction
•
•
•
•
Formation of a solid
Formation of water
Transfer of electrons
Formation of a gas
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Section 6.3
Balancing Chemical Equations
Synthesis Reaction (Combination)
• two or more simple substances combine to form a new,
more complex substance
• the reactants are generally either 2 elements or 2
compounds
• the product is always a compound
• example: 4Fe(s) + 3O2(g)  2Fe2O3(s)
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Section 6.3
Balancing Chemical Equations
Decomposition Reaction
• a complex substance is broken down into 2 or more
simpler substances
• the products can be any combination of elements and
compounds
• most reactions require energy in the form of heat, light,
or electricity
• EXs:
(NH4)2Cr2O7  Cr2O3 + 4H2O + N2
2NaClO3(s)  2NaCl(s) + 3O2(g)
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Section 6.3
Balancing Chemical Equations
Single-Replacement Reaction
• an uncombined element replaces an element that is part
of a compound
• whether one metal will displace another metal from a
compound can be determined by the relative reactivities
of the 2 metals. The activity series of metals lists metals
in order of decreasing reactivity. A reactive metal will
replace any metal listed below it in the activity series.
• the product will be a different element and a new
compound
• EX: Fe2O3(s) + 2Al(s)  Al2O3(s) + 2Fe(s)
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Section 6.3
Balancing Chemical Equations
Activity Series of Metals - A reactive metal will replace any metal listed below it in the activity series
Decreasing Reactivity
Lithium
Potassium
Barium
Calcium
Sodium
Magnesium
Aluminum
Manganese
Zinc
Chromium
Iron
Cobalt
Nickel
Tin
Lead
Hydrogen
Copper
Silver
Mercury
Platinum
Gold
*metals from lithium to sodium will replace hydrogen from acids and water; from magnesium to lead they will
replace hydrogen from acids only*
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Section 6.3
Balancing Chemical Equations
Double-Replacement Reaction
• involve an exchange of positive ions between 2 reacting
compounds
• generally involve the production of a precipitate and
another compound
• EX: Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
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Section 6.3
Balancing Chemical Equations
Using Solubility Rules
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Section 6.3
Balancing Chemical Equations
Using Solubility Rules
•
Predicting Precipitates
 Soluble solid
 Insoluble solid
 Slightly soluble solid
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Section 6.3
Balancing Chemical Equations
How to Predict Precipitates When Solutions of Two Ionic
Compounds Are Mixed
1. Write the reactants as they actually exist
before any reaction occurs. Remember that
when a salt dissolves, its ions separate.
2. Consider the various solids that could form. To
do this, simply exchange the anions of the
added salts.
3. Use the solubility rules to decide whether a
solid forms and, if so, to predict the identity of
the solid.
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Section 6.3
Balancing Chemical Equations
Combustion Reaction
• a chemical change in which oxygen reacts with another
substance, often producing energy in the form of heat
and light
• products will be carbon dioxide and water or a new
compound
• may also be a synthesis (combination) reaction
• EXs:
C12H22O11(s) + 3O2(g)  9C(s) + 3CO2(g) + H2O(g) + 5635 kJ
2Mg(s) + O2(g)  2MgO(s)
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