Principles of Chemistry

1
Principles of Chemistry
CHAPTER 6
BALANCING CHEMICAL EQUATIONS
2
Evidence of Chemical Reactions

• Chemical Reactions often give a visual signal
• color changes
• solid forms
• bubbles form
• heat and or flame is produced, or heat is absorbed

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3
Chemical Equations

• Chemical change involves a rearrangement in the
  ways in which atoms are grouped
• Chemical equation a representation of a chemical
  reaction
• reactants Chemical present before the reaction
• products Chemicals formed by the reaction

reactants ? products
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4
Reaction of Methane
CH4 O2 ? CO2 H2O

• Products should contain the same number of atoms
  as the reactants
• C - 1,1
• H - 4,2
• O - 2,3
• The equation is out of balance and the chemistry
  cant occur that way

H
H
H
H
H
H
C
C
O-
O-
O-
O-
O-
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5
Reaction of Methane
CH4 O2 O2 ? CO2 H2O

• We can correct the balance by adding an
  additional O2 molecule
• C - 1,1
• H - 4,2
• O - 4,3
• Notice the equation is still out of balance

H
H
H
H
H
H
O-
O-
C
C
O-
O-
O-
O-
O-
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6
Reaction of Methane
CH4 O2 O2 ? CO2 H2O H2O

• We can correct the balance by adding an
  additional H2O molecule
• C - 1,1
• H - 4,4
• O - 4,4
• CH4 2O2? CO2 2H2O

C
C
H
H
H
H
H
H
H
H
O-
O-
O-
O-
O-
O-
O-
O-
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7
Physical State

• The physical states of the reactants and products
  are identified using these symbols
• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous, dissolved in water

• CH4 (g) 2O2 (g)? CO2 (g) 2H2O (l)

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8
Balancing Chemical Equations

• Atoms are conserved in a chemical reaction
• An unbalanced chemical equation is not an
  accurate representation of the reaction that
  occurs
• The identities (formulas) of the compounds must
  never be changed when balancing a chemical
  equation
• Most chemical equations can be balanced by trial
  and error

H2 O2 ? H2O
wrong way H2 O2 ? H2O2
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9
How to Write and Balance Equations

• Read the description of the chemical reaction.
  Identify the products, reactants and their
  physical states
• Write the unbalanced equation that summarizes the
  information from step 1
• Balance the equation by inspection, starting with
  the most complicated molecule. Proceed element by
  element to determine what coefficients are
  necessary so that the same number of each type of
  atom appears on both sides of the equation
• Ensure that the coefficients used are the
  smallest integers that give the balanced equation

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10
Balancing Chemical Equations I

• Write a balanced chemical equation for the
  reaction of solid iron(III) oxide and nitric acid
  to form aqueous iron(III) nitrate and water

H
H
Fe2O3 HNO3 ? Fe(NO3)3 H2O
Fe3
Fe3
H
Fe3
N
O-
O-
O-
N
O-
O-
O-
N
O-
O-
O-
N
O-
O-
O-
O-
O-
O-
O-
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11
Balancing Chemical Equations I
Fe - 2, 2 O - 6, 19 H - 1, 2 N - 1, 6
Fe2O3 HNO3 ? 2Fe(NO3)3 H2O
Fe - 2, 2 O - 21, 19 H - 6, 2 N - 6, 6
Fe2O3 6HNO3 ? 2Fe(NO3)3 H2O
Fe - 2, 2 O - 21, 21 H - 6, 6 N - 6, 6
!!!
Fe2O3 6HNO3 ? 2Fe(NO3)3 3H2O
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12
References

• Zumdahl, Steven. DeCoste, Donald. Introductory
  Chemistry A foundation. Sixth Edition. Houghton
  Mifflin Company. ISBN 0-618-80327.
• Wikipedia (some graphics)

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Creative Commons Attribution 3.0 License