1. Chemical Equations Chapter 8
Hein and Arena
Eugene Passer Chemistry Department Bronx Community College
© John Wiley and Sons, Inc
Version 2.0
12th Edition

2. Chapter Outline 8.1 The Chemical Equation
8.4 Types of Chemical Equations
8.2 Writing and Balancing Chemical Equations
8.5 Heat in Chemical Reactions
8.3 Information in a Chemical Equation

3. Chemical equations provide us with the means to
Chemists use chemical equations to describe reactions they observe in the laboratory or in nature.
Chemical equations provide us with the means to
summarize the reaction
display the substances that are reacting
show the products
indicate the amounts of all component substances in a reaction.

4. 8.1 The Chemical Equation

5. Chemical reactions always involve change.
Atoms, molecules or ions rearrange to form different substances.
The substances entering the reaction are called reactants.
The substances formed in the reaction are called products.
During reactions, chemical bonds are broken and new bonds are formed.

6. A chemical equation is a shorthand expression for a chemical change or reaction.
A chemical equation uses the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction.

7. Al + Fe2O3  Fe + Al2O3 Al + Fe2O3  Fe + Al2O3 Chemical Equation
reactants
products
Al + Fe2O3  Fe + Al2O3
Al + Fe2O3  Fe + Al2O3
iron oxygen bonds break
aluminum oxygen bonds form

8. Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting.

9. coefficient 2
Al + Fe2O3  Fe + Al2O3

10. Conditions required to carry out the reaction may be placed above or below the arrow.

11. Al + Fe2O3  Fe + Al2O3
coefficient 2 
 heat

12. The physical state of a substance is indicated by symbols such as (l) for liquid.

13. 2Al(s) + Fe2O3(s)  2Fe(l) + Al2O3 (s) (s)
In a chemical reaction atoms are neither created nor destroyed.
2Al(s) + Fe2O3(s)  2Fe(l) + Al2O3 (s)
(s)
All atoms present in the reactant must also be present in the products.

14. Symbols Used in Chemical Reactions

15. placed between substances
symbol +
meaning
plus
placed between substances
location

16. between reactants and products
symbol 
meaning
yields
between reactants and products
location

17. symbol
(s)
solid
meaning
location
after formula

18. symbol
(l)
liquid
meaning
location
after formula

19. symbol
(g)
gas
meaning
location
after formula

20. symbol
(aq)
aqueous
meaning
after formula
location

21. symbol 
heat
meaning
written above or below 
location

22. symbol h
light energy
meaning
written above 
location

23. symbol 
gas formation
meaning
after formula
location

24. 8.2 Writing and Balancing Equations

25. To balance an equation adjust the number of atoms of each element so that they are the same on each side of the equation.
Never change a correct formula to balance an equation.

26. Steps for Balancing Equations

27. mercury(II) oxide → mercury + oxygen
Step 1 Identify the reaction. Write a description or word equation for the reaction.
Mercury (II) oxide decomposes to form mercury and oxygen.
mercury(II) oxide → mercury + oxygen

28. The formulas of the reactants and products can never be changed.
Step 2 Write the unbalanced (skeleton) equation.
The formulas of the reactants and products must be correct.
The reactants are written to the left of the arrow and the products to the right of the arrow.
HgO  Hg + O2
The formulas of the reactants and products can never be changed.

29. Step 3a Balance the equation.
Count and compare the number of atoms of each element on both sides of the equation.
Determine the elements that require balancing.

30. Element Reactant Side Product Side
Step 3a Balance the equation.
HgO → Hg + O2
Element Reactant Side Product Side Hg
There is one mercury atom on the reactant side and one mercury atom on the product side.
Mercury is balanced.

31. Element Reactant Side Product Side
Step 3a Balance the equation.
HgO  Hg + O2
Element Reactant Side Product Side O
There are two oxygen atoms on the product side and there is one oxygen atom on the reactant side.
Oxygen needs to be balanced.

32. Step 3b Balance the equation.
Balance each element one at a time, by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element.
A coefficient placed before a formula multiplies every atom in the formula by that coefficient.

33. Element Reactant Side Product Side
Step 3b Balance the equation.
HgO  Hg + O2 2
Element Reactant Side Product Side O
Place a 2 in front of HgO to balance O.
There are two oxygen atoms on the reactant side and there are two oxygen atoms on the product side.
Oxygen (O) is balanced.

34. Step 3c Balance the equation.
Check all other elements after each individual element is balanced to see whether, in balancing one element, another element became unbalanced.

35. Element Reactant Side Product Side
Step 3c Balance the equation.
2HgO  Hg + O2
Element Reactant Side Product Side Hg
Count and compare the number of mercury (Hg) atoms on both sides of the equation.
There are two mercury atoms on the reactant side and there is one mercury atom on the product side.
Mercury (Hg) is not balanced.

36. Element Reactant Side Product Side
Step 3c Balance the equation.
2HgO  Hg + O2 2
Element Reactant Side Product Side Hg
Place a 2 in front of Hg to balance mercury.
There are two mercury atoms on the reactant side and there are two mercury atoms on the product side.
Mercury (Hg) is balanced.

37. Element Reactant Side Product Side
 THE EQUATION IS BALANCED
2HgO  2Hg + O2
Element Reactant Side Product Side Hg O

38. sulfuric acid + sodium hydroxide → sodium sulfate + water
Balance the Equation
sulfuric acid + sodium hydroxide → sodium sulfate + water

39. H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)
Balance the Equation
H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) 2
Reactant Side Product Side
1 1
Na 1 2
O 1 1
H 3 2 2 2 4
Place a 2 in front of NaOH to balance Na.
There is one Na on the reactant side and there are two Na on the product side.

40.  THE EQUATION IS BALANCED 
H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) 2 2
Reactant Side Product Side
1 1
Na 2 2
O 2 1
H 4 2 2 4
Place a 2 in front of H2O to balance H.
There are 4 H on the reactant side and two H on the product side.

41. butane + oxygen → carbon dioxide + water
Balance the Equation
butane + oxygen → carbon dioxide + water

42. C4H10 (g) + O2 (g) → CO2(g) + H2O(l)
Balance the Equation
C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 4
Reactant Side Product Side
C 4 1
H 10 2 O 4 9
There are four C on the reactant side and there is one C on the product side.
Place a 4 in front of CO2 to balance C.

43. C4H10 (g) + O2 (g) → CO2(g) + H2O(l)5
Reactant Side Product Side
C 4 4
H 10 2 O 10 13
Place a 5 in front of H2O to balance H.
There are 10 H on the reactant side and there are two H on the product side.

44. C4H10 (g) + O2 (g) → CO2(g) + H2O(l)8
C4H10 (g) + O2 (g) → CO2(g) H2O(l) 4 10 5 5
Reactant Side Product Side
C 4 4
H 10 10 O 20 8 26
There is no whole number coefficient that can be
placed in front of O2 to balance O.
To balance O, double the coefficients of each substance other than oxygen.

45.  THE EQUATION IS BALANCED 
C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 2 8 10 13
Reactant Side Product Side
C 8 8
H 20 20 O 26
Place a 13 in front of O2 to balance O.
There are now 26 O on the product side.

46. 8.3 Information in a Chemical Equation

47. The meaning of a formula is context dependent.
The formula H2O can mean:
2H and 1 O atom
1 molecule of water
1 mol of water
6.022 x 1023 molecules of water
18.02 g of water

48. In an equation, formulas can represent units of individual chemical entities or moles.+
Cl2
2HCl →
1 molecule H2
1 molecule Cl2
2 molecules HCl
1 mol H2
1 mol Cl2
2 mol HCl

49. Formulas
Number of molecules
Number of atoms
Number of moles
Molar masses

50. 8.4 Types of Chemical Equations

51. Combination Decomposition Single-Displacement Double-Displacement

52. Combination Reactions

53. Two reactants combine to form one product.
A + B  AB

54. Examples

55. Metal + Oxygen → Metal Oxide
2Ca(s) + O2(g)  2CaO(s)
4Al(s) + 3O2(g)  2Al2O3(s)

56. Nonmetal + Oxygen → Nonmetal Oxide
S(s) + O2(g)  SO2(g)
N2(g) + O2(g)  2NO(g)

57. 2K(s) + F2(g)  2KF(s) 2Al(s) + 3Cl2(g)  2AlCl3(s)
Metal + Nonmetal → Salt
2K(s) + F2(g)  2KF(s)
2Al(s) + 3Cl2(g)  2AlCl3(s)

58. Na2O(s) + H2O(l)  2NaOH(aq)
Metal Oxide + Water → Metal Hydroxide
Na2O(s) + H2O(l)  2NaOH(aq)
CaO(s) + 2H2O(l)  2Ca(OH)2(aq)

59. Nonmetal Oxide + H2O(l) → Oxy-acid
SO3(g) + H2O(l)  H2SO4(aq)
N2O5(g) + H2O(l)  2HNO3(aq)

60. Decomposition Reactions

61. A single substance breaks down to give two or more different substances.
AB  A + B

62. Examples

63. 2Ag2O(s)  4Ag(s) + O2(g) 2PbO2(s)  2PbO(s) + O2(g)
Metal Oxide → Metal + Oxygen
2Ag2O(s)  4Ag(s) + O2(g)
Metal Oxide → Metal Oxide + Oxygen
2PbO2(s)  2PbO(s) + O2(g)

64. CaCO3(s)  CaO(s) + CO2(g)
Carbonate → CO2(g)
CaCO3(s)  CaO(s) + CO2(g)
Hydrogen Carbonate → CO2(g)
2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g)

65. 2NaNO3(s)  2NaNO2(s) + O2(g)
Miscellaneous Reactions
2H2O2(l)  2H2O(l) + O2(g)
2KClO3(s)  2KCl(s) + 3O2(g)
2NaNO3(s)  2NaNO2(s) + O2(g)

66. Single Displacement Reactions

67. One element reacts with a compound to replace one of the elements of that compound.
A + BC  AC + B

68. Metal + Acid → Hydrogen + Salt
Mg(s) + 2HCl(aq)  H2(g) + MgCl2(aq)
salt
2Al(s) + 3H2SO4(aq)  3H2(g) + Al2(SO4)3(aq)
salt

69. 2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq)
Metal + Water → Hydrogen + Metal Hydroxide
2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq)
metal hydroxide
Ca(s) + 2H2O(l)  H2(g) + Ca(OH)2(aq)
metal hydroxide

70. 3Fe(s) + 4H2O(g)  4H2(g) + Fe3O4(s)
Metal + Water → Hydrogen + Metal Oxide
3Fe(s) + 4H2O(g)  4H2(g) + Fe3O4(s)
metal oxide

71. The Activity Series

72. Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg
An atom of an element in the activity series will displace an atom of an element below it from one of its compounds .
Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg
Sodium (Na) will displace an atom below it from one of its compounds.
increasing activity

73. Examples Metal Activity Series

74. Mg(s) + PbS(s)  MgS(s) + Pb(s)
Metal Higher in Activity Series Displacing Metal Below It
Mg(s) + PbS(s)  MgS(s) + Pb(s)
Metals Mg Al Zn Fe Ni Sn Pb
Magnesium is above lead in the activity series.

75. Ag(s) + CuCl2(s)  no reaction
Metal Lower in Activity Cannot Displace Metal Above It
Ag(s) + CuCl2(s)  no reaction
Metals Pb H Cu Ag Hg
Silver is below copper in the activity series.

76. Example Halogen Activity Series

77. Cl2(g) + CaBr2(s)  CaCl2(aq) + Br2(aq)
Halogen Higher in Activity Series Displaces Halogen Below It
Cl2(g) + CaBr2(s)  CaCl2(aq) + Br2(aq)
Halogens F2 Cl2 Br2 I2
Chlorine is above bromine in the activity series.

78. Double Displacement Reactions

79. AB + CD  AD + CB A displaces C and combines with D
The reaction can be thought of as an exchange of positive and negative groups.
Two compounds exchange partners with each other to produce two different compounds.
B displaces D and combines with C
AB + CD  AD + CB

80. The Following Accompany Double Displacement Reactions
formation of a precipitate
release of gas bubbles
release of heat
formation of water

81. Examples

82. Acid Base Neutralization
acid + base → salt + water
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
H2SO4(aq) + 2NaOH(aq)  Na2SO4(aq) + 2H2O(l)

83. Formation of an Insoluble Precipitate
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)

84. CaO(s) + 2HCl(aq)  CaCl2(s) + H2O(l)
Metal Oxide + Acid
metal oxide + acid → salt + water
CuO(s) + 2HNO3(aq)  Cu(NO3)2(aq) + H2O(l)
CaO(s) + 2HCl(aq)  CaCl2(s) + H2O(l)

85. indirect gas formation
Formation of a Gas
H2SO4(aq) + 2NaCN(aq)  Na2SO4(aq) + 2HCN(g)
NH4Cl(aq) + NaOH(aq)  NaCl(aq) + NH4OH(aq)
indirect gas formation
NH4OH(aq)  NH3(g) + H2O(l)

86. 8.5 Heat in Chemical Reactions

87. Energy changes always accompany chemical reactions.
When this occurs, energy is released to the surroundings.
One reason why reactions occur is that the product attains a lower energy state than the reactants.

88. Exothermic reactions liberate heat.
The amounts of substances are expressed in moles.
Exothermic reactions liberate heat.
H2(g) + Cl2(g) → 2HCl(g) kJ (exothermic)
1 mol
2 mol
Endothermic reactions absorb heat.
N2(g) + O2(g) kJ → 2NO(g) (exothermic)
1 mol
2 mol

89. For life on Earth the sun is the major provider of energy.
The energy for plant photosynthesis is derived from the sun.
6CO2 + 6H2O kJ → C6H12O6 + 6O2
glucose
glucose

90. Energy of Activation

91. A certain amount of energy is always required for a reaction to occur.
The energy required to start a reaction is called the energy of activation.

92. This reaction will not occur unless activation energy is supplied.
CH4 + 2O2 → CO2 + 2H2O kJ
This reaction will not occur unless activation energy is supplied.
The activation energy can take the form of a spark or a flame.

93. 8.2
8.3

94. The End