1. Unit 8 Chemical Reactions
PAGE 123

2. Describing Chemical Reactions
A chemical reaction is the process by which one or more substances are changed into one or more different substances.
They are described by chemical equations

3. Describing Chemical Reactions
In a chemical reaction:
bonds are broken in the reactants
Bonds are formed in the products

4. Parts of a Chemical Equation

5. Reactants
original substances in a chemical reaction
written on the left hand side of a chemical equation

6. Products
The resulting substances produced in a chemical reaction.
written on the right hand side of the chemical equation

7. yields

8. Subscripts in (parenthesis)
represents the physical states of the compounds (elements)
Ex: (s)= solid, (l)=liquid, (g)= gas, (aq)= aqueous (dissolved in water)

10. Words or symbols are placed over/under the arrow(s) to indicate certain conditions under which the reaction is carried out.

11. : heat is applied
catalyst
substance that speeds up a chemical reaction without becoming part of the reaction.

14. Law of conservation of mass
According to the law of
conservation of mass,
the total mass of reactants must equal the total mass of products for any given chemical reaction.

15. Law of conservation of mass
Balance atoms
H2 (g) + O2 (g)  H2O (g) 2 2
Mass must be equal
50 g g 
95g

24. Translate Chemical Equations into Words

25. Chemical Equations
1) include all of the symbols appearing in the equation (with the exception of the coefficients)

26. Chemical Equations
3. Diatomic Elements: Elements that combine with each other when found alone in nature

28. Diatomic Elements
7 diatomic elements (know)
H2 O2 F2 Br2 I2 N2 Cl2

29. Example #2 reacts with Solid magnesium aqueous hydrochloric acid
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
reacts with
Solid magnesium
aqueous hydrochloric acid
aqueous magnesium
to produce
chloride
and hydrogen gas

30. Check for Understanding
CaCO3(s) CaO(s) CO2(g)
Solid calcium carbonate is heated and produces solid calcium oxide and gaseous carbon dioxide

31. Check for Understanding #2
3KOH(aq) + Fe(NO3)3(aq) Fe(OH)3(s) + 3KNO3(aq)
Aqueous potassium hydroxide reacts with aqueous iron (III) nitrate to produce solid iron (III) hydroxide and aqueous potassium nitrate

32. Writing Chemical Equations from a Written Description

33. Example #1
Aluminum metal reacts with oxygen in the air to form solid aluminum oxide. 4
Al (s)
+ O2 (g) 3  2
Al2O3 (s)

34. Example #2
When solid mercury(II) sulfide is heated with oxygen gas, liquid mercury metal and gaseous sulfur dioxide are produced.
HgS (s)
+ O2 (g)
Hg (l)
+ SO2 (g)

35. Check for Understanding
Oxygen gas can be produced by heating solid potassium chlorate in the presence of the catalyst manganese dioxide. Potassium chloride is a solid residue.
MnO2 2
KClO3 (s) 2
KCl (s)
+ O2 (g) 3

36. Check for Understanding
Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed.

37. Check for Understanding
Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed.
Ba(NO3)2 (aq)
+ K2CrO4 (aq)  2
KNO3 (aq)
+ BaCrO4 (s)

38. Types of Chemical Reactions

39. Synthesis (Combination) Reactions
Two or more substances combine to form a new compound. .
Formation of only ONE product.

40. Synthesis (Combination) Reactions

41. Examples:
2H O2 2H2O
CaO H2O Ca(OH)2

42. Decomposition Reactions
A single compound undergoes a reaction that produces two or more simpler substances.

43. Decomposition Reactions

44. Decomposition Reactions
Decomposition reactions are the opposite of synthesis reactions.
There is only one reactant.

45. Examples
CaCO3  CaO CO2
Na2CO3  Na2O + CO2

46. Single-Replacement Reactions
one element replaces a similar element in a compound.
a single element reacts with a compound.

47. Single-Replacement Reactions

48. Distinguishing Characteristics
Examples:
Pb HNO3  Pb(NO3) H2
F NaI  2NaF I2

49. Predict the Products
Zn + CuCl2 
ZnCl2 + Cu

50. Double Replacement Reactions
The ions of two compounds exchange places in an aqueous solution to form two new compounds.
Two ionic compounds “swap” ions.

51. Double Replacement Reactions

52. Usually forms a precipitate
precipitate: when two aqueous solutions are mixed and a solid product that is formed
AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)
Precipitate

53. Examples PbSO4 + 2KCl  K2SO4 + PbCl2
AgNO3(aq) + KCl(aq) ‑‑‑‑> AgCl(s) + KNO3(aq)

54. Predict the Products 2
HOH 2
KOH + H2SO4 
K2SO4 +
H2O

55. Combustion Reactions
A hydrocarbon (CxHy) combines with oxygen, releasing a large amount of energy in the form of light and heat.

56. General Form
CxHy O2  CO H2O

57. Example
C3H O2  3 CO H2O
CH4 + O2  CO2 + H2O

58. Predict the Products 1 2 6 5 2
C2H O2  10 4
CO2 + 12
H2O

59. 1) 2 Li H2O  2 LiOH + H2
single replacement / displacement

60. 2) NH4Cl  NH HCl
decomposition

61. double replacement / displacement
3) AgNO3 + NaCl  AgCl + NaNO3
double replacement / displacement

62. 4) 2 C6H O CO H2O
combustion

63. 5) N H2 2 NH3
combination / synthesis

64. Oxidation – Reduction Processes

65. Redox Reaction
Any chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons.

66. Examples of Redox Reactions
Combustion of gasoline
Burning of wood
Energy from food
Bleaching stains
Iron rusting

67. Oxidation and Reduction
Opposing reactions
Must occur together
Redox (Reduction – Oxidation)

68. Oxidation Electrons are fully or partially lost Mg0 (s)  Mg 2+ + 2e-
Electrons lost:
written as a product

69. Reduction S0(s) + 2 e-  S2- electrons gained: written as a reactant
Electrons are fully or partially gained
S0(s) + 2 e-  S2-
electrons gained:
written as a reactant

70. Mnemonic Device LEO: Loss of Electrons is Oxidation
LEO the lion goes GER
LEO: Loss of Electrons is Oxidation
GER: Gain of Electrons is Reduction

71. Mnemonic Device OIL RIG Oxidation Is Loss of electrons
Reduction Is Gain of electrons

72. Complete Redox Reaction
Add together the reduction half-reaction with the oxidation half-reaction to get the complete redox reaction.

73. Complete Redox Reaction
Mg0 (s)  Mg e- +
S0(s) + 2 e-  S2-
Mg0 (s) + S0(s)  Mg 2+ + S2- Mg + S 
Mg+2 + S -2

74. Assigning Oxidation Numbers

75. Oxidation Numbers
Numbers assigned to all of the elements involved in the reaction to determine if electrons have been transferred between atoms in a reaction.

76. Oxidation Numbers
The oxidation number is usually equal to the charge on the ion if it was formed.

77. Rule #1 Free elements are assigned an oxidation state of 0. Al = 0
Na = 0
H2 = 0

78. Rule #2
The oxidation state for any simple one-atom ion is equal to its charge.
Na+ = +1
Be2+= +2
F- = -1

79. Rule #3
The alkali metals (Li, Na, K, Rb, Cs and Fr) in compounds are always assigned an oxidation state of +1.
LiOH: Li = +1
Na2SO4: Na= +1

80. Rule #4
Fluorine in compounds is always assigned an oxidation state of -1.
HF: F= -1
MgF2: F= -1

81. Rule #5
The alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2. Similarly, Al & Ga are always +3.
MgF2: Mg = +2
CaO: Ca= +2
Al2O3: Al=+3

82. Rule #6 Hydrogen in compounds is assigned an oxidation state of +1.
Exception - Hydrides, ex. LiH (H=-1).
H2SO4: H=+1
HCl: H=+1

83. Rule #7 Oxygen in compounds is assigned an oxidation state of -2.
Exception - Peroxide, ex. H2O2 (O = -1).
H3PO4: O=-2
H2O: O=-2

84. Rule #8
The sum of the oxidation numbers of all atoms in a neutral compound is 0.
H2SO4: add up to 0
CO2: add up to 0

85. Rule #9
The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge on the ion.
SO42-: add up to -2
NH4+: add up to +1

86. Examples
Al(s):
Al = ____
Rule #1

87. Examples CaCl2: Rule #5 Rule #8 Ca+2 1 (+2) + 2 (Cl) = 0 -1 Cl = -1

88. Examples HNO3: Rule # 6 Rule # 7 +1 Rule # 8 +5 -2 N = +5 H= ____

89. Examples SO42-: Rule #7 Rule #9 +6 1(S) + 4(-2) = -2 -2 S= +6 S= ____

90. Examples
H2O:
H= ____
O= ____
Rule #6
Rule #7 +1 -2

91. Examples 1(N)+4(+1)=+1 (NH4)2CO3 N= -3 NH4+ CO32- 1(C)+3(-2)=-2 Rule 6
N=____ C=___
H=____ O=___ +4 +1 -2

92. Check for Understanding
AsO43-:
As= ____
O= ____ +5 -2

93. Check for Understanding
OH-
Cr(OH)3:
Cr= ____
O= ____
H= ____ +3 -2 +1

94. Check for Understanding
(NH4)3PO4:
N= ____
H=_____
P= ____
O= ____
NH4+
PO4-3 -3 +1 +5 -2

95. Oxidized Element Atoms that lose electrons in a chemical reaction.
Elements that lose electrons are undergoing oxidation and are said to be oxidized.

96. Oxidized Element
The substance that loses electrons is the oxidized element.
Atoms that are oxidized will have an increase in their oxidation number

97. Oxidized Element Increase in oxidation number
Mg0 (s) + S0(s)  Mg 2+ + S2-
Mg is oxidized

98. Reduced Element Atoms that gain electrons in a chemical reaction.
Elements that gain electrons are undergoing reduction and are said to be reduced.

99. Reduced Element
The substance that gains electrons is the reduced element.
Atoms that are reduced will have an decrease in their oxidation number

100. Reduced Element Decrease in oxidation number
Mg0 (s) + S0(s)  Mg 2+ + S2-
S is reduced

101. Oxidizing Agent The element or compound that is reduced.
It is called the oxidizing agent because it oxidizes the other element or compound.

102. S is the oxidizing agent
Decrease in oxidation number
Mg0 (s) + S0(s)  Mg 2+ + S2-
S is reduced
S is the oxidizing agent
(it is oxidizing Mg)

103. Reducing Agent The element or compound that is oxidized.
It is called the reducing agent because it reduces the other element or compound.

104. Mg is the reducing agent
Increase in oxidation number
Mg0 (s) + S0(s)  Mg 2+ + S2-
Mg is oxidized
Mg is the reducing agent
(it is reducing S)

105. Examples Are the following redox reactions? If yes:
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent?

106. Examples +2 -2 Mg(s) + S(s)  MgS Mg What element is oxidized?+2 -2
Mg(s) + S(s)  MgS Mg
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent?
Step # 2: Do the oxidation numbers change?
- if no  it is not a redox reaction
- if yes  it is a redox reaction
Step #1: Assign Oxidation Numbers
Yes – It is a redox S S Mg

107. Examples Yes – It is a redox +1 +2+1 +2
2AgNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2Ag(s)
NO3-
NO3-
Step # 2: Do the oxidation numbers change?
- if no  it is not a redox reaction
- if yes  it is a redox reaction
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? Cu Ag
Yes – It is a redox
Step #1: Assign Oxidation Numbers
AgNO3(aq)
Cu(s)

108. Check for Understanding
Are the following redox reactions?
If yes:
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent?

109. Check for Understanding+3 -2
4Fe(s) + 3O2(g)  2Fe2O3(s)
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? Fe
Yes O O2 Fe

110. Check for Understanding+1 -2 +2 -2 +1
Ca(s) + H2O(l)  Ca(OH)2(aq) + H2(g)
OH-
Yes Ca
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? H
H2O Ca

111. Check for Understanding+1 -1 +1 -2 +1 +1 -1 +1 -2
HCl + NaOH  NaCl + H2O
No – it is not a redox

112. Check for Understanding+3 -2 +2 -2 +4 -2
Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g)
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? C
Yes Fe
Fe2O3 CO

113. Check for Understanding+1 +2 +0
2AgNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2Ag (s)
NO3-
NO3-
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? Cu
Yes Ag
AgNO3 Cu

114. Check for Understanding+1 -1 +1 +1 +1 -1
NaCl + AgNO3  NaNO3 + AgCl
NO3-
NO3-
No – it is not a redox

115. Check for Understanding+1 -1
2 H2(g) + O2(g)  2 H2O(l)
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? H
Yes O O2 H2

116. Check for Understanding-4 +1 +4 -2 +1 -2
CH4 + 2 O2  CO H2O
What element is oxidized?
What element is reduced?
What is the oxidizing agent?
What is the reducing agent? C
Yes O O2
CH4