Unit 8 Chemical Reactions

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)

9
(No Transcript)
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.

12
(No Transcript)
13
(No Transcript)
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 45 g ?
95g
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
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

27
(No Transcript)
28
Diatomic Elements

• 7 diatomic elements (know)
• H2 O2 F2 Br2 I2 N2 Cl2

29
Example 2

• 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
Al2O3 (s)
2
34
Example 2

• When solid mercury(II) sulfide is heated with
  oxygen gas, liquid mercury metal and gaseous
  sulfur dioxide are produced.

SO2 (g)
HgS (s)
O2 (g)
Hg (l)
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
O2 (g)
KClO3 (s)
KCl (s)
2
2
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)
?
KNO3 (aq)
BaCrO4 (s)
2

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

• 2H2 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 2HNO3 ? Pb(NO3)2 H2
• F2 2NaI ? 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) ----gt AgCl(s) KNO3(aq)

54
Predict the Products
2
HOH
H2O
KOH H2SO4 ?
K2SO4

2
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 ? CO2 H2O

57
Example

• C3H8 5O2 ? 3 CO2 4 H2O
• CH4 O2 ? CO2 H2O

58
Predict the Products
1
2
6
5
H2O
C2H6 O2 ?
CO2

2
4
12
10
59

• 1) 2 Li 2 H2O ? 2 LiOH H2

single replacement / displacement
60

• 2) NH4Cl ? NH3 HCl

decomposition
61

• 3) AgNO3 NaCl ? AgCl NaNO3

double replacement / displacement
62

• 4) 2 C6H14 19 O2 12 CO2 14 H2O

combustion
63

• 5) N2 3 H2 2 NH3

combination / synthesis
64
Oxidation ReductionProcesses
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

• Electrons are fully or partially gained

S0(s) 2 e- ? S2-
electrons gained written as a reactant
70
Mnemonic Device

• 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 2 2e-

S0(s) 2 e- ? S2-
Mg0 (s) S0(s) ? Mg 2 S2-
Mg

S
?
Mg2

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 Al3

82
Rule 6

• Hydrogen in compounds is assigned an oxidation
  state of 1. 
• Exception - Hydrides, ex. LiH (H-1).
• H2SO4 H1
• HCl H1

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
0
87
Examples

• CaCl2
• Ca ____
• Cl____

Rule 5
Rule 8
Ca2
1 (2) 2 (Cl) 0
-1
Cl -1
88
Examples

• HNO3
• H ____
• N ____
• O ____

Rule 6
Rule 7
1
Rule 8
5
1(1) 1(N) 3(-2) 0
-2
N 5
89
Examples

• SO42-
• S ____
• O ____

Rule 7
Rule 9
6
1(S) 4(-2) -2
-2
S 6
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
Rule 7
C4
Rule 9
-3
4
N____ C___ H____ O___
1
-2
92
Check for Understanding

• S8
• S ____
• AsO43-
• As ____
• O ____

0
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
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
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
0
0
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
0
0
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
0
0
-2
3
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
1
-2
0
0
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
-2
1
1
-1
1
-1
1
-2
HCl NaOH ? NaCl H2O
No it is not a redox
112
Check for Understanding
0
-2
2
-2
3
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
0
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
0
0
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
-2
-2
-4
1
0
4
1
CH4 2 O2 ? CO2 2 H2O
What element is oxidized? What element is
reduced? What is the oxidizing agent? What is the
reducing agent?
C
Yes
O
O2
CH4