Post on 05-Jun-2015
CHAPTER 20CHAPTER 20
““Oxidation-Reduction Reactions”Oxidation-Reduction Reactions”
LEO SAYS GER
Section 20.1Section 20.1The Meaning of Oxidation and The Meaning of Oxidation and
Reduction (called “redox”)Reduction (called “redox”)
OBJECTIVES
Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons.
Section 20.1Section 20.1The Meaning of Oxidation and The Meaning of Oxidation and
Reduction (Redox)Reduction (Redox)
OBJECTIVES
State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.
Section 20.1Section 20.1The Meaning of Oxidation and The Meaning of Oxidation and
Reduction (Redox)Reduction (Redox)
OBJECTIVES
Describe what happens to iron when it corrodes.
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)
Early chemists saw “oxidation” reactions only as the combination of a material with oxygen to produce an oxide.
• For example, when methane burns in air, it oxidizes and forms oxides of carbon and hydrogen, as shown in Fig. 20.1, p. 631
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)
But, not all oxidation processes that use oxygen involve burning:
•Elemental iron slowly oxidizes to compounds such as iron (III) oxide, commonly called “rust”•Bleaching stains in fabrics•Hydrogen peroxide also releases oxygen when it decomposes
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)
A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur
simultaneously The substance gaining oxygen (or
losing electrons) is oxidized, while the substance losing oxygen (or gaining electrons) is reduced.
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) Today, many of these reactions may
not even involve oxygen Redox currently says that electrons
are transferred between reactants
Mg + S → Mg2+ + S2-
•The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg2+
•The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S2-
(MgS)
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)11
2
00
22
ClNaClNa
Each sodium atom loses one electron:
Each chlorine atom gains one electron:
eNaNa10
10 CleCl
LEO says GER :LEO says GER :
eNaNa10
Lose Electrons = Oxidation
Sodium is oxidized
Gain Electrons = Reduction
10 CleCl Chlorine is reduced
LEO says GER :LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent.
Mg(s) + S(s) → MgS(s)
Mg is oxidized: loses e-, becomes a Mg2+ ion
S is reduced: gains e- = S2- ion
Mg is the reducing
agent
S is the oxidizing agent
Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) Conceptual Problem 20.1, page 634
It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds?
In water, we learned that oxygen is highly electronegative, so:
the oxygen gains electrons (is reduced and is the oxidizing agent), and the hydrogen loses electrons (is oxidized and is the reducing agent)
Not All Reactions are Redox ReactionsNot All Reactions are Redox Reactions
- Reactions in which there has been no
change in oxidation number are NOT redox reactions.
Examples:
)()()()( 3
2511111
3
251
aqONNasClAgaqClNaaqONAg
)()()()(22
2
1
4
26
2
1
4
26
2
1121
lOHaqOSNaaqOSHaqHONa
CorrosionCorrosion•Damage done to metal is costly to prevent and repair•Iron, a common construction metal often used in forming steel alloys, corrodes by being oxidized to ions of iron by oxygen.
•This corrosion is even faster in the presence of salts and acids, because these materials make electrically conductive solutions that make electron transfer easy
CorrosionCorrosion•Luckily, not all metals corrode easily
•Gold and platinum are called noble metals because they are resistant to losing their electrons by corrosion•Other metals may lose their electrons easily, but are protected from corrosion by the oxide coating on their surface, such as aluminum – Figure 20.7, page 636•Iron has an oxide coating, but it is not tightly packed, so water and air can penetrate it easily
CorrosionCorrosion•Serious problems can result if bridges, storage tanks, or hulls of ships corrode
•Can be prevented by a coating of oil, paint, plastic, or another metal•If this surface is scratched or worn away, the protection is lost
•Other methods of prevention involve the “sacrifice” of one metal to save the second
•Magnesium, chromium, or even zinc (called galvanized) coatings can be applied
Section 20.2Section 20.2Oxidation NumbersOxidation Numbers
OBJECTIVES
Determine the oxidation number of an atom of any element in a pure substance.
Section 20.2Section 20.2Oxidation NumbersOxidation Numbers
OBJECTIVES
Define oxidation and reduction in terms of a change in oxidation number, and identify atoms being oxidized or reduced in redox reactions.
Assigning Oxidation NumbersAssigning Oxidation Numbers
• An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.
• Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element
Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers
1)The oxidation number of any uncombined element is zero.
2)The oxidation number of a monatomic ion equals its charge.
11
2
00
22
ClNaClNa
Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers
3)The oxidation number of oxygen in compounds is -2, except in peroxides, such as H2O2 where it is -1.
4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.
2
2
1
OH
Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers
5) The sum of the oxidation numbers of the atoms in the compound must equal 0.
2
2
1
OH2(+1) + (-2) = 0 H O
2
122
)(
HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H
Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers
6) The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its ionic charge.
3
2?
ONX + 3(-2) = -1N O
24
2?
OS
thus X = +5 thus X = +6
X + 4(-2) = -2S O
Reducing Agents and Oxidizing AgentsReducing Agents and Oxidizing Agents
• Conceptual Problem 20.2, page 641
• An increase in oxidation number = oxidation
• A decrease in oxidation number = reduction
eNaNa10
10 CleCl
Sodium is oxidized – it is the reducing agent
Chlorine is reduced – it is the oxidizing agent
Trends in Oxidation and ReductionTrends in Oxidation and ReductionActive metals:
Lose electrons easily Are easily oxidized Are strong reducing agents
Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents
Conceptual Problem 20.3, page 643
Section 20.3Section 20.3Balancing Redox EquationsBalancing Redox Equations
OBJECTIVES
Describe how oxidation numbers are used to identify redox reactions.
Section 20.3Section 20.3Balancing Redox EquationsBalancing Redox Equations
OBJECTIVES
Balance a redox equation using the oxidation-number-change method.
Section 20.3Section 20.3Balancing Redox EquationsBalancing Redox Equations
OBJECTIVES
Balance a redox equation by breaking the equation into oxidation and reduction half-reactions, and then using the half-reaction method.
Identifying Redox EquationsIdentifying Redox Equations In general, all chemical reactions can be assigned to one of two classes:
1)oxidation-reduction, in which electrons are transferred:• Single-replacement, combination,
decomposition, and combustion
2) this second class has no electron transfer, and includes all others:
• Double-replacement and acid-base reactions
Identifying Redox EquationsIdentifying Redox Equations In an electrical storm, nitrogen and
oxygen react to form nitrogen monoxide:
N2(g) + O2(g) → 2NO(g)
•Is this a redox reaction?
•If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction; therefore, the reaction as a whole must be a redox.
•Conceptual Problem 20.4, page 647
YES!
Balancing Redox EquationsBalancing Redox Equations It is essential to write a correctly balanced equation that represents what happens in a chemical reaction
• Fortunately, two systematic methods are available, and are based on the fact that the total electrons gained in reduction equals the total lost in oxidation. The two methods:
1)Use oxidation-number changes
2)Use half-reactions
Using half-reactionsUsing half-reactions A half-reaction is an equation showing
just the oxidation or just the reduction that takes place
they are then balanced separately, and finally combined
Step 1: write unbalanced equation in ionic form
Step 2: write separate half-reaction equations for oxidation and reduction
Step 3: balance the atoms in the half-reactions (More steps on the next screen.)
ChoosingChoosing a Balancing Method a Balancing Method
1) The oxidation number change method works well if the oxidized and reduced species appear only once on each side of the equation, and there are no acids or bases.
2) The half-reaction method works best for reactions taking place in acidic or alkaline solution.
Using half-reactionsUsing half-reactions continued
•Step 4: add enough electrons to one side of each half-reaction to balance the charges
•Step 5: multiply each half-reaction by a number to make the electrons equal in both
•Step 6: add the balanced half-reactions to show an overall equation
•Step 7: add the spectator ions and balance the equation
•Rules shown on page 651 – bottom
•Conceptual Problem 20.6, page 652
Electrochemical (Voltaic) Cells
•An apparatus that allows a redox reaction to occur by transferring electrons through an external connector
•Redox reactions that occur spontaneously may be employed to provide a source of electrical energy
•When the two half cells of a redox reaction are connected by an external conductor and a salt bridge that allows the migration of ions, a flow of electrons (electric current) is produced
• In a voltaic cell, a chemical reaction is used to produce a spontaneous electric current by converting chemical energy to electrical energy
Basic Concepts of Electrochemical Cells
Cathode
Cathode - The electrode where reduction occurs
•In an electrochemical (voltaic) cell, the cathode is the POSITIVE electrode
AnodeAnode – the electrode where
oxidation occurs• In an electrochemical
(voltaic) cell, the anode is the NEGATIVE electrode
•The anode is the more active metal (according to table J)
Electrolytic Cells• Sometimes, in combining half reaction,
the potential (Eo) for the overall reaction is negative. In this case, the reaction will not take place spontaneously.
• Redox reactions that do not occur spontaneously can be forced to take place by supplying energy with an externally applied electric current
• The use of an electric current to bring about a chemical reaction is called electrolysis
• In an electrolytic cell, an electric current is used to produce a chemical reaction
Basic Concepts of Electrolytic Cells
ElectrodesCathode – negative electrode
(reduction takes place here)• In electrolytic cells POSITIVE ions
are REDUCED at the cathode
ElectrodesAnode – Positive anode (oxidation
takes place here)• In electrolytic cells NEGATIVE
ions are OXIDIZED at the anode.
**NOTE: The charge of the electrode is the opposite in electrochemical cells**