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Introductory Chemistry , 3 rd Edition Nivaldo Tro. Chapter 16 Oxidation and Reduction. Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA. 2009, Prentice Hall. Oxidation – Reduction Reactions. Oxidation – reduction reactions are also called redox reactions .

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introductory chemistry 3 rd edition nivaldo tro

Introductory Chemistry, 3rd EditionNivaldo Tro

Chapter 16

Oxidation and

Reduction

Roy Kennedy

Massachusetts Bay Community College

Wellesley Hills, MA

2009, Prentice Hall

oxidation reduction reactions
Oxidation–Reduction Reactions
  • Oxidation–reduction reactions are also called redox reactions.
  • All redox reactions involve the transfer of electrons from one atom to another.
  • Spontaneous redox reactions are generally exothermic, and we can use their released energy as a source of energy for other applications.
    • Convert the heat of combustion into mechanical energy to move our cars.
    • Use electrical energy in a car battery to start our car engine.

Tro's Introductory Chemistry, Chapter 16

combustion reactions
Combustion Reactions
  • Combustion reactions are always exothermic.
  • In combustion reactions, O2 combines with all the elements in another reactant to make the products.

4 Fe(s) + 3 O2(g) → 2 Fe2O3(s) + energy

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) + energy

Tro's Introductory Chemistry, Chapter 16

reverse of combustion reactions
Reverse of Combustion Reactions
  • Since combustion reactions are exothermic, their reverse reactions are endothermic.
  • The reverse of a combustion reaction involves the production of O2.

energy + 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g)

energy + CO2(g) + 2 H2O(g) → CH4(g) + 2 O2(g)

  • Reactions in which O2 is gained or lost are redox reactions.

Tro's Introductory Chemistry, Chapter 16

oxidation and reduction one definition
Oxidation and Reduction:One Definition
  • When an element attaches to an oxygen during the course of a reaction it is generally being oxidized.
    • In CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g), C is being oxidized in this reaction, but H is not.
  • When an element loses an attachment to oxygen during the course of a reaction, it is generally being reduced.
    • In 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g), the Fe is being reduced.
  • One definition of redox is the gain or loss of O, but it is not the best.

Tro's Introductory Chemistry, Chapter 16

another oxidation reduction
Another Oxidation–Reduction
  • Consider the following reactions:

4 Na(s) + O2(g) → 2 Na2O(s)

2 Na(s) + Cl2(g) → 2 NaCl(s)

  • The reaction involves a metal reacting with a nonmetal.
  • In addition, both reactions involve the conversion of free elements into ions.

4 Na(s) + O2(g) → 2 Na+2O–(s)

2 Na(s) + Cl2(g) → 2 Na+Cl–(s)

Tro's Introductory Chemistry, Chapter 16

oxidation and reduction another definition

Ger

Leo

Oxidation and Reduction:Another Definition
  • In order to convert a free element into an ion, the atoms must gain or lose electrons.
    • Of course, if one atom loses electrons, another must accept them.
  • Reactions where electrons are transferred from one atom to another are redox reactions.
  • Atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced.

2 Na(s) + Cl2(g) → 2 Na+Cl–(s)

Na → Na+ + 1 e– (oxidation)

Cl2+ 2 e– → 2 Cl– (reduction)

Tro's Introductory Chemistry, Chapter 16

practice identify the element being oxidized and the element being reduced
Practice—Identify the Element Being Oxidized and the Element Being Reduced.
  • 2 C(s) + O2(g) → 2 CO(g)
  • Mg(s) + Cl2(g) → MgCl2(s)
  • Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s)

Tro's Introductory Chemistry, Chapter 16

practice identify the element being oxidized and the element being reduced continued
Practice—Identify the Element Being Oxidized and the Element Being Reduced, Continued.
  • 2 C(s) + O2(g) → 2 CO(g)
  • Mg(s) + Cl2(g) → MgCl2(s)
  • Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s)

C is oxidized because it is gaining an attachment to O.

O is reduced; there has to be reduction and it’s the only other element.

2+

0

0

Mg is oxidized because it is becoming a cation by losing electrons.

Cl is reduced because it is becoming an anion by gaining electrons.

Mg is oxidized because it is becoming a cation by losing electrons.

Fe2+ is reduced because it is gaining electrons to become neutral.

Tro's Introductory Chemistry, Chapter 16

oxidation reduction
Oxidation–Reduction
  • Oxidation and reduction must occur simultaneously.
    • If an atom loses electrons, another atom must take them.
  • The reactant that reduces an element in another reactant is called the reducing agent.
    • The reducing agent contains the element that is oxidized.
  • The reactant that oxidizes an element in another reactant is called the oxidizing agent.
    • The oxidizing agent contains the element that is reduced.

2 Na(s) + Cl2(g) → 2 Na+Cl–(s)

Na is oxidized, Cl is reduced.

Na is the reducing agent, Cl2 is the oxidizing agent.

Tro's Introductory Chemistry, Chapter 16

electron bookkeeping
Electron Bookkeeping
  • For reactions that are not metal + nonmetal, or do not involve O2, we need a method for determining how the electrons are transferred.
  • Chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reaction.
    • Although they look like them, oxidation states are not ion charges!
      • Oxidation states are imaginary charges assigned based on a set of rules.
      • Ion charges are real, measurable charges.

Tro's Introductory Chemistry, Chapter 16

rules for assigning oxidation states continued
Rules for Assigning Oxidation States, Continued
  • In their compounds, nonmetals have oxidation states according to the table below.
    • Nonmetals higher on the table take priority.

Tro's Introductory Chemistry, Chapter 16

practice assign an oxidation state to each element in the following
Practice—Assign an Oxidation State to Each Element in the Following:
  • F2
  • Mg2+
  • KCl
  • SO2
  • PO43−
  • BaO2

Tro's Introductory Chemistry, Chapter 16

practice assign an oxidation state to each element in the following continued
Practice—Assign an Oxidation State to Each Element in the Following, Continued:
  • F2F = 0 (Rule 1)
  • Mg2+ Mg = +2 (Rule 2)
  • KCl K = +1 (Rule 4a) and Cl = -1 (Rule 5)
  • SO2 O = -2 (Rule 5) and S = +4 (Rule 3a)
  • PO43−O = -2 (Rule 5) and P = +5 (Rule 3b)
  • BaOBa = +2 (Rule 4b) and O = -2

Tro's Introductory Chemistry, Chapter 16

oxidation and reduction a better definition

oxidation

reduction

Oxidation and Reduction:A Better Definition
  • Oxidation occurs when an atom’s oxidation state increases during a reaction.
  • Reduction occurs when an atom’s oxidation state decreases during a reaction.

CH4 + 2 O2 → CO2 + 2 H2O

-4 +1 0+4 –2 +1 -2

Tro's Introductory Chemistry, Chapter 16

slide16
Practice—Assign Oxidation States and Identify the Oxidizing and Reducing Agents in Each of the Following:
  • 3 H2S + 2 NO3– + 2 H+® 3S + 2 NO + 4 H2O
  • MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O

Tro's Introductory Chemistry, Chapter 16

slide17

oxidation

reduction

oxidation

reduction

Practice—Assign Oxidation States and Identify the Oxidizing and Reducing Agents in Each of the Following, Continued:

reducing

agent

oxidizing agent

  • 3 H2S + 2 NO3– + 2 H+® 3S + 2 NO + 4 H2O
  • MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O

+1 -2 +5 -2 +1 0 +2 -2 +1 -2

Oxidizing agent

reducing agent

+4 -2 +1 -1 +2 -1 0 +1 -2

Tro's Introductory Chemistry, Chapter 16

tendency to lose electrons
Tendency to Lose Electrons
  • Some metals have a greater tendency to lose electrons than others.
    • Metallic-free elements are always oxidized.
    • The greater the tendency of a metal to lose electrons, the easier it is to oxidize.
    • The greater the tendency of a metal to lose electrons, the harder it is to reduce its cations.
  • If Metal A has a greater tendency to lose electrons than Metal B, then:

A(s) + B+(aq)  A+(aq) + B(s),

but: A+(aq) + B(s)  no reaction.

Tro's Introductory Chemistry, Chapter 16

42

activity series of metals

displace H2

displace H2

displace H2

displace H2

K

Ba

Sr

Ca

Na

Mg

Al

Mn

Zn

Cr

Fe

Cd

Co

Ni

Sn

Pb

H

Sb

As

Bi

Cu

Hg

Ag

Pd

Pt

Au

K

Ba

Sr

Ca

Na

Mg

Al

Mn

Zn

Cr

Fe

Cd

Co

Ni

Sn

Pb

H

Sb

As

Bi

Cu

Hg

Ag

Pd

Pt

Au

K

Ba

Sr

Ca

Na

Mg

Al

Mn

Zn

Cr

Fe

Cd

Co

Ni

Sn

Pb

H

Sb

As

Bi

Cu

Hg

Ag

Pd

Pt

Au

K

Ba

Sr

Ca

Na

Mg

Al

Mn

Zn

Cr

Fe

Cd

Co

Ni

Sn

Pb

H

Sb

As

Bi

Cu

Hg

Ag

Pd

Pt

Au

from

cold

H2O

from

cold

H2O

from

cold

H2O

from

cold

H2O

from

steam

from

steam

from

steam

from

steam

react with O2 in the air to make oxides

react with O2 in the air to make oxides

react with O2 in the air to make oxides

react with O2 in the air to make oxides

Gold is at the

bottom, so it is

very unreactive.

from

acids

from

acids

from

acids

from

acids

Zn + 2 H+® H2 + Zn2+

Fe is above Cu, so Cu metal

will not displace Fe2+

Fe is below Zn, so Zn metal

will displace Fe2+.

Zn is above H,

so Zn will react with acids

Activity Series of Metals
  • Listing of metals by reactivity.
  • Free metal higher on the list displaces metal cation lower on the list.
  • Metals above H will dissolve in acid:

Zn + Fe2+® Fe + Zn2+

Cu + Fe2+® no reaction

slide20

Mg will react with Cu2+ to form Mg2+

and Cu metal.

Cu will not

react with Mg2+.

Mg is above

Cu on the

activity series.

Tro's Introductory Chemistry, Chapter 16

electrical current
Electrical Current
  • When we talk about the current of a liquid in a stream, we are discussing the amount of water that passes by in a given period of time.
  • When we discuss electric current, we are discussing the amount of electric charge that passes a point in a given period of time.
    • Whether as electrons flowing through a wire or ions flowing through a solution.

Tro's Introductory Chemistry, Chapter 16

50

redox reactions and current
Redox Reactions and Current
  • Redox reactions involve the transfer of electrons from one substance to another.
  • Therefore, redox reactions have the potential to generate an electric current.
  • In order to use that current, we need to separate the place where oxidation is occurring from the place that reduction is occurring.

Tro's Introductory Chemistry, Chapter 16

51

electric current flowing indirectly between atoms
Electric Current Flowing Indirectly Between Atoms

Tro's Introductory Chemistry, Chapter 16

53

electrochemical cells
Electrochemical Cells
  • Electrochemistry is the study of redox reactions that produce or require an electric current.
  • The conversion between chemical energy and electrical energy is carried out in an electrochemical cell.
  • Spontaneous redox reactions take place in a voltaic cell.
    • Also known as galvanic cells.
    • Batteries are voltaic cells.
  • Nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy.

Tro's Introductory Chemistry, Chapter 16

electrochemical cells continued
Electrochemical Cells, Continued
  • Oxidation and reduction reactions kept separate.
    • Half-cells.
  • Electron flow through a wire, along with ion flow through a solution, constitutes an electric circuit.
  • Requires a conductive solid (metal or graphite) electrode to allow the transfer of electrons.
    • Through external circuit.
  • Ion exchange between the two halves of the system.
    • Electrolyte.

Tro's Introductory Chemistry, Chapter 16

electrodes
Electrodes
  • Anode
    • Electrode where oxidation occurs.
    • Anions attracted to it.
    • Connected to positive end of battery in electrolytic cell.
    • Loses weight in electrolytic cell.
  • Cathode
    • Electrode where reduction occurs.
    • Cations attracted to it.
    • Connected to negative end of battery in electrolytic cell.
    • Gains weight in electrolytic cell.
      • Electrode where plating takes place in electroplating.

Tro's Introductory Chemistry, Chapter 16

current and voltage
Current and Voltage
  • The number of electrons that flow through the system per second is the current.
    • Electrode surface area dictates the number of electrons that can flow.
  • The amount of force pushing the electrons through the wire is the voltage.
    • The farther the metals are separated on the activity series, the larger the voltage will be.

Tro's Introductory Chemistry, Chapter 16

current
Current

The number of

electrons that pass

a point each second

is called the current

of the electricity.

The amount of

water that passes

a point each second

is called the current

of the river.

Tro's Introductory Chemistry, Chapter 16

voltage
Voltage

Voltage is the

force pushing

the electrons

down the wire.

Gravity is the

force pulling

the water down

the river.

Tro's Introductory Chemistry, Chapter 16

leclanch s acidic dry cell
LeClanché’s Acidic Dry Cell
  • Electrolyte in paste form.
    • ZnCl2 + NH4Cl.
      • Or MgBr2.
  • Anode = Zn (or Mg).
    • Zn(s) ®Zn2+(aq) + 2 e-
  • Cathode = graphite rod.
  • MnO2 is reduced.

2 MnO2(s) + 2 NH4+(aq) + 2 H2O(l) + 2 e- ® 2 NH4OH(aq) + 2 Mn(O)OH(s)

  • Cell voltage = 1.5 v.
  • Expensive, nonrechargeable, heavy, easily corroded.

Tro's Introductory Chemistry, Chapter 16

alkaline dry cell
Alkaline Dry Cell
  • Same basic cell as acidic dry cell, except electrolyte is alkaline KOH paste.
  • Anode = Zn (or Mg).

Zn(s) ®Zn2+(aq) + 2 e-

  • Cathode = brass rod.
  • MnO2 is reduced.

2 MnO2(s) + 2 NH4+(aq) + 2 H2O(l) + 2 e- ® 2 NH4OH(aq) + 2 Mn(O)OH(s)

  • Cell voltage = 1.54 v.
  • Longer shelf life than acidic dry cells and rechargeable; little corrosion of zinc.

Tro's Introductory Chemistry, Chapter 16

lead storage battery
Lead Storage Battery
  • Six cells in series.
  • Electrolyte = 6 M H2SO4.
  • Anode = Pb.

Pb(s) + SO42-(aq) ®PbSO4(s) + 2 e-

  • Cathode = Pb coated with PbO2.
  • PbO2 is reduced.

PbO2(s) + 4 H+(aq) + SO42-(aq) + 2 e- ® PbSO4(s) + 2 H2O(l)

  • Cell voltage = 2.09 v.
  • Rechargeable, heavy.

Tro's Introductory Chemistry, Chapter 16

fuel cells
Fuel Cells
  • Like batteries in which reactants are constantly being added.
    • So it never runs down!
  • Anode and cathode both Pt-coated metal.
  • Electrolyte is OH– solution.
  • Anode reaction: 2 H2 + 4 OH– → 4 H2O(l) + 4 e-.
  • Cathode reaction: O2 + 4 H2O + 4 e- → 4 OH–.
electrolysis
Electrolysis
  • Electrolysis is the process of using electricity to break a compound apart.
  • Electrolysis is done in an electrolytic cell.
  • Electrolytic cells can be used to separate elements from their compounds.
    • Generate H2 from water for fuel cells.
    • Recover metals from their ores.

Tro's Introductory Chemistry, Chapter 16

electrolytic cell
Electrolytic Cell
  • The + terminal of the battery = anode.
  • The - terminal of the battery = cathode.
  • Cations attracted to the cathode; anions attracted to the anode.
  • Cations pick up electrons from the cathode and are reduced; anions release electrons to the anode and are oxidized.
  • In electroplating, the work piece is the cathode.
    • Cations are reduced at the cathode and plate onto the surface.
    • The anode is made of the plate metal, the anode oxidizes and replaces the metal cations lost from the solution.

Tro's Introductory Chemistry, Chapter 16

electrolytic cell electroplating
Electrolytic Cell—Electroplating

Tro's Introductory Chemistry, Chapter 16

corrosion
Corrosion

Corrosion is the spontaneous oxidation of a metal by chemicals in the environment.

Since many materials we use are active metals, corrosion can be a very big problem.

Tro's Introductory Chemistry, Chapter 16

70

preventing corrosion
Preventing Corrosion

One way to reduce or slow corrosion is to coat the metal surface to keep it from contacting corrosive chemicals in the environment.

Paint.

Some metals, like Al, form an oxide that strongly attaches to the metal surface, preventing the rest from corroding.

Another method to protect one metal is to attach it to a more reactive metal that is cheap.

Sacrificial electrode.

Tro's Introductory Chemistry, Chapter 16

71

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