Chapter 16 Oxidation and Reduction. Tro - Chapter 162 Oxidation-Reduction Reactions...
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Transcript of Chapter 16 Oxidation and Reduction. Tro - Chapter 162 Oxidation-Reduction Reactions...
Chapter 16Oxidation
andReduction
Tro - Chapter 16 2
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 applicationsconvert the heat of combustion into mechanical energy to
move our carsuse electrical energy in a car battery to start our car engine
Tro - Chapter 16 3
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 - Chapter 16 4
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 - Chapter 16 5
Oxidation and ReductionOne 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 - Chapter 16 6
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 - Chapter 16 7
Oxidation and ReductionAnother Definition• in order to convert a free element into an ion, the
atoms must gain or lose electronsof 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– oxidationCl2 + 2 e– → 2 Cl– reduction
Leo
Ger
Tro - Chapter 16 8
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 - Chapter 16 9
Practice – Identify the Element being Oxidized and Element being Reduced;
and the Oxidizing and Reducing Agents
• 2 Mg(s) + O2(g) → 2 MgO(s)
• Fe(s) + Cl2(g) → FeCl2(s)
• Zn(s) + Fe2+(aq) → Zn2+(aq) + Fe(s)
Tro - Chapter 16 10
Practice – Identify the Element being Oxidized and Element being Reduced;
and the Oxidizing and Reducing Agents
• 2 Mg(s) + O2(g) → 2 MgO(s)
• Fe(s) + Cl2(g) → FeCl2(s)
• Zn(s) + Fe2+(aq) → Zn2+(aq) + Fe(s)
Mg is oxidized, O is reducedMg is the reducing agent, O2 is the oxidizing agent
Fe is oxidized, Cl is reducedFe is the reducing agent, Cl2 is the oxidizing agent
Zn is oxidized, Fe is reducedZn is the reducing agent, Fe2+ is the oxidizing agent
Tro - Chapter 16 11
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 reactioneven though 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 - Chapter 16 12
Rules for Assigning Oxidation States
• rules are in order of priority1. free elements have an oxidation state = 0
Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)
2. monatomic ions have an oxidation state equal to their charge
Na = +1 and Cl = -1 in NaCl
3. (a) the sum of the oxidation states of all the atoms in a compound is 0
Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0
Tro - Chapter 16 13
Rules for Assigning Oxidation States3. (b) the sum of the oxidation states of all the atoms in
a polyatomic ion equals the charge on the ion N = +5 and O = -2 in NO3
–, (+5) + 3(-2) = -1
4. (a) Group I metals have an oxidation state of +1 in all their compounds
Na = +1 in NaCl
4. (b) Group II metals have an oxidation state of +2 in all their compounds
Mg = +2 in MgCl2
Tro - Chapter 16 14
Rules for Assigning Oxidation States5. in their compounds, nonmetals have oxidation
states according to the table below nonmetals higher on the table take priority
Nonmetal Oxidation State Example
F -1 CF4
H +1 CH4
O -2 CO2
Group 7A -1 CCl4
Group 6A -2 CS2
Group 5A -3 NH3
Tro - Chapter 16 15
Practice – Assign an Oxidation State to each Element in the following
• Br2
• K+
• LiF
• CO2
• SO42-
• Na2O2
Tro - Chapter 16 16
Practice – Assign an Oxidation State to each Element in the following
• Br2 Br = 0, (Rule 1)
• K+ K = +1, (Rule 2)
• LiF Li = +1, (Rule 4a) & F = -1, (Rule 5)
• CO2 O = -2, (Rule 5) & C = +4, (Rule 3a)
• SO42- O = -2, (Rule 5) & S = +6, (Rule 3b)
• Na2O2 Na = +1, (Rule 4a) & O = -1, (Rule 3a)
Tro - Chapter 16 17
Oxidation and ReductionA 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
oxidationreduction
Tro - Chapter 16 18
Identify the Oxidizing and Reducing Agents in Each of the Following
3 H2S + 2 NO3– + 2 H+ S + 2 NO + 4 H2O
MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O
Tro - Chapter 16 19
Identify the Oxidizing and Reducing Agents in Each of the Following
3 H2S + 2 NO3– + 2 H+ S + 2 NO + 4 H2O
MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O
+1 -2 +5 -2 +1 0 +2 -2 +1 -2
ox agred ag
+4 -2 +1 -1 +2 -1 0 +1 -2
oxidationreduction
oxidation
reduction
red agox ag
Tro - Chapter 16 20
Balancing Redox Reactions1) assign oxidation states and
determine element oxidized and element reduced
2) separate into oxidation & reduction half-reactions
3) balance half-reactions by massa) first balance atoms other than O and
Hb) then balance O by adding H2O to
side that lacks Oc) finally balance H by adding H+ to
side that lacks H
Fe2+ + MnO4– → Fe3+ + Mn2+
+2 +7 -2 +3 +2oxid
red
Fe2+ → Fe3+
MnO4– → Mn2+
Fe2+ → Fe3+
MnO4– → Mn2+
MnO4– → Mn2+ + 4H2O
MnO4– + 8H+ → Mn2+ + 4H2O
Tro - Chapter 16 21
Balancing Redox Reactions4) balance each half-reaction
with respect to charge by adjusting the numbers of electrons
a) electrons on product side for oxid.b) electrons on reactant side for red.
5) balance electrons between half-reactions
6) add half-reactions, canceling electrons and common species
7) Check
Fe2+ → Fe3+ + 1 e-
MnO4– + 8H+ + 5 e- → Mn2+ + 4H2O
MnO4– + 8H+ → Mn2+ + 4H2O
+7 +2
Fe2+ → Fe3+ + 1 e-
MnO4– + 8H+ + 5 e- → Mn2+ + 4H2O
} x 5
5 Fe2+ → 5 Fe3+ + 5 e-
MnO4– + 8H+ + 5 e- → Mn2+ + 4H2O
5 Fe2+ + MnO4– + 8H+ → Mn2+ + 4H2O + 5 Fe3+
Tro - Chapter 16 22
Practice – Balance the Following EquationCu+ + I2 → Cu2+ + I–
Tro - Chapter 16 23
Practice – Balance the Following EquationCu+ + I2 → Cu2+ + I– +1 0 +2 -1
oxidred
ox: Cu+ → Cu2+ red: I2 → I–
ox: Cu+ → Cu2+ red: I2 → 2 I–
ox: Cu+ → Cu2+ + 1 e- red: I2 + 2 e- → 2 I–
ox: Cu+ → Cu2+ + 1 e- } x 2 red: I2 + 2 e- → 2 I–
2 Cu+ + I2 → 2 Cu2+ + I2
Tro - Chapter 16 24
Practice – Balance the Following Equation I– + Cr2O7
2- → Cr3+ + I2
Tro - Chapter 16 25
Practice – Balance the Following Equation I– + Cr2O7
2- → Cr3+ + I2+6 0+3-1
oxidred
ox: I– → I2 red: Cr2O72– → Cr3+
-2
ox: 2 I– → I2 red: Cr2O72– → 2 Cr3+
red: Cr2O72– → 2 Cr3+ + 7 H2O
red: Cr2O72– + 14H+→ 2Cr3+ + 7H2O
ox: 2 I– → I2 + 2e- red: Cr2O72– + 14H+ + 6e- → 2Cr3+ + 7H2O
ox: 2 I– → I2 + 2e-}x3 red: Cr2O72– + 14H+ + 6e- → 2Cr3+ + 7H2O
Cr2O72– + 14 H+ + 6 I– → 2 Cr3+ + 7 H2O + 3 I2
Tro - Chapter 16 26
Will a Reaction Take Place?
• reactions that are energetically favorable are said to be spontaneousthey can happen, but the activation energy may
be so large that the rate is very slow
• the relative reactivity of metals can be used to determine if some redox reactions are spontaneous
Tro - Chapter 16 27
Single Displacement Reactions• also known as single replacement reactions• a more active free element displaces a less
active element in a compoundmetals displace metals or H
Cu + 2 AgNO3 Cu(NO3)2 + 2 Ag
Mg + 2 HCl MgCl2 + H2
nonmetals displace nonmetals
2 KI + Br2 2 KBr + I2
carbon displaces metals from oxides
3 C + Fe2O3 3 CO + 2 Fe
• always redox
Tro - Chapter 16 28
KBaSrCaNaMgAlMnZnCrFeCdCoNiSnPbHSbAsBiCuHgAgPdPtAu
displace H2
fromcoldH2O
fromsteam
fromacids
reac
t wit
h O
2 in
the
air
to m
ake
oxid
es
Fe is above Cu, so Cu metalwill not displace Fe2+
KBaSrCaNaMgAlMnZnCrFeCdCoNiSnPbHSbAsBiCuHgAgPdPtAu
displace H2
fromcoldH2O
fromsteam
fromacids
reac
t wit
h O
2 in
the
air
to m
ake
oxid
es
Gold is at thebottom, so it isvery unreactive.
KBaSrCaNaMgAlMnZnCrFeCdCoNiSnPbHSbAsBiCuHgAgPdPtAu
displace H2
fromcoldH2O
fromsteam
fromacids
reac
t wit
h O
2 in
the
air
to m
ake
oxid
es
Zn is above H,so Zn will react with acids
Zn + Fe2+ Fe + Zn2+
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
Cu + Fe2+ no reactionZn + 2 H+ H2 + Zn2+
KBaSrCaNaMgAlMnZnCrFeCdCoNiSnPbHSbAsBiCuHgAgPdPtAu
displace H2
fromcoldH2O
fromsteam
fromacids
reac
t wit
h O
2 in
the
air
to m
ake
oxid
es
Fe is below Zn, so Zn metalwill displace Fe2+
Tro - Chapter 16 29
Mg is aboveCu on theActivity SeriesMg will react with
Cu2+ to form Mg2+
and Cu metal
but Cu will not react with Mg2+
Tro - Chapter 16 30
Predict the Products & Balance the Equation• Mg + H3PO4
• Cu + H2SO4
• Al + Fe2+
Tro - Chapter 16 31
• 3 Mg + 2 H3PO4 Mg3(PO4)2 + 3 H2
• Cu + H2SO4 no reaction
• 2 Al + 3 Fe2+ Al3+ + 3 Fe
Predict the Products & Balance the Equation
Tro - Chapter 16 32
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 cellsbatteries are voltaic cells
• nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy
Tro - Chapter 16 33
Electrochemical Cells• 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 systemelectrolyte
Tro - Chapter 16 34
Electrodes• Anode
electrode where oxidation occursanions attracted to itconnected to positive end of battery in electrolytic
cellloses weight in electrolytic cell
• Cathodeelectrode where reduction occurscations attracted to itconnected to negative end of battery in electrolytic
cellgains weight in electrolytic cell
electrode where plating takes place in electroplating
Tro - Chapter 16 35
Voltaic Cell
Tro - Chapter 16 36
Current and Voltage
• the number of electrons that flow through the system per second is the currentElectrode surface area dictates the number of
electrons that can flow
• the amount of force pushing the electrons through the wire is the voltagethe farther the metals are separated on the
activity series, the larger the voltage will be
Tro - Chapter 16 37
Current
The amount of water that passesa point each secondis called the currentof the river.
The number of electrons that passa point each secondis called the currentof the electricity.
Tro - Chapter 16 38
Voltage
Gravity is the force pullingthe water downthe river.
Voltage is the force pushingthe electrons down the wire.
Tro - Chapter 16 39
Dead Battery
As the reactionproceeds, thereactants getconsumed andthe voltaic cell“dies”. The current decreasesuntil electronscan no longerflow throughthe wire.
Tro - Chapter 16 40
LeClanche’ Acidic Dry Cell• electrolyte in paste form
ZnCl2 + NH4Clor 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 - Chapter 16 41
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 reduced2 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 - Chapter 16 42
Lead Storage Battery• 6 cells in series
• electrolyte = 6 M H2SO4
• anode = PbPb(s) + SO4
2-(aq) PbSO4(s) + 2 e-
• cathode = Pb coated with PbO2
• PbO2 is reducedPbO2(s) + 4 H+(aq) + SO4
2-(aq) + 2 e- PbSO4(s) + 2 H2O(l)
• cell voltage = 2.09 v
• rechargeable, heavy
Tro - Chapter 16 43
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–
Tro - Chapter 16 44
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 - Chapter 16 45
Preventing Corrosion• one way to reduce or slow corrosion is to coat
the metal surface to keep it from contacting corrosive chemicals in the environmentpaintsome 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 cheapsacrificial electrode
Tro - Chapter 16 46
Nonspontaneous Redox Reaction
• the reverse of a spontaneous reaction is nonspontaneous
• to get it to run, an outside energy source must be supplied
• nonspontaneous redox reactions can be made to work by using a battery to force the electrons to flow in the nonspontaneous direction
Tro - Chapter 16 47
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 compoundsgenerate H2 from water for fuel cells recover metals from their ores
Tro - Chapter 16 48
Electrolytic Cell• the + terminal of the battery = anode• the - terminal of the battery = cathode• cations attracted to the cathode, anions 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 workpiece is the cathodecations are reduced at cation and plate to the surface the anode is made of the plate metal, the anode oxidizes and
replaces the metal cations in the solution
Tro - Chapter 16 49
Electrolytic Cell - Electroplating