Chapter 16 Oxidation and Reduction. Tro - Chapter 162 Oxidation-Reduction Reactions...

transcript

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

• 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

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

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

Fe2+ → Fe3+ + 1 e-

MnO4– + 8H+ + 5 e- → Mn2+ + 4H2O

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+

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

Fe is above Cu, so Cu metalwill not displace Fe2+

displace H2

fromcoldH2O

fromsteam

fromacids

Gold is at thebottom, so it isvery unreactive.

displace H2

fromcoldH2O

fromsteam

fromacids

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+

displace H2

fromcoldH2O

fromsteam

fromacids

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

Chapter 16 Oxidation and Reduction. Tro - Chapter 162 Oxidation-Reduction Reactions...

Documents