Oxidation and ReductionOxidation and ReductionChapter 20Chapter 20

Types of Chemical Types of Chemical ReactionsReactions

Type I: ions or molecules react with no apparent Type I: ions or molecules react with no apparent change in the electronic structure of the particles. change in the electronic structure of the particles.

Type II: ions or atoms undergo changes of electronic Type II: ions or atoms undergo changes of electronic structure. Electrons may be transferred from one structure. Electrons may be transferred from one particle to another. On the other hand, the sharing particle to another. On the other hand, the sharing of the electrons may be somewhat changed. of the electrons may be somewhat changed.

Type II reactions involving electron changes are Type II reactions involving electron changes are called oxidation-reduction reactions. called oxidation-reduction reactions.

It is these "redox" reactions which we will now It is these "redox" reactions which we will now discuss. discuss.

Before we indicate what oxidation-reduction Before we indicate what oxidation-reduction reactions are, we will briefly indicate what they are reactions are, we will briefly indicate what they are not.not.

What Redox is NOTWhat Redox is NOT

In the BaSOIn the BaSO44 reaction in Table 26-1, the substances are all ionic. reaction in Table 26-1, the substances are all ionic. Since there is no change in the charge of these ions in the reaction, Since there is no change in the charge of these ions in the reaction,

there are no electron changes. there are no electron changes. This reaction is not an oxidation-reduction reaction.This reaction is not an oxidation-reduction reaction. The production of a (BaS0The production of a (BaS044) is nearly always a result of a non-redox ) is nearly always a result of a non-redox

reaction. reaction. Most acid-base reactions are also the non-redox type.Most acid-base reactions are also the non-redox type. Since nearly every other kind of reaction is an oxidation-reduction Since nearly every other kind of reaction is an oxidation-reduction

reaction, redox reactions are important in the laboratory. reaction, redox reactions are important in the laboratory. They are also important in life processes and in industry.They are also important in life processes and in industry.

OxidationOxidation The term oxidation was first applied to the combining of The term oxidation was first applied to the combining of

oxygen with other elements. oxygen with other elements. There were many known instances of this behavior:There were many known instances of this behavior:

– Iron rusts Iron rusts – Carbon burns Carbon burns

In rusting, oxygen combines slowly with iron to form FeIn rusting, oxygen combines slowly with iron to form Fe22OO33. . In burning, oxygen unites rapidly with carbon to form COIn burning, oxygen unites rapidly with carbon to form CO22. . Observation of these reactions gave rise to the terms "slow" Observation of these reactions gave rise to the terms "slow"

and "rapid" oxidation.and "rapid" oxidation. Chemists recognize, however, that other nonmetallic Chemists recognize, however, that other nonmetallic

elements unite with substances in a manner similar to that of elements unite with substances in a manner similar to that of oxygen. oxygen. – Hydrogen, antimony, and sodium all burn in chlorine, and iron will Hydrogen, antimony, and sodium all burn in chlorine, and iron will

burn in fluorine. burn in fluorine. Since these reactions were similar, chemists formed a more Since these reactions were similar, chemists formed a more

general definition of oxidation:general definition of oxidation:– Electrons were removed from each free element by the reactants Electrons were removed from each free element by the reactants

OO22 or Cl or Cl22. . Thus oxidation is defined as the process by which Thus oxidation is defined as the process by which

electrons are apparently removed from an atom or ion.electrons are apparently removed from an atom or ion.

ReductionReduction A reduction reaction was originally limited to the A reduction reaction was originally limited to the

type of reaction in which ores were "reduced" from type of reaction in which ores were "reduced" from their oxides. their oxides. – Iron oxide was "reduced" to iron by carbon monoxide. Iron oxide was "reduced" to iron by carbon monoxide. – Copper(II) oxide could be "reduced" to copper by hydrogen. Copper(II) oxide could be "reduced" to copper by hydrogen.

In these reactions, oxygen is removed, and the free In these reactions, oxygen is removed, and the free element is produced. element is produced.

The free element can be produced in other ways:The free element can be produced in other ways:– An iron nail dropped into a copper(II) sulfate solution causes An iron nail dropped into a copper(II) sulfate solution causes

a reaction which produces free copper. a reaction which produces free copper. – An electric current passing through molten sodium chloride An electric current passing through molten sodium chloride

produces free sodium. produces free sodium. The similarity between oxidation and reduction The similarity between oxidation and reduction

reactions led chemists to formulate a more reactions led chemists to formulate a more generalized definition of reduction. generalized definition of reduction.

By definition, reduction is the process by By definition, reduction is the process by which electrons are apparently added to which electrons are apparently added to atoms or ions.atoms or ions.

OIL RIG—the Texas DefinitionOIL RIG—the Texas Definition

OOxidation xidation iis s LLoss (of electrons), oss (of electrons), RReduction eduction iis s GGain (of electrons)ain (of electrons)

Oxidizing and Reducing Oxidizing and Reducing AgentsAgents

In an oxidation-reduction reaction, electrons are transferred.In an oxidation-reduction reaction, electrons are transferred. – All the electrons exchanged in an oxidation-reduction reaction must be All the electrons exchanged in an oxidation-reduction reaction must be

accounted for. accounted for. It seems reasonable, therefore, that It seems reasonable, therefore, that both oxidation and reduction must both oxidation and reduction must

occur at the same time in a reaction. occur at the same time in a reaction. Electrons are lost and gained at the same time and the number Electrons are lost and gained at the same time and the number

lost must equal the number gained.lost must equal the number gained.

The substance in the reaction which gives up electrons is called The substance in the reaction which gives up electrons is called the reducing agent. The reducing agent contains the atoms which the reducing agent. The reducing agent contains the atoms which are oxidized (the atoms which lose electrons)are oxidized (the atoms which lose electrons). . – Zinc is a good example of a reducing agent. It is oxidized to the zinc ion, ZnZinc is a good example of a reducing agent. It is oxidized to the zinc ion, Zn2+2+

The substance in the reaction which gains electrons is called the The substance in the reaction which gains electrons is called the oxidizing agent. It contains the atoms which are reduced (the oxidizing agent. It contains the atoms which are reduced (the atoms which gain electrons).atoms which gain electrons). – Dichromate ion, CrDichromate ion, Cr220077

2-2-, is a good example of an oxidizing agent. It is reduced to , is a good example of an oxidizing agent. It is reduced to the chromium ion, Crthe chromium ion, Cr3+3+

If a substance gives up electrons readily, it is said to be a strong reducing If a substance gives up electrons readily, it is said to be a strong reducing agent. Its oxidized form, however, is normally a poor oxidizing agent. agent. Its oxidized form, however, is normally a poor oxidizing agent.

If a substance gains electrons readily, it is said to be a strong oxidizing If a substance gains electrons readily, it is said to be a strong oxidizing agent. Its reduced form is a weak reducing agent.agent. Its reduced form is a weak reducing agent.

Redox of nails and copperRedox of nails and copper

Oxidation NumbersOxidation Numbers How is it possible to determine whether an oxidation-reduction How is it possible to determine whether an oxidation-reduction

reaction has taken place? reaction has taken place? We do so by determining whether any electron shifts have taken We do so by determining whether any electron shifts have taken

place during the reaction. place during the reaction. To indicate electron changes, we look at the oxidation numbers of To indicate electron changes, we look at the oxidation numbers of

the atoms in the reaction. the atoms in the reaction. The oxidation number is the charge an atom appears to The oxidation number is the charge an atom appears to

have when we assign a certain number of electrons to have when we assign a certain number of electrons to given atoms or ions.given atoms or ions.

Any change of oxidation numbers in the course of a Any change of oxidation numbers in the course of a reaction indicates an oxidation-reduction reaction has reaction indicates an oxidation-reduction reaction has taken place.taken place.

Oxidation numbers are assigned according to the apparent Oxidation numbers are assigned according to the apparent charge of the element (aka, valence!)charge of the element (aka, valence!)

For example, suppose iron, as a reactant in a reaction, has an For example, suppose iron, as a reactant in a reaction, has an oxidation number of 2+. oxidation number of 2+. – If iron appears as a product with an oxidation number other than 2+, If iron appears as a product with an oxidation number other than 2+,

say 3+, or 0, then a redox reaction has taken place.say 3+, or 0, then a redox reaction has taken place.

Determining Oxidation Determining Oxidation NumbersNumbers

For all compounds, whether covalent, polar covalent, or For all compounds, whether covalent, polar covalent, or ionic, we treat as ionic, we treat as ionicionic for counting electrons and for for counting electrons and for oxidation-reduction reactions.oxidation-reduction reactions.

Rule 1Rule 1:: Sum of the oxidation numbers of all the atoms in Sum of the oxidation numbers of all the atoms in the chemical species equals the charge on the species.the chemical species equals the charge on the species.Neutral compoundsNeutral compounds: Sum of oxidation numbers = 0: Sum of oxidation numbers = 0Ionic speciesIonic species: Sum of oxidation numbers = charge of the : Sum of oxidation numbers = charge of the ionion

Rule 2Rule 2:: In Binary Compounds, the more Electronegative In Binary Compounds, the more Electronegative (EN) element is assigned to have a negative oxidation (EN) element is assigned to have a negative oxidation number. (See EN trends.)number. (See EN trends.)

Rule 3Rule 3:: Atoms may have only certain oxidation numbers. Atoms may have only certain oxidation numbers. The range is:The range is:

Maximum oxidation number possible = + Group number.Maximum oxidation number possible = + Group number.Minimum oxidation number possible = (Group number - 8) Minimum oxidation number possible = (Group number - 8) (this number will be negative)(this number will be negative)

Determining Oxidation Determining Oxidation Numbers (Cont)Numbers (Cont)

Atoms which will have known oxidation numbers are:Atoms which will have known oxidation numbers are: Atoms asAtoms as Elements:Elements: Ex. H Ex. H22, O, O22, P, P44, Fe, Fe

Oxidation number = 0 Oxidation number = 0 Monoatomic Ions:Monoatomic Ions:

CationsCations: Ex. Na: Ex. Na++, Al, Al3+3+ (main group metals) (main group metals)Oxidation number = + Group NumberOxidation number = + Group NumberAnionsAnions: Cl: Cl--, O, O2-2- Oxidation number = Group Number - 8 Oxidation number = Group Number - 8

HydrogenHydrogenCombined with NonmetalsCombined with Nonmetals: Ex. NH: Ex. NH33, H, H22O, HClO, HClOxidation number = +1Oxidation number = +1Combined with MetalsCombined with Metals: Ex. NaH, CaH: Ex. NaH, CaH22 (hydrides) (hydrides)Oxidation number = -1 Oxidation number = -1

Oxygen Oxygen (Unless O(Unless O222-2-, peroxide), peroxide)

Oxidation number = -2Oxidation number = -2

COCO: (Sum will equal 0 since it is a neutral molecule): (Sum will equal 0 since it is a neutral molecule)

O will have a -2 ox. number.O will have a -2 ox. number. 1 C + 1 O = 01 C + 1 O = 0

(C?) + (-2) = 0(C?) + (-2) = 0C? = +2C? = +2

Oxidation number of C in CO is +2Oxidation number of C in CO is +2Oxidation number of O in CO is -2 (known)Oxidation number of O in CO is -2 (known)

Check ox. number to see if it falls within Check ox. number to see if it falls within range:range:+2 is in between the maximum value of C, +2 is in between the maximum value of C, +4, (Gr#) and the minimum value of C, - +4, (Gr#) and the minimum value of C, - 4, (Gr# - 8).4, (Gr# - 8).So okay.So okay.

CrCr22OO772-2-: (Sum of all oxidation numbers will equal -2 : (Sum of all oxidation numbers will equal -2

since it is an ion.)since it is an ion.)

2 Cr + 7 O = -22 Cr + 7 O = -22(Cr?) + 7(-2) = -22(Cr?) + 7(-2) = -22(Cr?) + (-14) = -22(Cr?) + (-14) = -22(Cr?) = +122(Cr?) = +12Cr? = +6Cr? = +6

Oxidation number of each Cr in Oxidation number of each Cr in CrCr22OO772-2- is +6 is +6

Oxidation number of each O in Oxidation number of each O in CrCr22OO772-2- is -2 is -2

(known)(known) Check ox. number to see if it falls within Check ox. number to see if it falls within

range:range:+6 is the maximum value that Cr can have +6 is the maximum value that Cr can have (Gr#). So okay.(Gr#). So okay.

CSCS22: (Sum will equal 0 since it is a neutral molecule): (Sum will equal 0 since it is a neutral molecule)

C will have the positive oxidation number since it C will have the positive oxidation number since it is less EN than Sis less EN than SS will have a -2 charge since it is Gr # 6, (6 - 8 = -S will have a -2 charge since it is Gr # 6, (6 - 8 = -2)2)

C + 2 S = 0C + 2 S = 0(C?) + 2 (-2) = 0(C?) + 2 (-2) = 0(C?) + (-4) = 0(C?) + (-4) = 0C? = +4C? = +4

Oxidation number of C in CS2 is +4Oxidation number of C in CS2 is +4Oxidation number of each S in CS2 is -2 (known)Oxidation number of each S in CS2 is -2 (known)

Check ox. number to see if it falls within range:Check ox. number to see if it falls within range:+4 is the maximum value that C can have, (Gr#). +4 is the maximum value that C can have, (Gr#). So okay.So okay.

HNOHNO33(aq) + H(aq) + H33AsOAsO33(aq)  (aq)    NO(g) + H  NO(g) + H33AsOAsO44(aq) + H(aq) + H

22O(l)O(l) Step #1:  Step #1:  Try to balance the atoms by inspection.Try to balance the atoms by inspection. The H and O atoms are difficult to balance in this equation. You might arrive The H and O atoms are difficult to balance in this equation. You might arrive

at the correct balanced equation using a “trial and error” technique, but if you at the correct balanced equation using a “trial and error” technique, but if you do not discover the correct coefficients fairly quickly, proceed to Step #2. do not discover the correct coefficients fairly quickly, proceed to Step #2.

Step #2:Step #2:  Is the reaction redox?  Is the reaction redox? The N atoms change from +5 to +2, so they are reduced. This information is The N atoms change from +5 to +2, so they are reduced. This information is

enough to tell us that the reaction is redox. (The As atoms, which change from enough to tell us that the reaction is redox. (The As atoms, which change from +3 to +5, are oxidized.)+3 to +5, are oxidized.)

Step #3:Step #3:  Determine the net increase in oxidation number for the element   Determine the net increase in oxidation number for the element that is oxidized and the net decrease in oxidation number for the element that that is oxidized and the net decrease in oxidation number for the element that is reduced.is reduced.

As  +3 to +5     Net Change = +2 As  +3 to +5     Net Change = +2 N  +5 to +2      Net Change = -3N  +5 to +2      Net Change = -3 Step #4:Step #4:  Determine a ratio of oxidized to reduced atoms that would yield a   Determine a ratio of oxidized to reduced atoms that would yield a

net increase in oxidation number equal to the net decrease in oxidation net increase in oxidation number equal to the net decrease in oxidation number. number.

As atoms would yield a net increase in oxidation number of +6. (Six electrons As atoms would yield a net increase in oxidation number of +6. (Six electrons would be lost by three arsenic atoms.) 2 N atoms would yield a net decrease would be lost by three arsenic atoms.) 2 N atoms would yield a net decrease of -6. (Two nitrogen atoms would gain six electrons.) Thus the ratio of As of -6. (Two nitrogen atoms would gain six electrons.) Thus the ratio of As atoms to N atoms is 3:2. atoms to N atoms is 3:2.

Step #5:Step #5: To get the ratio identified in Step 5, add coefficients to the formulas  To get the ratio identified in Step 5, add coefficients to the formulas which contain the elements whose oxidation number is changing.which contain the elements whose oxidation number is changing.

2HNO2HNO33(aq) + 3H(aq) + 3H33AsOAsO33(aq) (aq)   NO(g) + H  NO(g) + H33AsOAsO44(aq) + H(aq) + H22O(l) O(l) Step #6:Step #6:  Balance the rest of the equation by inspection.  Balance the rest of the equation by inspection. 2HNO3(aq) + 3H3AsO3(aq)   2HNO3(aq) + 3H3AsO3(aq)      2NO(g) + 3H   2NO(g) + 3H33AsOAsO44(aq) + H(aq) + H22O(l)O(l)

Cu(s) + HNOCu(s) + HNO33(aq)  (aq)    Cu(NO  Cu(NO33))22(aq) + NO(g) + H(aq) + NO(g) + H22O(l)O(l) The nitrogen atoms and the oxygen atoms are difficult to balance The nitrogen atoms and the oxygen atoms are difficult to balance

by inspection, so we will go to Step #3. by inspection, so we will go to Step #3. The copper atoms and some of the nitrogen atoms change their The copper atoms and some of the nitrogen atoms change their

oxidation numbers. These changes indicate that this reaction is a oxidation numbers. These changes indicate that this reaction is a redox reaction. We next determine the changes in oxidation redox reaction. We next determine the changes in oxidation number for the atoms oxidized and reduced. number for the atoms oxidized and reduced.

Cu  0 to +2      Net Change = +2 Cu  0 to +2      Net Change = +2 Some N  +5 to +2     Net Change = -3Some N  +5 to +2     Net Change = -3 We need three Cu atoms (net change of +6) for every 2 nitrogen We need three Cu atoms (net change of +6) for every 2 nitrogen

atoms that change (net change of -6). Although the numbers for atoms that change (net change of -6). Although the numbers for the ratio determined in Step #5 are usually put in front of reactant the ratio determined in Step #5 are usually put in front of reactant formulas, this equation is somewhat different.  Because some of formulas, this equation is somewhat different.  Because some of the nitrogen atoms are changing and some are not, we need to be the nitrogen atoms are changing and some are not, we need to be careful to put the 2 in front of a formula in which all of the careful to put the 2 in front of a formula in which all of the nitrogen atoms are changing or have changed. We therefore place nitrogen atoms are changing or have changed. We therefore place the 2 in front of the NO(g) on the product side. The 3 for the the 2 in front of the NO(g) on the product side. The 3 for the copper atoms can be placed in front of the Cu(s). copper atoms can be placed in front of the Cu(s).

          3Cu(s) + HNO3Cu(s) + HNO33(aq)  (aq)    Cu(NO  Cu(NO33))22(aq) + 2NO(g) + H(aq) + 2NO(g) + H22O(l) O(l) We balance the rest of the atoms, being careful to keep the ratio We balance the rest of the atoms, being careful to keep the ratio

of Cu to NO 3:2.of Cu to NO 3:2.        

3Cu(s) + 8HNO3Cu(s) + 8HNO33(aq)  (aq)    3Cu(NO  3Cu(NO33))22(aq) + 2NO(g) + 4H(aq) + 2NO(g) + 4H22O(l)O(l)

SummarySummary 1. An oxidation-reduction reaction involves an apparent transfer of electrons 1. An oxidation-reduction reaction involves an apparent transfer of electrons

from one particle to another.from one particle to another. 2. Oxidation is the process by which electrons are apparently removed from an 2. Oxidation is the process by which electrons are apparently removed from an

atom or group of atoms.atom or group of atoms. 3. Reduction is the process by which electrons are apparently added to atoms or 3. Reduction is the process by which electrons are apparently added to atoms or

groups of atoms. groups of atoms. 3. Any substance in a reaction which loses electrons is a reducing agent. 3. Any substance in a reaction which loses electrons is a reducing agent. 4. Any substance in a reaction which gains electrons is an oxidizing agent. 4. Any substance in a reaction which gains electrons is an oxidizing agent. 5. If a substance gives up electrons readily, it is a strong reducing agent. Its 5. If a substance gives up electrons readily, it is a strong reducing agent. Its

oxidized form is usually a poor oxidizing agent. oxidized form is usually a poor oxidizing agent. 6. If a substance acquires electrons readily, it is a strong oxidizing agent. Its 6. If a substance acquires electrons readily, it is a strong oxidizing agent. Its

reduced form is usually a poor reducing agent.reduced form is usually a poor reducing agent. 7. Oxidation number is the charge an atom appears to have when we assign a 7. Oxidation number is the charge an atom appears to have when we assign a

certain number of electrons to that atom. certain number of electrons to that atom. 8. Six rules for assigning oxidation numbers:8. Six rules for assigning oxidation numbers:

– a. The oxidation number of any free element is O.a. The oxidation number of any free element is O.b. The oxidation number of any single-atom ion is equal to the that ion.b. The oxidation number of any single-atom ion is equal to the that ion.c. The oxidation number of hydrogen is usually 1+. c. The oxidation number of hydrogen is usually 1+.

– d. The oxidation number of oxygen in most compounds is 2-. d. The oxidation number of oxygen in most compounds is 2-. – e. The sum of the oxidation numbers of all the atoms in a particle equal the apparent e. The sum of the oxidation numbers of all the atoms in a particle equal the apparent

charge of that particle.charge of that particle.– f. In compounds, elements of Group IA and Group IIA have an oxidation number f. In compounds, elements of Group IA and Group IIA have an oxidation number

numerically equal to their group in the periodic table.numerically equal to their group in the periodic table. 9. In all chemical reactions, charge, number and kind of atoms, and number of 9. In all chemical reactions, charge, number and kind of atoms, and number of

electrons are conserved. Knowing these quantities, you can do a redox equation. electrons are conserved. Knowing these quantities, you can do a redox equation. 10. Redox reactions are more easily balanced by splitting the equation into half-10. Redox reactions are more easily balanced by splitting the equation into half-

reactions.reactions.