Oxidation and Reduction Chapter 20

Types of Chemical Reactions Type I: ions or molecules react with no apparent change in the electronic structure of the particles. Type I: ions or molecules react with no apparent change in the electronic structure of the particles. Type II: ions or atoms undergo changes of electronic structure. Electrons may be transferred from one particle to another. On the other hand, the sharing of the electrons may be somewhat changed. Type II: ions or atoms undergo changes of electronic structure. Electrons may be transferred from one particle to another. On the other hand, the sharing of the electrons may be somewhat changed. Type II reactions involving electron changes are called oxidation-reduction reactions. Type II reactions involving electron changes are called oxidation-reduction reactions. It is these "redox" reactions which we will now discuss. It is these "redox" reactions which we will now discuss. Before we indicate what oxidation-reduction reactions are, we will briefly indicate what they are not. Before we indicate what oxidation-reduction reactions are, we will briefly indicate what they are not.

What Redox is NOT In the BaSO 4 reaction in Table 26-1, the substances are all ionic. In the BaSO 4 reaction in Table 26-1, the substances are all ionic. Since there is no change in the charge of these ions in the reaction, there are no electron changes. Since there is no change in the charge of these ions in the reaction, 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 (BaS0 4 ) is nearly always a result of a non-redox reaction. The production of a (BaS0 4 ) is nearly always a result of a non-redox 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 reaction, redox reactions are important in the laboratory. Since nearly every other kind of reaction is an oxidation-reduction 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.

Oxidation The term oxidation was first applied to the combining of oxygen with other elements. The term oxidation was first applied to the combining of oxygen with other elements. There were many known instances of this behavior: There were many known instances of this behavior: –Iron rusts –Carbon burns In rusting, oxygen combines slowly with iron to form Fe 2 O 3. In rusting, oxygen combines slowly with iron to form Fe 2 O 3. In burning, oxygen unites rapidly with carbon to form CO 2. In burning, oxygen unites rapidly with carbon to form CO 2. Observation of these reactions gave rise to the terms "slow" and "rapid" oxidation. Observation of these reactions gave rise to the terms "slow" and "rapid" oxidation. Chemists recognize, however, that other nonmetallic elements unite with substances in a manner similar to that of oxygen. Chemists recognize, however, that other nonmetallic elements unite with substances in a manner similar to that of oxygen. –Hydrogen, antimony, and sodium all burn in chlorine, and iron will burn in fluorine. Since these reactions were similar, chemists formed a more general definition of oxidation: Since these reactions were similar, chemists formed a more general definition of oxidation: –Electrons were removed from each free element by the reactants O 2 or Cl 2. Thus oxidation is defined as the process by which electrons are apparently removed from an atom or ion. Thus oxidation is defined as the process by which electrons are apparently removed from an atom or ion.

Reduction A reduction reaction was originally limited to the type of reaction in which ores were "reduced" from their oxides. A reduction reaction was originally limited to the type of reaction in which ores were "reduced" from their oxides. –Iron oxide was "reduced" to iron by carbon monoxide. –Copper(II) oxide could be "reduced" to copper by hydrogen. In these reactions, oxygen is removed, and the free element is produced. In these reactions, oxygen is removed, and the free 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 a reaction which produces free copper. –An electric current passing through molten sodium chloride produces free sodium. The similarity between oxidation and reduction reactions led chemists to formulate a more generalized definition of reduction. The similarity between oxidation and reduction reactions led chemists to formulate a more generalized definition of reduction. By definition, reduction is the process by which electrons are apparently added to atoms or ions. By definition, reduction is the process by which electrons are apparently added to atoms or ions.

OIL RIG—the Texas Definition Oxidation is Loss (of electrons), Reduction is Gain (of electrons) Oxidation is Loss (of electrons), Reduction is Gain (of electrons)

Oxidizing and Reducing Agents 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 accounted for. It seems reasonable, therefore, that both oxidation and reduction must occur at the same time in a reaction. It seems reasonable, therefore, that both oxidation and reduction must occur at the same time in a reaction. Electrons are lost and gained at the same time and the number lost must equal the number gained. Electrons are lost and gained at the same time and the number lost must equal the number gained. The substance in the reaction which gives up electrons is called the reducing agent. The reducing agent contains the atoms which are oxidized (the atoms which lose electrons). The substance in the reaction which gives up electrons is called the reducing agent. The reducing agent contains the atoms which are oxidized (the atoms which lose electrons). –Zinc is a good example of a reducing agent. It is oxidized to the zinc ion, Zn 2+ The substance in the reaction which gains electrons is called the oxidizing agent. It contains the atoms which are reduced (the atoms which gain electrons). The substance in the reaction which gains electrons is called the oxidizing agent. It contains the atoms which are reduced (the atoms which gain electrons). –Dichromate ion, Cr , is a good example of an oxidizing agent. It is reduced to the chromium ion, Cr 3+ 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. 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. If a substance gains electrons readily, it is said to be a strong oxidizing agent. Its reduced form is a weak reducing agent. If a substance gains electrons readily, it is said to be a strong oxidizing agent. Its reduced form is a weak reducing agent.

Redox of nails and copper

Oxidation Numbers How is it possible to determine whether an oxidation-reduction reaction has taken place? How is it possible to determine whether an oxidation-reduction reaction has taken place? We do so by determining whether any electron shifts have taken place during the reaction. We do so by determining whether any electron shifts have taken place during the reaction. To indicate electron changes, we look at the oxidation numbers of the atoms in the reaction. To indicate electron changes, we look at the oxidation numbers of the atoms in the reaction. The oxidation number is the charge an atom appears to have when we assign a certain number of electrons to given atoms or ions. The oxidation number is the charge an atom appears to have when we assign a certain number of electrons to given atoms or ions. Any change of oxidation numbers in the course of a reaction indicates an oxidation-reduction reaction has taken place. Any change of oxidation numbers in the course of a reaction indicates an oxidation-reduction reaction has taken place. Oxidation numbers are assigned according to the apparent charge of the element (aka, valence!) Oxidation numbers are assigned according to the apparent charge of the element (aka, valence!) For example, suppose iron, as a reactant in a reaction, has an oxidation number of 2+. For example, suppose iron, as a reactant in a reaction, has an oxidation number of 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.

Determining Oxidation Numbers For all compounds, whether covalent, polar covalent, or ionic, we treat as ionic for counting electrons and for oxidation-reduction reactions. For all compounds, whether covalent, polar covalent, or ionic, we treat as ionic for counting electrons and for oxidation-reduction reactions. Rule 1: Sum of the oxidation numbers of all the atoms in the chemical species equals the charge on the species. Neutral compounds: Sum of oxidation numbers = 0 Ionic species: Sum of oxidation numbers = charge of the ion Rule 1: Sum of the oxidation numbers of all the atoms in the chemical species equals the charge on the species. Neutral compounds: Sum of oxidation numbers = 0 Ionic species: Sum of oxidation numbers = charge of the ion Rule 2: In Binary Compounds, the more Electronegative (EN) element is assigned to have a negative oxidation number. (See EN trends.) Rule 2: In Binary Compounds, the more Electronegative (EN) element is assigned to have a negative oxidation number. (See EN trends.) Rule 3: Atoms may have only certain oxidation numbers. The range is: Rule 3: Atoms may have only certain oxidation numbers. The range is: Maximum oxidation number possible = + Group number. Minimum oxidation number possible = (Group number - 8) (this number will be negative) Maximum oxidation number possible = + Group number. Minimum oxidation number possible = (Group number - 8) (this number will be negative)

Determining Oxidation Numbers (Cont) Atoms which will have known oxidation numbers are: Atoms which will have known oxidation numbers are: Atoms as Elements: Ex. H 2, O 2, P 4, Fe Oxidation number = 0 Atoms as Elements: Ex. H 2, O 2, P 4, Fe Oxidation number = 0 Monoatomic Ions: Cations: Ex. Na +, Al 3+ (main group metals) Oxidation number = + Group Number Anions: Cl -, O 2- Oxidation number = Group Number - 8 Monoatomic Ions: Cations: Ex. Na +, Al 3+ (main group metals) Oxidation number = + Group Number Anions: Cl -, O 2- Oxidation number = Group Number - 8 Hydrogen Combined with Nonmetals: Ex. NH 3, H 2 O, HCl Oxidation number = +1 Combined with Metals: Ex. NaH, CaH 2 (hydrides) Oxidation number = -1 Hydrogen Combined with Nonmetals: Ex. NH 3, H 2 O, HCl Oxidation number = +1 Combined with Metals: Ex. NaH, CaH 2 (hydrides) Oxidation number = -1 Oxygen (Unless O 2 2-, peroxide) Oxidation number = -2 Oxygen (Unless O 2 2-, peroxide) Oxidation number = -2

CO: (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 = 0 (C?) + (-2) = 0 C? = +2 1 C + 1 O = 0 (C?) + (-2) = 0 C? = +2 Oxidation number of C in CO is +2 Oxidation number of O in CO is -2 (known) Oxidation number of C in CO is +2 Oxidation number of O in CO is -2 (known) Check ox. number to see if it falls within range: +2 is in between the maximum value of C, +4, (Gr#) and the minimum value of C, - 4, (Gr# - 8). So okay. Check ox. number to see if it falls within range: +2 is in between the maximum value of C, +4, (Gr#) and the minimum value of C, - 4, (Gr# - 8). So okay.

Cr 2 O 7 2- : (Sum of all oxidation numbers will equal -2 since it is an ion.) 2 Cr + 7 O = -2 2(Cr?) + 7(-2) = -2 2(Cr?) + (-14) = -2 2(Cr?) = +12 Cr? = +6 2 Cr + 7 O = -2 2(Cr?) + 7(-2) = -2 2(Cr?) + (-14) = -2 2(Cr?) = +12 Cr? = +6 Oxidation number of each Cr in Cr 2 O 7 2- is +6 Oxidation number of each O in Cr 2 O 7 2- is -2 (known) Oxidation number of each Cr in Cr 2 O 7 2- is +6 Oxidation number of each O in Cr 2 O 7 2- is -2 (known) Check ox. number to see if it falls within range: +6 is the maximum value that Cr can have (Gr#). So okay. Check ox. number to see if it falls within range: +6 is the maximum value that Cr can have (Gr#). So okay.

CS 2 : (Sum will equal 0 since it is a neutral molecule) C will have the positive oxidation number since it is less EN than S S will have a -2 charge since it is Gr # 6, (6 - 8 = -2) C will have the positive oxidation number since it is less EN than S S will have a -2 charge since it is Gr # 6, (6 - 8 = -2) C + 2 S = 0 (C?) + 2 (-2) = 0 (C?) + (-4) = 0 C? = +4 C + 2 S = 0 (C?) + 2 (-2) = 0 (C?) + (-4) = 0 C? = +4 Oxidation number of C in CS2 is +4 Oxidation number of each S in CS2 is -2 (known) Oxidation number of C in CS2 is +4 Oxidation number of each S in CS2 is -2 (known) Check ox. number to see if it falls within range: +4 is the maximum value that C can have, (Gr#). So okay. Check ox. number to see if it falls within range: +4 is the maximum value that C can have, (Gr#). So okay.

HNO 3 (aq) + H 3 AsO 3 (aq)  NO(g) + H 3 AsO 4 (aq) + H 2 O(l) Step #1: Try to balance the atoms by inspection. Step #1: Try to balance the atoms by inspection. 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 do not discover the correct coefficients fairly quickly, proceed to Step #2. 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 do not discover the correct coefficients fairly quickly, proceed to Step #2. Step #2: Is the reaction redox? Step #2: Is the reaction redox? 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 +3 to +5, are oxidized.) 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 +3 to +5, are oxidized.) Step #3: Determine the net increase in oxidation number for the element that is oxidized and the net decrease in oxidation number for the element that is reduced. Step #3: Determine the net increase in oxidation number for the element that is oxidized and the net decrease in oxidation number for the element that is reduced. As +3 to +5 Net Change = +2 As +3 to +5 Net Change = +2 N +5 to +2 Net Change = -3 N +5 to +2 Net Change = -3 Step #4: Determine a ratio of oxidized to reduced atoms that would yield a net increase in oxidation number equal to the net decrease in oxidation number. Step #4: Determine a ratio of oxidized to reduced atoms that would yield a net increase in oxidation number equal to the net decrease in oxidation number. 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 of -6. (Two nitrogen atoms would gain six electrons.) Thus the ratio of As atoms to N atoms is 3:2. 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 of -6. (Two nitrogen atoms would gain six electrons.) Thus the ratio of As atoms to N atoms is 3:2. Step #5: To get the ratio identified in Step 5, add coefficients to the formulas which contain the elements whose oxidation number is changing. Step #5: To get the ratio identified in Step 5, add coefficients to the formulas which contain the elements whose oxidation number is changing. 2HNO 3 (aq) + 3H 3 AsO 3 (aq)  NO(g) + H 3 AsO 4 (aq) + H 2 O(l) 2HNO 3 (aq) + 3H 3 AsO 3 (aq)  NO(g) + H 3 AsO 4 (aq) + H 2 O(l) Step #6: Balance the rest of the equation by inspection. Step #6: Balance the rest of the equation by inspection. 2HNO3(aq) + 3H3AsO3(aq)  2NO(g) + 3H 3 AsO 4 (aq) + H 2 O(l) 2HNO3(aq) + 3H3AsO3(aq)  2NO(g) + 3H 3 AsO 4 (aq) + H 2 O(l)

Cu(s) + HNO 3 (aq)  Cu(NO 3 ) 2 (aq) + NO(g) + H 2 O(l) The nitrogen atoms and the oxygen atoms are difficult to balance by inspection, so we will go to Step #3. The nitrogen atoms and the oxygen atoms are difficult to balance by inspection, so we will go to Step #3. The copper atoms and some of the nitrogen atoms change their oxidation numbers. These changes indicate that this reaction is a redox reaction. We next determine the changes in oxidation number for the atoms oxidized and reduced. The copper atoms and some of the nitrogen atoms change their oxidation numbers. These changes indicate that this reaction is a redox reaction. We next determine the changes in oxidation 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 = -3 Some N +5 to +2 Net Change = -3 We need three Cu atoms (net change of +6) for every 2 nitrogen atoms that change (net change of -6). Although the numbers for the ratio determined in Step #5 are usually put in front of reactant formulas, this equation is somewhat different. Because some of 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 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 copper atoms can be placed in front of the Cu(s). We need three Cu atoms (net change of +6) for every 2 nitrogen atoms that change (net change of -6). Although the numbers for the ratio determined in Step #5 are usually put in front of reactant formulas, this equation is somewhat different. Because some of 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 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 copper atoms can be placed in front of the Cu(s). 3Cu(s) + HNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2NO(g) + H 2 O(l) 3Cu(s) + HNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2NO(g) + H 2 O(l) We balance the rest of the atoms, being careful to keep the ratio of Cu to NO 3:2. We balance the rest of the atoms, being careful to keep the ratio of Cu to NO 3:2. 3Cu(s) + 8HNO 3 (aq)  3Cu(NO 3 ) 2 (aq) + 2NO(g) + 4H 2 O(l) 3Cu(s) + 8HNO 3 (aq)  3Cu(NO 3 ) 2 (aq) + 2NO(g) + 4H 2 O(l)

Summary 1. An oxidation-reduction reaction involves an apparent transfer of electrons from one particle to another. 1. An oxidation-reduction reaction involves an apparent transfer of electrons from one particle to another. 2. Oxidation is the process by which electrons are apparently removed from an atom or group of atoms. 2. Oxidation is the process by which electrons are apparently removed from an atom or group of atoms. 3. Reduction is the process by which electrons are apparently added to atoms or groups of atoms. 3. Reduction is the process by which electrons are apparently added to atoms or 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 oxidized form is usually a poor oxidizing agent. 5. If a substance gives up electrons readily, it is a strong reducing agent. Its oxidized form is usually a poor oxidizing agent. 6. If a substance acquires electrons readily, it is a strong oxidizing agent. Its reduced form is usually a poor reducing agent. 6. If a substance acquires electrons readily, it is a strong oxidizing agent. Its reduced form is usually a poor reducing agent. 7. Oxidation number is the charge an atom appears to have when we assign a certain number of electrons to that atom. 7. Oxidation number is the charge an atom appears to have when we assign a 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. b. The oxidation number of any single-atom ion is equal to the that ion. c. The oxidation number of hydrogen is usually 1+. –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 charge of that particle. –f. In compounds, elements of Group IA and Group IIA have an oxidation number numerically equal to their group in the periodic table. 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. 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. 10. Redox reactions are more easily balanced by splitting the equation into half-reactions. 10. Redox reactions are more easily balanced by splitting the equation into half-reactions.