Unit VII Study Guide: Nature of Oxidation and Reduction

The oxidation number for H in NaBH4 is

-1

The oxidation process involves

1.) complete loss of electrons 2.) Shift of electrons away from an atom in a covalent bond 3.) Gain of oxygen 4.) Loss of hydrogen in a molecular compound 5.) Increase in oxidation number

The reduction process involves

1.) gain of hydrogen 2.) Shift of electrons toward an atom in a covalent bond 3.) Loss of oxygen 4.) Gain of hydrogen in a molecular compound 5.) Decrease in oxidation number

The green color on the Statue of Liberty is due to

CuCO3 (copper(II) carbonate)

True or False: Reduction involves oxygen transfer to a metal.

False Reason: 2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g) The removal of oxygen from the Fe2O3 means that it is being reduced to Fe. Oxygen was transferred to carbon not a metal.

True or False: Covering a metal surface with oil or paint is the best way to prevent corrosion.

False Reason: Covering the surface of a metal object with paint or oil will prevent corrosion by sealing the metal off from any surrounding oxygen. Unfortunately, these protective coatings may eventually wear off or get scratched, exposing the metal surface and allowing corrosion to begin. Corrosion-sensitive metals can also be coated with another metal that is resistant to corrosion.

True or False: The neutralization of HCl by NaOH is a redox reaction.

False Reason: HCl + NaOHHOH + NaCl H = +1, Cl = -1 + Na = +1, O = -2, H = +1H = +1, O = -2 + Na = +1, Cl = -1 H: +1+1 no change Cl: -1-1 no change Na: +1+1 no change O: -2-2 no change

True or False: In a polyatomic ion, the sum of oxidation numbers of the atoms equals zero.

False Reason: In a polyatomic ion, the sum of the oxidation numbers of all atoms is equal to the overall charge on the ion.

True or False: A shift of electrons towards an atom in a covalent bond is an oxidation of that atom.

False Reason: It is reduction not oxidation.

True or False: The half reaction Mg --} Mg2+ represents a reduction process

False Reason: Mg loses two electrons to form Mg2+. Oxidation is the losing of electrons. Mg is oxidized not reduced.

True or False: The change in N from HNO3 to NaNO3 represents an oxidation of N.

False Reason: N is in the same ion, NO3-, so there will be no change in the oxidation number of N (+5 in both).

True or False: Coefficients are needed only for those materials directly involved in the redox process.

False Reason: Occasionally, a coefficient may need to be placed in front of a formula that was not involved in the redox process.

True or False: Oxidation of Fe2+ by Cr2O7(2-) readily occurs in basic solution.

False Reason: The oxidation of Fe2+ by Cr2O7(2-) does not readily occur in basic solution

True or False: Reactions in a basic medium require addition of hydroxide ions.

False: Reason: For reactions that occur in basic solution rather than acidic solution, the steps are primarily the same. However, after finishing step 6, add an equal number of OH- ions to both sides of the equation.

Balance the following equation using the half-reaction method (Be sure to show the steps of the method): Cu + HNO3 ---} Cu2+ + NO

Final Answer: 3Cu + 2HNO3 + 6H+ 3Cu2+ + 2NO + 4H2O Identify what is being oxidized and what is being reduced. Left: Cu = 0; H = +1; N = +5; O = -2 Right: Cu = +2; N = +2; , O = -2 Cu went from 0 to +2, losing two electrons. Copper is oxidized. N went from +5 to +2, gaining three electrons. Nitrogen is reduced. Break the reaction into two half-reactions: Oxidation: Cu Cu2+ Reductions: HNO3 NO Balance each half-reaction by both stoichiometry and electron charge. This is accomplished by adding substances to the reaction. The only rule is that the only substances you can add must already be in the solution. These include water (H2O), H+ ions (in acid solutions), OH- (in basic solutions) and electrons. Start with the oxidation half-reaction. The half-reaction is already balanced atomically. To balance electronically, two electrons must be added to the product side: Cu Cu2+ + 2e- Now, balance the reduction half-reaction. This reaction requires work. The first step is to balance all atoms except oxygen and hydrogen. HNO3 NO There is only one nitrogen atom on both sides, so nitrogen is already balanced. The second step is to balance the oxygen atoms and the product side has only one oxygen. Add two water molecules to the product side. HNO3 NO + 2H2O The third step is to balance the hydrogen atoms. This is accomplished by adding H+ ions to the side that needs more hydrogen. The reactant side has one hydrogen atom while the product side has four. Add 3 H+ ions to the reactant side. HNO3 + 3H+ NO + 2H2O The equation is balanced atomically, but not electrically. The final step is to balance the charge by adding electrons to the more positive side of the reaction. On the reactant side the overall charge is +3, while the product side is neutral. To counteract the +3 charge, add three electrons to the reactant side. HNO3 + 3H+ + 3e- NO + 2H2O Equalize the electron transfer. In redox reactions, the number of electrons gained must equal the number of electrons lost. To accomplish this, each reaction is multiplied by whole numbers to contain the same number of electrons. The oxidation half-reaction has two electrons while the reduction half-reaction has three electrons. The lowest common denominator between them is six electrons. Multiply the oxidation half-reaction by 3 and the reduction half-reaction by 2: 3Cu 3Cu2+ + 6e- 2HNO3 + 6H+ + 6e- 2NO + 4H2O Recombine the half-reactions. This is accomplished by adding the two reactions together. Once they are added, cancel out anything that appears on both sides of the reaction. 3Cu ---} 3Cu2+ + 6e- 2HNO3 + 6H+ ---} + 6e- 2NO + 4H2O 3Cu + 2HNO3 + 6H+ ---} + 6e- 3Cu2+ + 6e- + 2NO + 4H2O Both sides have 6 electrons that can be cancelled. 3Cu + 2HNO3 + 6H+ ---} 3Cu2+ + 2NO + 4H2O

Balance the following equation using the half-reaction method (Be sure to show the steps of the method): ClO3- + SO2 ---} SO42- + Cl-

Final Answer: ClO3- + 3H2O + 3SO2 3SO42- + Cl- + 6H+ Identify what is being oxidized and what is being reduced. Left: Cl = +5; O = -2; S = +4 Right: S = +6; O = -2; Cl = -1 S went from +4 to +6 losing 2 electrons. Sulfur is oxidized. Cl went from +5 to -1 gaining 4 electrons. Chlorine is reduced. Break the reaction into two half-reactions: Oxidation: SO2 ---} SO42- Reductions: ClO3- ---} Cl- Balance each half-reaction by both stoichiometry and electron charge. This is accomplished by adding substances to the reaction. The only rule is that the only substances you can add must already be in the solution. These include water (H2O), H+ ions (in acid solutions), OH- (in basic solutions) and electrons. Start with the oxidation half-reaction. SO2 + 2H2O SO42- + 4H+ To balance electronically, two electrons must be added to the product side: SO2 + 2H2O SO42- + 4H+ + 2e- Now, balance the reduction half-reaction. This reaction requires work. The first step is to balance all atoms except oxygen and hydrogen. ClO3- Cl- There is only one chlorine atom on both sides, so chlorine is already balanced. The second step is to balance the oxygen atoms and the product side has three oxygen. Add three water molecules to the product side. ClO3- Cl- + 3H2O The third step is to balance the hydrogen atoms. This is accomplished by adding H+ ions to the side that needs more hydrogen. The reactant side has no hydrogen atoms while the product side has six. Add 6 H+ ions to the reactant side. ClO3- + 6H+ Cl- + 3H2O The equation is balanced atomically, but not electrically. The final step is to balance the charge by adding electrons to the more positive side of the reaction. On the reactant side the overall charge is +5, while the product side is -1. ClO3- + 6H+ + 6e- Cl- + 3H2O Equalize the electron transfer. In redox reactions, the number of electrons gained must equal the number of electrons lost. To accomplish this, each reaction is multiplied by whole numbers to contain the same number of electrons. The oxidation half-reaction has two electrons while the reduction half-reaction has three electrons. The lowest common denominator between them is six electrons. Multiply the oxidation half-reaction by 3 and the reduction half-reaction by 2: SO2 + 2H2O ---} SO42- + 4H+ + 2e- ClO3- + 6H+ ---} + 6e- Cl- + 3H2O Recombine the half-reactions. This is accomplished by adding the two reactions together. Once they are added, cancel out anything that appears on both sides of the reaction. 3SO2 + 6H2O ---} 3SO42- + 12H+ + 6e- ClO3- + 6H+ + 6e- ---} Cl- + 3H2O 3SO2 + 6 3H2O + ClO3- + 6H++ 6e- ---} 3SO42- + 12 6H+ + 6e- + Cl- + 3H2O Both sides have 6 electrons that can be cancelled. ClO3- + 3H2O + 3SO2 ---} 3SO42- + Cl- + 6H+

Determine the change in oxidation numbers for the iron and manganese: Fe2+ + MnO4- Fe3+ + Mn2+

For the oxidation number of Mn in MnO4- Mn + 4O = -1 (overall charge of ion) Mn + 4(-2) = -1 Mn + (-8) = -1 Mn = +7 Fe2+ Fe3+; +1; oxidation Mn7+ Mn2+; -5; reduction

Balance the following equation using the oxidation number change method (Be sure to show the steps of the method): HNO3 + H3AsO3 ---} NO + H3AsO4 + H2O

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Balance the following equation using the oxidation number change method (Be sure to show the steps of the method): Al + MnO2 ---} Al2O3 + Mn

Identify the oxidation number of every atom: Left hand side: Al = 0; Mn = +4; O = -2 Right hand side: Al = +3; O = -2; Mn = 0 Determine the change in oxidation number for each atom that changes. Al: 0 --} +3; Change = +3 Mn: +4 --} 0; Change = -4 Make the total increase in oxidation number equal to the total decrease in oxidation number: We need 4 atoms of Al for every 3 atoms of Mn. Place these numbers as coefficients in front of the formulas containing those atoms. 4Al + 3MnO2 --} Al2O3 + Mn Balance all remaining atoms 4Al + 3MnO2 --} 2Al2O3 + 3Mn

In a reaction, KNO2 is converted to KNO3. The oxidation-reduction change was

N+3 --} N+5

True or False: Addition of H to the O=O double bond is a reduction process

True

True or False: Double-displacement reactions are not redox reactions.

True

True or False: Electrons are added to each half-reaction to balance the charges.

True

True or False: Monitoring how the oxidation numbers change is good practice.

True

True or False: Net ionic equations help identify oxidation and reduction processes.

True

True or False: Oxidation numbers are assigned to each atom in the equation.

True

True or False: S is the oxidizing agent in the formation of ZnS from Zn and S.

True

True or False: Sodium hypochlorite oxidizes stains to make their removal easier.

True

True or False: Some oxidation reactions do not involve oxygen.

True

True or False: The change in Cl from KClO3 to KCl represents a reduction of Cl.

True

True or False: The half-reaction method is more useful for reactions involving ions in water.

True

True or False: The oxidation number for S in compounds depends on the structure of the compound.

True

True or False: The oxidation number of an atom in an element is zero.

True

True or False: The reaction of sodium metal with water to form NaOH and hydrogen gas is a redox reaction.

True

True or False: The total increase in oxidation number must equal the total decrease in oxidation number.

True

True or False: In the reaction Mg + HCl ---} MgCl2 + H2, Mg is oxidized

True Reason: Mg = 0 + H = +1, Cl = -1 Mg = +2, Cl = -1 + H = 0 Mg: 0 +2 is oxidation

A redox reaction must involve a change in oxidation number for at least ____ of the atoms in the reaction.

Two

Since redox reactions involve the transfer of electrons, could this phenomenon be used in electrical circuits? Why?

Yes, this is the principle behind how batteries work.

The following reaction illustrates the process of corrosion of zinc metal to form "white rust" 2Zn + 2H2O + O2- Zn(OH)2 Which material is oxidized and which is reduced?

Zn is oxidized to Zn2+ and O is reduced to O2-

All of the following statements about reduction are true except a.) bleaches reduce stains by removal of oxygen atoms b.) reduction is the removal of oxygen from a substance c.) iron oxide can be reduced to pure iron d.) every reduction process requires a simultaneous oxidation process

a.) bleaches reduce stains by removal of oxygen atoms

The fifth step in balancing equations using the half-reaction method is to

add electrons where needed

The sixth step in balancing equations using the half-reaction method is to

add the two half-reactions together

The first step in the oxidation-number change method is to

assign oxidation numbers

The only halogen that does not have a variable oxidation number is a.) Cl b.) F c.) I d.) Br

b.) F

All of the following are iron oxidation states that are components of rust except a.) Fe b.) Fe+ c.)Fe2+ d.)Fe3+

b.) Fe+

For the reaction below, all of the following statements are correct except one Mn(NO3)2 + NaBiO3 + HNO3 ---} HMnO4 + Bi(NO3)3 + NaNO3 + H2O a.) Mn is oxidized b.) Na is reduced c.)Bi is reduced d.)N is unchanged

b.) Na is reduced

The third step in balancing equations using the half-reaction method is to

balance each half-reaction

Oxygen can have all of the following oxidations numbers except a.) +1 b.) +2 c.) -3 d.) -2

c.) -3

Single replacement reactions are redox reactions because of all of the following except a.) a free element is transformed to an ionized material b.) an ionized material is transformed to a free element c.) a cation becomes an anion d.) a material with an oxidation number of zero experiences a change in oxidation number

c.) a cation becomes an anion

All of the following are true about redox reactions except a.) electron loss is oxidation b.) electron gain is reduction c.) electron loss is reduction d.) electrons that are lost must be gained by another particle

c.) electron loss is reduction

Step five in the oxidation-number change method is to

check to see if the equation is balanced in terms of both atoms and charges

In the formation of water from O2 and H2 all of the following are true except a.) hydrogen atoms are oxidized b.) oxygen atoms are reduced c.) in the -O-H bond, the electrons are more attracted to O d.) H atoms in water have a greater electron density than they did in H2

d.) H atoms in water have a greater electron density than they did in H2

All of the following statements about oxidation are true except a.) combustion is an oxidation process b.) oxidation involves the addition of oxygen c.) rusting of iron is an oxidation process d.) treating iron ore with carbon is an oxidative process

d.) treating iron ore with carbon is an oxidative process

The second step in the oxidation-number change method is to

identify what is oxidized and what is reduced

An oxidizing agent

oxidizes another atom

A reducing agent

reduces another atom

Oxidation involves

shifting electrons from another atom to oxygen

The production of ZnS involves

transfer of two electrons from Zn to S

Step three in the oxidation-number change method is to

use lines to connect atoms undergoing oxidation or reduction

The second step in balancing equations using the half-reaction method is to

write the half-reactions

The first step in balancing equations using the half-reaction method is to

write the unbalanced ionic equation