Chapter 20 Oxidation-Reduction Reactions
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Transcript of Chapter 20 Oxidation-Reduction Reactions
Chapter 20Oxidation-Reduction Reactions
Some Common Reactions
The combustion of gasoline in an automobile engine requires oxygen
Burning of wood in a fireplace requires oxygen
The reactions that break down food in your body and release energy use oxygen
The oxide of hydrogen is water
Charcoal oxidizes when it burn forming CO2
Bleaching stains in fabric is still oxidation even though it does not burn.
Oxidation
When methane burns in air, it oxidizes and forms oxides of carbon and hydrogen.
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
When elemental iron turns to rust, it slowly oxidizes to compounds such as iron (III) oxide.
4Fe(s) + 3O2(g) 2Fe2O3(s)
ReductionOriginally reduction meant a loss of oxygen
from a compound
2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g) iron oxide carbon iron carbon dioxide
Reduction of iron ore to metallic iron involves the removal of oxygen from iron (III) oxide.
Involves heating the ore with carbon.
QuestionWhat happens to magnesium and oxygen
when they react to form magnesium oxide?
+2 -22Mg + O2 2MgO
magnesium oxygen magnesium oxide
Magnesium loses electrons to form Mg2+
Oxygen gains electrons to form O2-
Electron Shift in Redox ReactionsThe modern concept of oxidation and reduction
have been extended to include many reactions that do not even involve oxygen.
Oxygen is the most electronegative element (besides fluorine)
When oxygen bonds with an atom of a different element (except fluorine), electrons from that atom shift toward oxygen.
Redox ReactionsRedox reactions are currently understood to
involve any shift of electron between reactants.
OxidationOxidation – a process that involves a complete or partial loss of electrons or a gain of oxygen.
• Results in an increase in the oxidation number of an atom
ReductionReduction – a process that involves a complete or partial gain of electrons or the loss of oxygen.
• Results in a decrease in the oxidation number of an atom
Redox Reactions
Oxidation and reduction always occur simultaneously.
The substance gaining oxygen or losing electrons is oxidized
The substance losing oxygen or gaining electrons is reduced.
Redox Reactions
During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal.
Mg + S Mg2+S2- magnesium sulfur magnesium sulfide
2 electrons are transferred from a magnesium atom to a sulfur atom.
Magnesium atoms become more stable by the loss of electrons. Sulfur atoms become more stable by the gain of electrons
Redox Reactions
Mg + S Mg2+S2- magnesium sulfur magnesium sulfide
Oxidation: Mg Mg2+ + 2e- (loss of electrons)
Reduction: S + 2e- S2- (gain of electrons)
Magnesium atom is said to be oxidized to a magnesium ion
Sulfur atom is said to be reduced to a sulfide ion.
Redox Reactions
When a metal combines with oxygen, it loses electrons
When oxygen is removed from the oxide of a metal, the metal gains electrons.
This knowledge is what led to the broader definition of oxidation and reduction as an exchange of electrons.
Redox Reactions
Reducing agentReducing agent – the substance that loses the electrons
Mg + S MgS reducing agent oxidized
Oxidizing agentOxidizing agent – the substance that accepts electrons
Mg + S MgS oxidizing agent reduced
Sample Problem
Silver nitrate reacts with copper to form copper nitrate and silver. From the equation below, determine what is oxidized and what is reduced. Identify the oxidizing agent and the reducing agent.
2AgNO3 + Cu Cu(NO3)2 + 2Ag
Rewrite the equation in ionic form
2Ag+ + 2NO3- + Cu Cu2+ + 2NO3
- + 2Ag
Sample Problem
2Ag+ + 2NO3- + Cu Cu2+ + 2NO3
- + 2Ag
Oxidation: 2 e- are lost from copper when it becomes a Cu2+
Reduction: 2e- are gained by two silver ions which become neutral silver atoms.
2Ag+ + 2NO3- + Cu Cu2+ + 2NO3
- + 2Ag oxidizing reducingAgent agentreduced oxidized
Sample Problem
2Na + S 2Na+ + S2- oxidized reduced reducing agent oxidizing agent
4Al + 3O2 4Al3+ + 3O2- Oxidized reduced Reducing agent oxidizing agent
2I- I2 + 2e-
oxidation
Zn2+ + 2e- Zn
reduction
Redox with Covalent CompoundsSome reactions involve covalent compounds. In
these compounds complete electron transfer does not occur.
2H2H22 (g) + O2 (g) 2H2O (l)
In each reactant hydrogen molecule, the bonding electrons are shared equally between the hydrogen atoms.
In water, however, the bonding electrons are pulled toward oxygen because it is much more electronegative than hydrogen.
Redox with Covalent Compounds
2H2H22 (g) + O2 (g) 2H2O (l)
There is a shift of bonding electrons away from hydrogen, even though there is not a complete transfer.
Hydrogen is oxidized because it undergoes a partial loss of electrons.
Redox with Covalent Compounds2H2 (g) + OO2 2 (g) 2H2O (l)
In oxygen, the bonding electrons are share equally between oxygen atoms in the reactant oxygen molecule.
When oxygen bonds to hydrogen in the water molecule, there is a shift of electrons toward oxygen.
Oxygen is thus reduced because it undergoes a partial gain of electrons.
Redox with Covalent Compounds
H H O O H O shift of bonding
e- shared e- shared e- away from H
equally equally H and toward O
H is reducing O is oxidizing agent agent
Redox with Covalent Compounds
Some reactions involving covalent reactants or products, the partial electron shifts are less obvious.
General guideline for covalent reactants or products:
• for carbon compounds, the addition of oxygen or the removal of hydrogen is always oxidation
Processes Leading to Oxidation & Reduction
Processes Leading to Oxidation & Reduction
Oxidation ReductionComplete loss of electrons (ionic reactions)
Complete gain of electrons (ionic reactions)
Shift of electrons away from an atom in a covalent bond
Shift of electrons toward an atom in a covalent bond
Gain of oxygen Loss of oxygen
Loss of hydrogen by a covalent compound
Gain of hydrogen by a covalent compound
Increase in oxidation Increase in oxidation numbernumber
Decrease in oxidation Decrease in oxidation numbernumber
Corrosion
Iron, often used in the form of the alloy steel, corrodes by being oxidized to ions of iron by oxygen.
2Fe(s) + O2 (g) + 2H2O (l) 2Fe(OH)2 (s)
4Fe(OH)2(s) + O2 (g) + 2H2O (l) 4Fe(OH)3 (s)
• Equations describe the corrosion of iron to iron hydroxides in moist conditions.
Corrosion
Water in the environment accelerates the rate of corrosion.
Corrosion occurs more rapidly in the presence of salts and acids.
Salts and acids produce electrically conducting solutions that make electron transfer easier.
Resistance to Corrosion
Not all metals corrode easily.
Gold and platinum are called noble metals because they are very resistant to losing their e- by corrosion.
Other metals lose electrons easily but are protected from extensive corrosion by the oxide coating formed on their surface.
• Aluminum oxidizes quickly in air to form a coating of very tightly packed aluminum oxide particles.
Resistance to Corrosion
Iron also forms a coating when it corrodes
But the coating of iron oxide that forms is not tightly packed.
Water and air can penetrate the coating and attack the iron metal below it.
Corrosion continues until the iron object becomes only a pile of rust.
Controlling Corrosion
To prevent corrosion, the metal surface can be coated with oil, paint, plastic or another metal.
Coatings exclude air and water from the surface, preventing corrosion.
If coating is scratched or worn away, however, the exposed metal will begin to corrode.
Controlling Corrosion
Another method of corrosion control
One metal is “sacrificed” or allowed to corrode, in order to save a second metal.
To protect an iron object, a piece of magnesium (or other active metal) may be placed in electrical contact with the iron.
When oxygen and water attack the iron object, the iron atoms lose electrons as the iron being to be oxidized.
Controlling Corrosion
Because magnesium is a better reducing agent than iron and is more easily oxidized
the magnesium immediately transfers electrons to the iron, preventing their oxidation to iron ions.
Magnesium is “sacrificed” by oxidation and protects the iron in the process.
Controlling Corrosion
Sacrificial zinc and magnesium blocks are sometimes attached to piers and ship hulls to prevent corrosion damage in areas submerged in water.
Underground pipelines and storage tanks may be connected to magnesium block for protection
It is easier and cheaper to replace a block of magnesium or zinc than to replace a bride or a pipeline.
Question
Can you identify the common chemical characteristic of all metal corrosion?
The transfer of electrons from metals to oxidizing agents.
Questions
Define oxidation and reduction in terms of the gain or loss of oxygen.
Oxidation is the gain of oxygenReduction is the loss of oxygen
Define oxidation and reduction in terms of the gain or loss of electrons.
LEO the lion goes GER
Loss of Electrons is Oxidation Gain of Electrons is Reduction
Questions
What happens to the atoms in an iron nail that corrodes?
Iron atoms are oxidized when iron corrodes
How do you identify the oxidizing agent and the reducing agent in a redox reaction?
The species reduced is the oxidizing agent. The species oxidized is the reducing agent.
Questions
Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds)
2Na(s) + Br2(l) 2NaBr(s)
Na oxidized, Br2 reduced
H2(g) + Cl2(g) 2HCl(g)
H2 oxidized, Cl2 reduced
Questions
Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds)
2Li(s) + F2(g) 2LiF(s)
Li oxidized, F2 reduced
S(s) + Cl2(g) SCl2(g)
S oxidized, Cl2 reduced
Questions
Use electron transfer or electron shift to identify what is oxidized and what is reduced in each reaction. (use electronegativity values for molecular compounds)
N2(g) + 2O2(g) 2NO2(s)
N2 oxidized, O2 reduced
Mg(s) + Cu(NO3)2(aq) Mg(NO3)2(aq) + Cu(s)
Mg oxidized, Cu reduced
Questions
Identify the reducing agent and the oxidizing agent for each reaction.
2Na(s) + Br2(l) 2NaBr(s)
Na reducing agent, Br2 oxidizing agent
H2(g) + Cl2(g) 2HCl(g)
H2 reducing agent, Cl2 oxidizing agent
Questions
Identify the reducing agent and the oxidizing agent for each reaction.
2Li(s) + F2(g) 2LiF(s)
Li reducing agent, F2 oxidizing agent
S(s) + Cl2(g) SCl2(g)
S reducing agent, Cl2 oxidizing agent
Questions
Identify the reducing agent and the oxidizing agent for each reaction.
N2(g) + 2O2(g) 2NO2(s)
N2 reducing agent, O2 oxidizing agent
Mg(s) + Cu(NO3)2(aq) Mg(NO3)2(aq) + Cu(s)
Mg reducing agent, Cu oxidizing agent
End of Section 20.1
Oxidation Numbers
Oxidation numberOxidation number is a + or – number assigned to an atom to indicate its degree of oxidation or reduction.
General Rule
A bonded atom’s oxidation # is the charge that it would have if the e- in the bond were assigned to the atom of the more electronegative element.
Rules for Assigning Oxidation Numbers
1.The oxidation number of a monatomic ion is = in magnitude and sign to its ionic charge.
Bromide (Br1-) is -1 Iron III (Fe3+) is +3
2.The oxidation number of hydrogen in a compound is +1, except in metal hydrides, such as NaH, where it is -1
3.The oxidation number of oxygen in a compound is -2 except in peroxides, such as H2O2, where it is -1 and in compounds with the more electronegative fluorine, where it is positive.
Rules for Assigning Oxidation Numbers
4.The oxidation number of an atom in uncombined (elemental) form is 0.
Potassium metal (K) is 0 Nitrogen Gas (N2) is 0
5.For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0
6.For polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion.
Some Thought
Determining oxidation numbers of elements in compounds is a way for chemists to keep track of electron transfer during redox reactions
What are other examples where items are numbered to keep track of movement?
The numbers on a sports player’s jersey
The area codes assigned to telephone numbers in different regions
Binary Ionic Compounds
In binary ionic compounds, such as NaCl and CaCl2, the oxidation numbers of the atoms equal their ionic charges.
Na1+ + Cl-1 NaCl oxidation # +1 -1 neutral
Ca2+ + Cl-1 CaCl2 oxidation # +2 -1 neutral
Note the sign I put before the oxidation number
Molecular Compounds
No ionic charges are associated with atoms of molecular compounds.
However, oxygen is reduced in the formation of water for example.
In water the two shared e- in the H – O bond are
shifted toward the O and away from the H.
Imagine the e- contributed by the two H atoms are completely transferred to the O.
Molecular Compounds
The charge that would result from the transfer are the oxidation numbers of the bonded elements.
The oxidation number of O is -2 and the oxidation number of each hydrogen is +1
Oxidation numbers are often written above the chemical symbols in a formula.
+1 -2H2O
Multiple Oxidation Numbers
Many elements can have several different oxidation numbers.
+1 +6 -2K2CrO4 – Potassium Chromate
+1 +12 -2K2CrO7 – Potassium Dichromate
Sample Problems
What is the oxidation number of each kind of atom in the following ions and compounds?
+4 -2 SO2 +4 -2 CO3
2-
+1 +6 -2 Na2SO4
-3 +1 -2 (NH4)2S
Sample Problems
Determine the oxidation number of each element in the following.
+3 -2 S2O3 +1 -1 Na2O2 +5 -2 P2O5
+5 -2 NO3
-
Sample Problems
Determine the oxidation number of chlorine in each of the following substances.
+1 +5 -2 KClO3 0 Cl2 +2 +7 -2 Ca(ClO4)2
+1 -2 Cl2O
Sample Problems
What are the oxidation numbers of iodine in the following?
+1 +7 -2 HIO4 +1 +5 -2 HIO3 +1 +1 -2 HIO
0 I2
+1 -1
HI
Oxidation Number Changes
When copper wire is placed in a solution of silver nitrate the following reaction occurs
+1 +5 -3 0 +2 +5 -2 02AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)
Ag is reduced from +1 to 0
Cu is oxidized from 0 to +2
Oxidation Number Changes
An increase in the oxidation number of an atom or ion indicates oxidation.
A decrease in the oxidation number of an atom or ion indicates reduction.
+2 +6 -2 0 0 +2 +6 -2 2CuSO4(aq) + Fe2(s) 2Cu(s) + 2FeSO4
Cu is reduced from +2 to 0
Fe is oxidized from 0 to +2
Sample ProblemIdentify which atoms are oxidized and which are
reduced in the following reaction. Also identify the oxidizing agent and the reducing agent.
+1 -1 0 +1 -1 0 2HBr(aq) + Cl2(g) 2HCl(aq) + Br2(l)
Cl is reduced from 0 to -1, so Cl2 is the oxidizing agent
Br is oxidized from -1 to 0, so Br1- is the reducing agent
Sample Problem
Identify which atoms are oxidized and which are reduced in each reaction
0 0 +1 -2 O2(g) + 2H2(g) 2H2O(l)
O2 is reduced from 0 to -2
H2 is oxidized from 0 to +1
Sample Problem
Identify which atoms are oxidized and which are reduced in each reaction
+1 +5 -2 +1 +3 -2 0 2KNO3(s) 2KNO2(s) + O2(g)
N is reduced from +5 to +3
O is oxidized from -2 to 0
Sample Problem
Identify the oxidizing agent and the reducing agent in each equation.
0 0 +1 -2 O2(g) + 2H2(g) 2H2O(l)
O2 is reduced, thus O2 is the oxidizing agent
H2 is oxidized, thus H2 is the reducing agent
Sample Problem
Identify the oxidizing agent and the reducing agent in each equation.
+1 +5 -2 +1 +3 -2 0 2KNO3(s) 2KNO2(s) + O2(g)
N is reduced, thus N is the oxidizing agent
O is oxidized, thus O is the reducing agent
Questions
What is the general rule for assigning oxidation numbers?
The oxidation number is the charge a bonded atom
would have if the electrons in the bond were assigned to the more electronegative element
How is a change I oxidation number related to the process of oxidation and reduction?
An increase in oxidation number indicates oxidation; a decrease in oxidation number indicates reduction.
Sample Problem
Identify which atoms are oxidized and which are reduced in each reaction. Also identify the oxidizing agent and the reducing agent.
0 0 +1 -2 2Na(s) + Cl2(g) 2NaCl(s)
Cl2 is reduced, thus Cl2 is the oxidizing agent
Na is oxidized, thus Na is the reducing agent
Sample Problem
Identify which atoms are oxidized and which are reduced in each reaction. Also identify the oxidizing agent and the reducing agent.
0 0 +1 -2 2HNO3(aq) + 6HI(aq) 2NO(g) + 3I2(s) + 4H2O(l)
O2 is reduced, thus O2 is the oxidizing agent
H2 is oxidized, thus H2 is the reducing agent
End of section 20.2
Identifying Redox ReactionsIn general, all chemical reaction can be
assigned to one of two classes1. Redox reactions in which electrons are
transferred from one reacting species to another. a. Many single-replacement reactions,
combination reactions, decomposition reactions and combustion reactions are redox reactions.
2. All other reactions in which no electron transfer occurs. a. Double-replacement reactions and acid-base
reactions are not redox reactions
What Kind of Reaction Is It?
2Mg(s) + O2(g) 2MgO(s)
Combination reaction – two or more substances react to form a single new substance
2HgO (s) 2Hg (l) + O2 (g)
Decomposition reaction – a single compound breaks down into two or more simpler products.
Many are redox reactions
What Kind of Reaction Is It?
Zn(s) + Cu(NO3)2(aq) Cu(s) + Zn(NO3)2 (aq)
Single Replacement Reaction – one element replaces a second element in a compound.
2C6H18(l) + 25O2 (g) 16CO2(g) + 18H2O(l)
Combustion Reactions – an element or a compound reacts with oxygen often producing energy in the form of heat and light..
Many are redox reactions
What Kind of Reaction Is It?
Na2S(aq) + Cd(NO3)2(aq) CdS(s) + 2NaNO3(aq)
Double Replacement Reaction – involving an exchange of positive ions between two compounds.
2C6H18(l) + 25O2 (g) 16CO2(g) + 18H2O(l)
Combustion Reactions – an element or a compound reacts with oxygen often producing energy in the form of heat and light..
Many are not redox reactions
Identifying Redox Reactions
If the oxidation number of an element in a reacting species change, then that element has undergone either oxidation or reduction.
Many reactions in which color changes occur are redox reactions.
Balancing Redox Equations
Many redox reaction are too complex to be balanced by trial and error.
Two systematic methods are available to balance redox reactions
The two methods are based on the fact that the total number of electrons gained in reduction must equal the total number of electrons lost in oxidation.
One method used oxidation number changes, and the other used half reactions.
Using Oxidation Number Changes to Balance Redox Equations
Oxidation Number Method
You balance by comparing the increases and the decreases in oxidation numbers.
+3 -2 +2 -2 0 +4 -2
Fe2O3(s) + CO(g) Fe(S) + CO2(g)
Step 1 – assign oxidation numbers to all the atoms in the equation.
Using Oxidation Number Changes to Balance Redox Equations
Oxidation Number Method
C oxidized (+2)
+3 -2 +2 -2 0 +4 -2Fe2O3(s) + CO(g) Fe(S) + CO2(g)
Fe reduced (-3)
Step 2 – Identify which atoms are oxidized and which are reduced
Using Oxidation Number Changes to Balance Redox Equations
Oxidation Number Method (+2) oxidized
+3 -2 +2 -2 0 +4 -2Fe2O3(s) + CO(g) Fe(S) + CO2(g)
(-3) reduced
Step 3 – Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction.
Using Oxidation Number Changes to Balance Redox Equations
In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balance by the total decrease in the oxidation number of the species reduced.
Using Oxidation Number Changes to Balance Redox Equations
Oxidation Number Method 3 x (+2) = +6
+3 -2 +2 -2 0 +4 -2Fe2O3(s) + 33CO(g) 22Fe(S) + 33CO2(g)
2 x (-3) = -6
Step 4 – Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.
Using Oxidation Number Changes to Balance Redox Equations
Oxidation Number Method
Fe2O3(s) + 3CO(g) 2Fe(S) + 3CO2(g)
Step 5 – Make sure that the equation is balanced for both atoms and charge.
Sample ProblemOxidation Number Method
S oxidized (+4) x 3= +12
+1 +6 -2 +1 -2 0 +1 -2 +1 +3 -2 +4 -222K2Cr22O7(aq) + H2O(l) + 33S(s) KOH(aq) + 22Cr22O3(s) + 33SO2 (g)
Cr reduced (-3) x 4 = -12
4 Cr atoms must be reduced for each 3 S atom that are oxidized
Sample ProblemOxidation Number Method
S oxidized (+4) x 3= +12
+1 +6 -2 +1 -2 0 +1 -2 +1 +3 -2 +4 -22K2Cr2O7) + 22H2O + 3S 44KOH + 2Cr2O3 + 3SO2
Cr reduced (-3) x 4 = -12
Check the equation and balance by inspection
4 in front of KOH balances potassium2 in front of H2O balances hydrogen and oxygen.
Sample ProblemsBalance each redox equation using the oxidation number change method
KClO3(s) KCl (s) + O2 (g)
2KClO3(s) 2KCl (s) + 3O2 (g)
HNO2 (aq) + HI (aq) NO (g) + I2 (s) + H2O (l)
2HNO2 (aq) + 2HI (aq) 2NO (g) + I2 (s) + 2H2O (l)
Sample ProblemsBalance each redox equation using the oxidation number change method
Bi2S3 + HNO3 Bi(NO3)3 + NO + S + H2O
Bi2S3 + 8 HNO3 2Bi(NO3)3 + 2NO + 3S + 4H2O
SbCl5 + KI SbCl3 + KCl + I2
SbCl5 + 2KI SbCl3 + 2KCl + I2
Using Half Reactions to Balance Redox Equations
Half Reaction Method
Half reaction is an equation showing just the oxidation or just the reduction that takes place
You write and balance the oxidation and reduction half reaction separately before combining them into a balanced redox equation
Then you balance the electrons gained in the reduction with the electrons lost in the oxidation.
Using Half Reactions to Balance Redox Equations
Half Reaction Method
S + HNO3 SO2 + NO + H2O (unbalanced)
S + H+ + NO3- SO2 + NO + H2O
In this case only HNO3 is ionized. The products are covalent compounds
Step 1 – write the unbalanced equation in ionic form
Using Half Reactions to Balance Redox Equations
Half Reaction Method
Oxidation Half Reaction 0 +2
S SO2 Reduction Half Reaction +5 +2
NO3- NO
Step 2 – Write separate half reactions for the oxidation and reduction processes.
Using Half Reactions to Balance Redox Equations
Half Reaction Method0 +2
S SO2
Sulfur is already balanced, but oxygen is not.
The reaction takes place in acid solution, so H2O and H+ are present and can be used to balance oxygen and hydrogen as needed.
2H2H22OO + S SO2 + 4H4H++
Step 3 – Balance the atoms in the half reactions.
Using Half Reactions to Balance Redox Equations
Half Reaction Method +5 +2
NO3- NO
Nitrogen is already balanced, but oxygen is not.
4H4H++ + NO3- NO + 2H2H22OO
Using Half Reactions to Balance Redox Equations
Half Reaction Method 0 +4
2H2O + S SO2 + 4H+ + 4e4e - -
S is oxidized going from 0 to +4, a loss of 4 e-
+5 +2 3e3e-- + 4H+ + NO3
- NO + 2H2O
N is reduced going from +5 to +2, a gain of 3 e-
Step 4 – Add enough electrons to one side of each half reaction to balance the charges.
Using Half Reactions to Balance Redox Equations
Half Reaction Method 0 +4
(x 3)(x 3) 6H2O + 3S 3SO2 + 12H+ + 12e -
S is oxidized going from 0 to +4, a loss of 4 e-
+5 +2 (x 4) 12(x 4) 12e-- + 16H+ + 4NO3
- 4NO + 8H2O
N is reduced going from +5 to +2, a gain of 3 e-
Step 5 – Multiply each half reaction by an appropriate number to make the numbers of electrons equal in both.
Using Half Reactions to Balance Redox Equations
Half Reaction Method6H2O + 3S 3SO2 + 12H+ + 12e -
12e- + 16H+ + 4NO3
- 4NO + 8H2O
6H2O + 3S + 16H+ + 4NO3- + 12e - 3SO2 + 12H+ +
4NO + 8H2O + 12e –
3S + 4H+ + 4NO3- 3SO2 + 4NO + 2H2O
Step 6 – Add the balanced half reactions to show an overall equation and then subtract the terms that appear in both sides of the equation
Using Half Reactions to Balance Redox Equations
Half Reaction Method
3S + 4HNO3- 3SO2 + 4NO + 2H2O
Spectator ionSpectator ion – are present during a reaction, but do not participate in or change during a reaction.
Because none of the ions in the reactants appear in the products, there are no spectator ions in this particular example.
Step 7 – Add the spectator ions and balance the equation.
Sample Problem
Half Reaction Method
KMnO4 + HCl MnCl2 + Cl2 + H2O + KCl
K+ + MnO4- + H+ + Cl- Mn2+ + 2Cl- + Cl2 + H2O + K+ + Cl-
Step 1 – write the unbalanced equation in ionic form
Sample Problem
Half Reaction Method
Reduction Half Reaction +7 +2
MnO4- Mn2+
Oxidation Half Reaction -1 0
2Cl- Cl2
Step 2 – Write separate half reactions for the oxidation and reduction processes.
Sample Problem
Reduction Half Reaction +7 +2
8H+ + MnO4- Mn2+ + 4H4H22OO
Oxidation Half Reaction -1 0
2Cl- Cl2
Solution is acidic, so H2O and H+ ions are used to balance equation. (If solution is basic, H2O and OH- are used)
Step 3 – Balance the atoms in the half reactions.
Sample Problem
Half Reaction Method +7 +2
5e5e-- + 8H+ + MnO4- Mn2+ + 4H2O
Mn is reduced going from +7 to +2, a gain of 5 e-
-1 02Cl- Cl2 + 2e-2e-
Cl is oxidized going from -1 to 0, a loss of 2 e-
Step 4 – Add enough electrons to one side of each half reaction to balance the charges.
Sample Problem
Half Reaction Method +7 +2
(x 2)(x 2) 10e- + 16H+ + 2MnO4- 2Mn2+ + 8H2O
Mn is reduced going from +7 to +2, a gain of 5 e-
-1 0(x5)(x5) 10Cl- 5Cl2 + 10e-
Cl is oxidized going from -1 to 0, a loss of 2 e-
Step 5 – Multiply each half reaction by an appropriate number to make the numbers of electrons equal in both.
Sample Problem
Half Reaction Method10e- + 16H+ + 2MnO4
- 2Mn2+ + 8H2O
10Cl- 5Cl2 + 10e-
10e- + 16H+ + 2MnO4- + 10Cl- 2Mn2+ + 8H2O + 5Cl2
+ 10e-
16H+ + 2MnO4- + 10Cl- 2Mn2+ + 8H2O + 5Cl2
Step 6 – Add the balanced half reactions to show an overall equation and then subtract the terms that appear in both sides of the equation
Sample Problem
Half Reaction Method
16H+ + 6Cl- + 2MnO4- + 2K+ + 10Cl-
5Cl2 + 2Mn2+ + 4Cl- + 8H2O + 2K+ + 2Cl-
16H+ = 6Cl- + 10Cl- (from the HCl in original equation)
2MnO4- = 2K+ (from the KMnO4 in original equation)
2Mn2+ = 4Cl- (from the MnCl2 in original equation)
2K+ + 2Cl- (from the KCl in original equation)
Step 7 – Add the spectator ions and balance the equation.
Sample Problem
Half Reaction Method
16H+ + 16Cl- + 2MnO4- + 2K+
5Cl2 + 2Mn2+ + 6Cl- + 8H2O + 2K+
Adding spectator and non-spectator Cl- on each side gives
16HCl + 2KMnO4 5Cl2 + 2MnCl2 + 8H2O + 2KCl
Step 7 – Add the spectator ions and balance the equation.
Sample Problem 2
The following takes place in basic solution. Balance the equation using the half reaction method
Zn + NO3
- NH3 + Zn(OH4)2-
4Zn + NO3- + 6H2O + 7OH-
4Zn(OH4)2- + NH3
Sample Problem 3
The following takes place in basic solution. Balance the equation using the half reaction method
Zn + As2O3 AsH3 + Zn2+
6Zn + As2O3 + 9H2O 6Zn2+ + 2AsH3 + 12OH-
Choosing a Balancing Method
In some redox reactions, the same element is both oxidized and reduced. (called a disproportional reaction)
3Br2 + 6KOH 5KBr + KBrO3 + 3H2O
Br is reduced from 0 to -1Br is oxidized from 0 to + 5
Equations like above and equation for reactions that take place in acidic or alkaline solution are best balanced by the half reaction method.
End of Chapter 20