Slide 1 / 83 This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org New Jersey Center for Teaching and Learning Progressive Science Initiative Slide 2 / 83 www.njctl.org Unit 3: Presentation E Redox Reactions AP Chemistry Slide 3 / 83 REDOX Reactions Photosynthesis is a classic REDOX reaction where electrons are transferred from water to carbon dioxide to create a new sugar molecule. Slide 4 / 83 Oxidation and Reduction REDOX reactions involve the transfer of electrons from one species to another. The substance that gains electrons is said to be reduced while the substance that loses electrons is said to be oxidized. Cu(s) + Ag + (aq) --> Cu + (aq) + Ag(s) In this reaction, a copper atom loses an electron to a silver ion. Cu(s) got oxidized Ag + (aq) got reduced Slide 5 / 83 Oxidation and Reduction Oxidation Reduction Loses electrons (LEO) gains electrons (GER) oxidation state increases oxidation state decreases electrons are product Na(s) --> Na + (aq) + e - electrons are reactant Mg 2+ (aq) + 2e - --> Mg(s) Slide 6 / 83 Oxidation States In order to determine whether a substance got oxidized or reduced in a reaction, the oxidation state of each species must be known. If the substance is found in its neutral standard state, its oxidation state will be zero. Na(s) = 0 or F2(g) = 0 or C(graphite) = 0 If the substance is a molecular compound made of more than one element, the sum of the oxidation states must equal 0 C O 2 The more electronegative of the two receives a charge equivalent to it's normal ionic charge. -2 The less electronegative of the two receives the + charge necessary to cancel out the negative charge +4
Transcript
Slide 1 / 83
This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others.
Click to go to website:www.njctl.org
New Jersey Center for Teaching and Learning
Progressive Science Initiative
Slide 2 / 83
www.njctl.org
Unit 3: Presentation E
Redox Reactions
AP Chemistry
Slide 3 / 83
REDOX Reactions
Photosynthesis is a classic REDOX reaction where electrons are transferred from water to carbon dioxide to create a new
sugar molecule.
Slide 4 / 83
Oxidation and ReductionREDOX reactions involve the transfer of electrons from one
species to another. The substance that gains electrons is said to be reduced while the substance that loses electrons is said
to be oxidized.
Cu(s) + Ag+(aq) --> Cu+
(aq) + Ag(s)
In this reaction, a copper atom loses an electron to a silver ion.
Oxidation StatesFor ionic species, the sum of the oxidation states must equal
the charge on the ion.
S O32-
+4 -2For ionic compounds, first separate the compound into its
ions, then determine charge on each element.
Mg Cr2O7+2 -2
Cr2 O7 + 6 -2
Slide 8 / 83
Oxidation States
Peroxides and hydrides
In a peroxide (H2O2 or Na2O2) O carries an oxidation state of -1.
In a hydride (NaH or CaH2), H carries an oxidation state of -1.
Slide 9 / 83
1 Oxidation/Reduction reactions study the transfer of ________ between atoms and molecules.
A protons
B neutrons
C H+ ions
D OH- ions
E electrons
Slide 10 / 83
2 Which of the substances found in the reaction below would contain an element with an oxidation number = 0?10Au(s) + 36H+(aq) + 6NO3-(aq) --> 3N2(g) + 18H2O(l) + 10Au3+(aq)
A AuB HNO3
C Au and N2
D Au and H2OE N2 and Au3+
Slide 11 / 83
3 In the substance glucose (C6H12O6), the sum total of the oxidation states must equal _____.
A 0
B +1
C The sum total of the number of valence electrons present in each element
D +3E 24
Slide 12 / 83
4 In the compound IBr, which element would be the most electronegative and what would it's oxidation state be?
I Br
A I, -1
B Br, -1
C I, 0
D Br, 0
E Br, +1
Slide 13 / 83
5 The oxalate ion, C2O42-, is one of the common components of kidney stones! Which is the more electronegative atom in the ion and what is it's oxidation state?
A C, +4
B C, -2
C C, -4
D O, -2E O, -4
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6 The polyatomic ion cyanide (CN-) is toxic and used in the mining of gold in America's desert southwest. Which of the following could be a correct set of oxidation states for C and N in the ion?
A C = +4, N = -3B C = +4, N = 0C C = +2, N = -3
D C = 0, N = +1
E C = -3, N = +4
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7 What is the oxidation state of O in the compound N2O (laughing gas) and what would be the total negative charge from all the oxygens?
N N OA -2, -2
B -1, 0
C -2, 0
D -2, -4E -2, -4
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8 What would be the oxidation state of the fluorine atom in the compound OF2 and what would be the total negative charge from the Flourines?
A -1, -1B 0, -1C -1, -2D -1, -3E -1, 0
Slide 17 / 83 Slide 18 / 83
10 What is the oxidation number of each oxygen atom in the compound MnO 2 ?
A -2
B -1
C 0
D +1
E +2
Slide 19 / 83
11 What is the oxidation state of S in the S2O32- ion? The S2O32- ion can do great harm to metals like iron.
A -2
B +2
C -1D +4E 0
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12 What is the oxidation state of each atom in CaC2O4?
A Ca = 0, C = +2, O = -2
B Ca = +2, C = +2, O = -2C Ca = +4, C = +4, O =-2D Ca = +2, C = +3, O = -2E Ca = +2, C = +4, O = -2
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13 In 2004, Coca cola had to recall their Dasani brand water because of the presence of the bromate ion in the drinking water. What would be the oxidation state of all atoms in CuBrO3?
A Cu = +2, Br = -1, O = -2
B Cu = +1, Br = +3, O = -2C Cu = +2, Br = +4, O = -2D Cu = +1, Br = +5, O =-2
E Cu = 0, Br = +6, O =-2
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14 Chloroform, CHCl3, has wide uses. It is used as an anesthetic, a refrigerant, and in the making of teflon. What is the oxidation state of carbon in chloroform?
A -2
B -1
C 0
D +2
E -3
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15 In which of the following compounds would oxygen have an oxidation state of -1?
A Li2O2
B Mg(OH)2
C CH3OH
D O2 E Li2O
Slide 24 / 83
REDOX ReactionsTo determine which substances got oxidized or reduced, look at
the change in the substances oxidation state.
H+(aq) + I O3
-(aq) + F-
(g) --> I2(s) + F2(aq) + H2 O(l)
+1 +5 -2 -1 0 0 +1 -2
IO3-(aq) got reduced = +5 --> 0
F2(g) got oxidized = 0 --> -1
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16 Which of the following represents a reduction?
A An elements charge going from -3 --> -2
B An elements charge going from -2 -->-3
C An elements charge going from 0 --> +2
D An elements charge going from +1 --> +3
E None of these
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17 Which of the following represents an oxidation?
A Al3+(aq) --> Al(s)
B F2(g) --> 2F-(aq)
C Mn7+ --> Mn2+
D Fe2+(aq) --> Fe3+
(aq)
E O-(aq) --> O2-
(aq)
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18 In which of the following equations does O get oxidized?
A 2H2O2 -- 2H2O + O2
B 2H2O --> 2H2 + O2
C CH4 + 2O2 --> CO2 + 2H2OD 3Fe + 3O2 --> 2Fe2O3
E A and B
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19 Which of the following is NOT an oxidation/reduction reaction?
A Ca + 2H+ --> Ca2+ + H2
B H2 + Cl2 -->2HCl
C 4Al + 3O2 --> 2Al2O3
D CaO + CO2 --> CaCO3
E Mg3N2 --> 3Mg + N2
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20 Which of the following would be TRUE regarding the reaction that occurs below. Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) --> 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq)
A Cr2O72- gets oxidized to Cr3+
B Fe2+ gets reduced to Fe3+
C H+ gets reduced to H2O
D Cr2O72- gets oxidized to H2O
E Fe2+ gets oxidized to Fe3+
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Oxidizing and Reducing Agents
Any substance that gets oxidized is called a reducing agent or reducer becomes the electrons lost will be gained by another
species.
Any substance that gets reduced is called an oxidizing agent or oxidizer becomes the electrons gained will be lost by
another species.
Remember: Reducing Agents are Oxidized
Oxidation Agents are Reduced.
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Oxidizing and Reducing Agents
Good reducing agent must lose electrons easily. Neutral metals or substances with low ionization energies fit the bill.
For example: Na, I-, Fe2+
Good oxidizing agents must gain electrons easily. Neutral or positive non-metals or highly positive metals fit the bill here.
For example: MnO4-, CrO4
2-, F2, O2, HNO3
Slide 32 / 83
21 Identify the oxidizing agent in the following reaction.
Al(s) + 6HCl(aq) --> AlCl3(aq) + H2(g)
A AlB AlCl3C HCl
D H2
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22 Which substance is oxidized in the following reaction? (First, assign oxidation numbers.)
Cu + S ➝ CuS
A Cu
B S
C Cu and S
D CuS
E This is not a redox reaction.
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23 Which substance is the reducing agent below?
Cu + S ➝ CuS
A Cu
B S
C Cu and S
D CuS
E This is not a redox reaction.
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24 Which substance is oxidized in the following reaction? (First, assign oxidation numbers.)
Ca + Fe3+ ➝ Ca2+ + Fe
A Ca
B Fe3+
C Ca2+
D Fe
E This is not a redox reaction.
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25 Which substance is the reducing agent?
3 K + Al(NO3)3 ➝ Al + 3 KNO3
A K
B Al(NO3)3
C KNO3
D This is not a redox reaction.
Slide 37 / 83
26 Which of the following would make the best oxidizing agent?
A H2O
B IO3-
C IO2-
D I2
E I-
Slide 38 / 83
27 Which of the following would make the best reducing agent?
A F2(g)
B Cl2(g)
C K(s)
D Al(s)
E C(s)
Slide 39 / 83
28 Good oxidizing agents will have low ionization energies.
Yes
No
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29 Which of the following would make the best oxidizing agent?
A HNO
B HNO2
C HNO3
D Na
E H2O
Slide 41 / 83
30 Which of the following would make the best reducing agent?
A F-
B Cl-
C Br-
D I-
E O2-
Slide 42 / 83
Balancing REDOX Reactions
In a redox reaction, both the atoms AND electrons must be balanced.
Example: Cu2+(aq) + Ag(s) --> Cu(s) + Ag+
(aq)
Gained 2 e- Lost 1 e-
Even though there is one of each atom on each side, the equation is not balanced because the number of electrons lost does not equal the number gained. This can be changed by reacting more Ag atoms so more electrons are lost.
Balanced: Cu2+(aq) + 2Ag(s) --> Cu(s) + 2Ag+
(aq)
Gained 2 e- Lost 2 e-
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Balancing REDOX Reactions
Example: Balance the following:
Al(s) + Fe2+(aq) --> Al3+
(aq) + Fe(s)
2Al(s) + 3Fe2+(aq) --> 2 Al3+(aq) + 3Fe(s)
Lost 6e- Gain 6e-
move for answer
Slide 44 / 83
31 What is the correct set of coefficients when the reaction below is balanced?
Cu+(aq) + Mg(s) --> Cu(s) + Mg2+(aq)
A 1,2,1,2
B 1,1,1,1C 2,1,2,1D 2,2,1,1E 1,1,2,2
Slide 45 / 83
Balancing REDOX Reactions
REDOX reactions occur in solutions just like most of the reactions we do in Chemistry. REDOX reactions are different because the medium in which the reaction occurs affects the results of the reactions.
Since REDOX reactions depend on the movement of electrons, certain chemical species - like rogue H + ions (aka protons) - can have an impact on how the reaction occurs.
Therefore, when balancing a REDOX reaction it is important to note if there is an excess or deficit of H + ions. In other words, is the solution in which the reaction is occurring Acidic (excess of H +) or Basic (deficit of H+). Acidic and Basic solutions have slightly different rules for balancing in each scenario.
Slide 46 / 83
Guidelines for Balancing REDOX Reactions1. Determine Oxidation states of all elements
2. Determine which element was oxidized, which was reduced
3. Write half reactions for the element that was oxidized and the element that was reduced
4. Using coefficients, balance the elements (all non-hydrogen or non-oxygen elements) that are being oxidized/reduced in each half reaction
5. Balance remaining oxygen atoms in water molecules in each half reaction
6. Balance the remaining hydrogen atoms with H+ ions in each half reaction
7. Balance charge with electrons (e-) in each half reaction
8. Multiple coefficients in each half reaction by a number to make sure electrons are balanced
9. Recombine the two half reactions by adding them together. Cancel out any redundant chemical species - electrons should always cancel out
Slide 47 / 83
MnO4-(aq) + Fe2+(aq) --> Fe3+(aq) + MnO2(s)
MnO4-
O:-2 Mn + (4*-2)=-1
Therefore:Mn: +7Fe: +2
MnO2
O:-2 Mn + (2*-2)=0
Therefore:Mn:+4Fe: +3
Step 1) Determine Oxidation states of all elements
Acidic REDOX Reactions Example
Slide 48 / 83
MnO4-(aq) + Fe2+(aq) --> Fe3+(aq) + MnO2(s)
MnO4-
O:-2 Mn + (4*-2)=-1
Therefore:Mn: +7Fe: +2
MnO2
O:-2 Mn + (2*-2)=0
Therefore:Mn:+4Fe: +3
Step 2) Determine which element got oxidized and which element got reduced
Because Mn went from an oxidation state of +7 to +4, it must have been reduced.
Because Fe went from an oxidation state of +2 to +3, it must have been oxidized. (It is able to accept more oxygen now since it is more positive)
Question: Mn is the oxidizing agent and Fe is the reducing agent.
Acidic REDOX Reactions Example
Which agent is the oxidizing agent?
Slide 49 / 83
MnO4-(aq) + Fe2+(aq) --> Fe3+(aq) + MnO2(s)
Step 3) Write half reactions for the element that was oxidized and the element that was reduced
MnO4-(aq) --> MnO2(s)
Fe2+(aq) --> Fe3+(aq)
Reduction half reaction
Oxidation half reaction
Note: A half reaction is literally just one half of a reaction. It is useful sometimes in REDOX reactions to look at the reduction half and the oxidation half separately, and to write a half reaction just look at the change in the species reducing (reduction half reaction) and the change in the species oxidizing (oxidation half reaction).
REMEMBER: Since a half reaction is only PART of a chemical reaction it may include invalid chemical species (like lone electons, cations, or anions). When you originally write half reactions they may not be balanced. THAT IS OKAY!
Acidic REDOX Reactions Example
Slide 50 / 83
Step 4) Using coefficients, balance the elements that are being oxidized/reduced in each half reaction
MnO4-(aq) --> MnO2(s) Fe2+(aq) --> Fe3+(aq)
Reduction half reaction Oxidation half reaction
Mn is already balanced! Fe is already balanced!
Acidic REDOX Reactions Example
Slide 51 / 83
Step 5) Balance the remaining Oxygen atoms with water molecules in each half reaction
Determine which element was oxidized, which was reduced
Reduced: NO3-(N went from +5 -->+4) Gained 1 e-
Oxidized: I2 (I went from 0 -->+5) Lost 5 e -
move for answer
move for answer
Balancing REDOX Reactions in Acid Solutions
Slide 58 / 83
Using coefficients, balance the elements that are being oxidized/reduced
NO3-(aq) + I2(s) --> IO3-(aq) + NO2(g)
NO3-(aq) + I2(s) --> 2IO3
-(aq) + NO2(g)
Multiply coefficients by a number to make sure electrons are balanced
10NO3-(aq) + I2(s) --> 2IO3
-(aq) + 10NO2(g)
Gain 10 e- Lose 10 e-
move for answer
move for answer
Balancing REDOX Reactions in Acid Solutions
Slide 59 / 83
Balance remaining oxygen atoms with water molecules
NO3-(aq) + I2(s) --> IO3-(aq) + NO2(g)
10NO3-(aq) + I2(s) --> 2IO3
-(aq) + 10NO2(g) + 4H2O(l)
Balance remaining H atoms with H+ ions
8H+(aq) + 10NO3
-(aq) + I2(s) --> 2IO3
-(aq) + 10NO2(g) + 4H2O(l)move for answer
move for answer
Balancing REDOX Reactions in Acid Solutions
Slide 60 / 83
NO3-(aq) + I2(s) --> IO3-(aq) + NO2(g)
Reactants Products
N atoms = 10 N atoms = 10
I atoms = 2 I atoms = 2
O atoms = 30 O atoms = 30
H atoms = 8 H atoms = 8
Total charge = 2- Total charge = 2-
Check that all atoms and charges are balanced
8H+(aq) + 10NO3
-(aq) + I2(s) --> 2IO3
-(aq) + 10NO2(g) + 4H2O(l)
Move for answer
Balancing REDOX Reactions in Acid Solutions
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32 What is the proper coefficient in front of CO2 after the equation below is balanced in an acidic medium?H+(aq) + MnO4− (aq) + C2O42− (aq) ➝ Mn2+ (aq) + CO2 (aq) + H2O(l)
A 1B 2
C 4D 6E 10
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33 What is the coefficient of the bromide ion after the following REDOX reaction is balanced?
A 1
B 2
C 4
D 5
E 6
N2H4(l) + BrO3-(aq) --> N2(g) + Br-(aq) + H2O(l)
Slide 63 / 83
34 For the reaction represented below, how many electrons will be transferred when the equation is properly balanced?
Cu + NO3- --> NO2 + Cu2+
A 2B 4
C 5D 6E 8
Slide 64 / 83
Guidelines for Balancing Basic REDOX Reactions
To balance REDOX reactions that occur under basic conditions, follow the same steps for acidic conditions with one key difference.
Recall, in an acidic solution there are excess H+ ions, which is why it is okay to leave H+ in the final reaction
However, in a basic solution there are no excess H+ ions, so we must eliminate the H+ ions. We do that by adding something that exists in excess in a basic solution - OH- ions.
Balancing Basic REDOX Reactions BONUS STEP:
10. Add OH- ions to both sides of your reaction to cancel out H+ ions and convert them to water. *Remember, cancel out any redundant chemical species you create in doing this!
Slide 65 / 83
Basic REDOX Reactions ExampleStep 10) Add OH- ions to both sides of your reaction to cancel out
excess H+ ions and convert them to water. *Remember, cancel out any redundant chemical species you create in doing this!
MnO4-(aq) + 4H+(aq) + 3Fe2+
(aq)--> MnO2(s) + 2H2O(l) + 3Fe3+(aq)
4 OH- ions are needed on each side to cancel out the H+ ionsMnO4
35 When the following reaction is balanced in basic solution, what is the proper coefficient in front of the hydroxide ion and which side of the reaction is it on?
MnO4-(aq) + IO3-(aq) --> MnO2(s) + IO4-(aq)
A 3, product sideB 2, product sideC 2, reactant sideD 3, reactant sideE 1 reactant side
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36 When the equation below is balanced in basic solution, what are the proper coefficients for the hydroxide and bromide ions respectively?
Br2(l) + OH-(aq) --> Br-(aq) + BrO3-(aq) + H2O(l)
A 6OH- and 5Br-
B 3OH- and 2Br-
C 6OH- and 4Br-
D 2OH- and 2Br-
E 5OH- and 3Br-
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37 What is the coefficient of the hydroxide ion after the following REDOX reaction has been balanced?
REDOX StoichiometryTitrations can be utilized in REDOX reactions to determine the
concentration of an analyte.
By oxidizing or reducing an analyte with a standardized reductant or oxidant titrant, the amount of analyte can be determined.
standardized titrant
unknown analyte
Slide 77 / 83
REDOX StoichiometryThe end point of a titration is often accompanied by a color
change as a substance is either formed or created. It is then important to know some colors for some important oxidants and
reductants.
MnO4-(aq) = purple
CrO42-
(aq) = yellow
Cr2O72-
(aq) = orange
Slide 78 / 83
REDOX Stoichiometry
Transition metal ions also form complex ions that have particular colors.
Fe3+(aq) = yellow/brown
Co2+(aq) = pink
Cu2+(aq) = blue
Cr3+(aq) = green
Slide 79 / 83
REDOX Stoichiometry
Let's examine a titration of potassium permanganate with iron(II) nitrate in acidic medium
8H+(aq) + MnO4
-(aq) + 5Fe2+
(aq) --> Fe3+(aq) + Mn2+
(aq) + 4H2O(l)
As the permanganate disappears, so does the purple color and as the Fe3+ is made, the yellow/brown color appears.
When the purple is gone, the permanganate has all reacted.
Excess MnO4-(aq) Excess Fe3+
(aq)
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38 How many mL of 2M HCl would need to be added to 10 mL of 0.34 M solution of potassium permanganate in order for all of the permanganate to be titrated by the reaction below?
