Download Name: Chemistry Honors Date: Period: ____ Reduction/Oxidation

Name: ________________________
Date: ______________ Period: ____
Chemistry Honors
Reduction/Oxidation and Electrochemistry
Reduction-Oxidation Reactions and Electrochemistry
Read this!
What is electrochemistry and how can it be helpful to us?
This is a question that could have a multitude of answers! As the name may suggest,
electrochemistry deals with the chemistry of electrons: here electrons are and how they can move.
Electrochemistry is the study of electron movement as it relates to chemical reactions. From
photosynthesis to a rusting ship, electrochemical processes can be found at every level of life.
Before we can dive into this field of chemistry, we need to take a closer look at what reductionoxidation reactions are.
We have learned the definition of reduction-oxidation reactions. What exactly does it mean to be
oxidized or reduced?
We have also learned how to track electrons in simple reactions that include ionic compounds. How
can we track the movement of electrons in chemical reactions involving covalent compounds?
Remember, oxidation and reduction (redox) occurs in every type of chemical reaction except double
replacement.
Model 1: In order to track electron movement we have to watch for changes in charge. But for
molecular substances we must use oxidation numbers. Assigning oxidation number is a system
developed by scientists. They can be positive or negative numbers, but do not confuse them with
charges of ions.
There are a few rules to assign oxidation numbers:
1.
2.
3.
4.
Elemental substances = 0
Alkali metals = +1
Alkaline earth metals = +2
Metals in an ionic compound = the charge of the metal
These first four rules should be easy to remember since they are in line with what we would
do to assign charges to the elements an metal ions.
5.
6.
7.
8.
Fluorine in a compound = -1
Oxygen in a compound = -2 (except for peroxides, then it is -1; with fluorine, +2)
Hydrogen = +1 (except in hydride compounds, then -1)
The oxidation states of other elements can be determined because the sum of the
oxidation #’s of all of the atoms in a substance is equal to the net charge of the
substance.
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Adapted from Chemistry: Electro Chemistry and Galvanic Cells and POGIL
What is a hydride?
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Practice Assigning Oxidation Numbers
Identify the oxidation number each atom, by writing the oxidation number above each atom.
Example
-3 +1
NH3 << this is not a hydride.
1. CO2
2. NaCl
3. CaCO3
4. SO42-
5. H2O2 (hydrogen peroxide)
6. SO3
7. NaH
8. Br2
9. Zn
10. BaSO3
11. NH4+1
12. N2O5
13. KMnO4
14. CH4
15. H3PO4
16. Cr2O7-2
17. Cu
18. ZnC2O4
19. Ti+4
20. NH4Cl
Visit the link below for a short review by Hank!
https://www.youtube.com/watch?v=lQ6FBA1HM3s
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Reduction Oxidation POGIL
Read this!
Just like the formation of precipitate can be shown more clearly by writing a net ionic
equation, electron transfers can be illustrated by writing half reaction equations. They
help us to keep track of how many electrons are gained or loss by a particular species
during a chemical reaction. These half-reactions can be useful in understanding how
batteries work, and how other reactions that occur in nature and in your body work.
Model 2: Strategy for Balancing Redox Equations
Example Equation: Cu(NO3)2 + Al  Al(NO3)3 + Cu
Step 1: Assign oxidation numbers.
+2+5-2
0
+3+5+-2 0
Cu(NO3)2 + Al  Al(NO3)3 + Cu
Step 2: Identify the species with an increase in charge (the oxidized species) from the equation, and add
the necessary # of electrons to the product side.
Al  Al3 + 3e-
(oxidation half-reaction)
Step 3: Identify the species with the reduction in charge (the reduced species) from the equation, and
add the necessary # of electrons to the reactant side.
Cu+2 + 2e-  Cu
(reduction half-reaction)
Note: The overall net charge on both sides of each half reaction is equal.
Step 4: Multiply each half-reaction by a factor to make the number of electrons gained in one halfreaction equal to the number of electrons lot in the other. Add the two half-reactions together.
Step 5: Check to be sure both atoms and charge are balanced on both sides of the equation.
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Reduction Oxidation POGIL
Questions:
1. In the model, are electrons lost or gained during oxidation?
Are electrons lost or gained during reduction?
2. In the model, what happens to the oxidation number during oxidation?
What happens to the oxidation number during reduction?
3. What is the relationship between the number of electrons gained and the number of electrons
lost in the reaction?
4. What part of the equation can you change if the reaction is not balanced as written?
Practice
1. 2 H2O (l)  2 H2 (g) + O2 (g)
Species oxidized:
Species Reduced:
Write the half reaction for each species:
a. Oxidation half reaction:
b. Reduction half reaction:
c. Balanced equation
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Reduction Oxidation POGIL
Follow the same process for the following processes.
2. 3 Mg + 2 AlCl3  3 MgCl2 + 2Al
a. Oxidation half reaction:
b. Reduction half reaction:
3. Na2O + 2 Li  Li2O + 2Na
a. Oxidation half reaction:
b. Reduction half reaction:
4. Zn + AgNO3  Zn(NO3)2 + Ag
a. Oxidation half reaction:
b. Reduction half reaction:
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Reduction Oxidation POGIL
5. Redox reactions save lives! Airbags in automobiles are inflated with nitrogen gas produced by
two redox reactions. The gas generator in some bags contains sodium azide (NaN3) and iron (III)
oxide (Fe2O3). The mixture is automatically ignited during a head-on collision. When this
happens, the sodium azide decomposes in a redox reaction to form sodium and nitrogen. The
sodium produced by this reaction then reacts with the iron (IIIO oxide as represented in the
unbalanced equation:
Na (s) + Fe2O3 (s)  Na2O (s) + Fe (s)
Write the unbalanced oxidation and reduction half-reactions for the above redox reaction.
Balance the equation for the reaction shown in problem 1.
Reflection on Learning
Name three insights that your team discovered about oxidation and reduction half-reactions after
examining the model.
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Reduction Oxidation POGIL
Read this!
In certain situations, we are concerned with determining which substance is causing the oxidation or
reduction of another substance. In these situations, we identify the oxidizing agent and/or reducing
agent.
The oxidizing agent causes the oxidation of the other substance. This substance is the one being
reduced.
The reducing agent causes the reduction of the other substance. This substance is the one being
oxidized.
1. Assign oxidation numbers
2. Determine which substance is being oxidized/reduced.
3. Which is the oxidizing agent? Which is the reducing agent?  This step depends only on your
understanding of the vocabulary.
Practice
1. Identify the species being oxidized and reduced in each of the following reactions:
a. Cr+ + Sn4+
Cr3+ + Sn2+
Reducing Agent:
b. 3 Hg2+ + 2 Fe (s)
Oxidizing Agent:
3 Hg2 + 2 Fe3+
Reducing Agent:
c. 2 As (s) + 3 Cl2 (g)
Reducing Agent:
Oxidizing Agent:
2 AsCl3
Oxidizing Agent:
2. Would you use an oxidizing agent or reducing agent in order for the following reactions to
occur?
a. ClO3-
ClO2
b. SO42-
c. Mn2+
MnO2
d. Zn
S2ZnCl2
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Reduction Oxidation POGIL
Read this!
Reactions often times take place in aqueous solutions. Dissolved ions, neutral atoms, and molecular
liquids and gases are all involved. Many times the solutions are also acidic or basic and can include H+
or OH- ions. These reactions are very hard to balance using the method used before because both
atoms and charge must be balanced at the same time. Below is another set of rules to make balancing
these equations easier.
Model 3: Balancing Redox Equations in Acidic Solution
Directions Perform each step as it is listed.
___Fe (s) + ___NO3-1 (aq) + ___ H+ (aq)  ___ Fe+2(aq) + ___NO (g) + ___H2O (l)
Step 1. Assign Oxidation Numbers
___Fe (s) + ___NO3-1 (aq) + ___ H+ (aq)  ___ Fe+2(aq) + ___NO (g) + ___H2O (l)
Step 2. Identify the species being oxidized and reduced from the changes in their oxidation numbers.
Oxidized:
Reduced:
Step 3. Write the two unbalanced equation for the oxidation and reduction half reactions. Write the
compounds as compounds do not separate them as done previously.
Oxidation:
Reduction:
Step 4. Balance elements in the half reaction except oxygen and hydrogen.
Oxidation:
Reduction:
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Reduction Oxidation POGIL
Step 5. Balance oxygen by adding H2O to the appropriate side, and balance hydrogen by adding H+ to
the other side.
Oxidation:
Reduction:
Step 6. Balance the electric charges by adding electrons to the reactant side for the reduction and the
product side for the oxidation until the net charge on both sides of the equation is equal.
Oxidation:
Reduction:
Step 7 and 8. Multiply all species in either 1 or both of the half reactions that result in equal numbers of
electrons in the two half reactions and then add the two equation and includes physical states. Simplify
by canceling out species that appear on both sides of the arrow and check to make sure charge and # of
atoms are balanced.
Oxidation:
Reduction:
Finished reaction:
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Reduction Oxidation POGIL
Practice: Balance the following reactions in acidic solution. You must show all work!!
1. Cr2O72- (aq) + NO2-1 (aq)  Cr3+ (aq) + NO3- (aq)
2. As (s) + ClO3-1 (aq)  H3AsO3 (aq) + HClO (aq)
3. MnO4-1 (aq) + Cl-1 (aq)  Mn2+ (aq) + Cl2 (g)
4. Cl2 (g) + S2O32- (aq)  Cl-1 (aq) + SO42- (aq)
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Reduction Oxidation POGIL
Electrochemistry
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As you have learned, chemical reactions can be either endothermic or exothermic. This energy does not
only have to be in the form of heat energy; it can also be electricity. These reactions can also occur
spontaneously or non-spontaneously. In the case of any two metals on the activity series, the more active
metal is more readily oxidized. In other words, if the “free metal” is more active, the reaction will occur
spontaneously.
An electrochemical process is any conversion of chemical energy into electrical energy or vice versa. In
order to use a redox reaction as an energy source the two half reactions must be physically separated
from each other. An electric current may also be used to produce a chemical change. Both situations are
considered electrochemical cells. They are devices in which chemical energy is converted into electrical
energy or electrical energy is converted into chemical energy.
Model 4 – Voltaic/Galvanic Cells
Read this!
Voltaic cells are electrochemical cells used to convert chemical energy into electrical energy. The
reactions that occur in voltaic cells are spontaneous. A functioning voltaic cell must have a cathode, an
anode, and a salt bridge. When the voltaic is in operation, electrons flow through an external circuit and
ions diffuse through a salt bridge. The salt bridge can look different but they are usually made of agar, a
jelly like substance. They must be porous to allow ions to pass from half-cell (the reduction or oxidation
reaction) to the other.
Model 4A. The Zn/Cu Cell
Go to the following website:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
Pay close attention to the working model; make sure you click on each of the buttons at the
bottom to get a micro-scale view of each electrode and the salt bridge opening in each beaker.
Answer the following questions about the model shown on the web site.
Questions
1. In the model shown on the web site, which label (anode or cathode) is attached to the zinc
metal and to the copper metal?
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Reduction Oxidation POGIL
2. Which way do electrons flow through the wire, from the anode to the cathode or from the
cathode to the anode?
3. What is happening to the zinc atoms in the zinc half-cell? Zinc ions?
4. What is happening to the copper atoms in the copper half-cell? Copper ions?
5. Is the reaction occurring at the anode oxidation or reduction?
6. Is the reaction at the cathode oxidation or reduction?
7. Which ions move through the salt bridge?
8. Why do you think positive ions move through the salt bridge from the anode compartment to
the cathode compartment?
9. Why do you think negative ions move through the salt bridge from the cathode compartment
to the anode compartment?
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Reduction Oxidation POGIL
10. Based on your answers on the previous page, define cathode:
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11. Based on your answers on the previous page, define anode:
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12. Write the half-reaction to show the change in zinc as the cell is running.
13. Write the half-reaction to show the change in copper as the cell is running.
14. List the ions that flow into and out of the salt bridge in each half-cell.
15. What would happen if the salt bridge were to be removed from the set-up?
16. What characteristic of NaNO3 (aq) makes it useful as the solution in the salt bridge?
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Reduction Oxidation POGIL
Problem:
Two half-cells are prepared by a student in the laboratory and are connected as shown in the diagram
below:
Half-cell 1 contains a tin electrode in a solution of Sn(NO3)2 (aq).
Half-cell 2 contains an aluminum electrode in a solution of Al(NO3)3 (aq).
The salt bridge contains a solution of NaNO3 (aq).
When the switch is closed the following reaction occurs: 2 Al0 + 3 Sn2+ → 2 Al3+ + 3 Sn0
1. Label the electrode and solution in each of the half-cells.
2. Write the oxidation half-reaction as shown in the equation above.
3. Write the reduction half-reaction as shown in the equation above.
4. Based on your answers to previous questions, decide which electrode is the anode and which
electrode is the cathode. Place the appropriate label in the diagram.
5. When the switch is closed the circuit will be completed. Use an arrow to mark the direction of
electron flow in the cell.
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Reduction Oxidation POGIL
Model 5 – Electrolytic Cells
Read this!
Electrolytic cells convert electrical energy into chemical energy- that is –they use electricity to
force nonspontaneous reactions to occur.
In an electrolytic cell the anode is positive and the cathode is negative.
The cathode is connected to the negative terminal of the battery and the anode is connected to
the positive end of the battery.
An electrolytic cell generally looks like this:

Use the remaining space to draw a venn diagram to compare and
Contrast electrolytic and voltaic cells.
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Reduction Oxidation POGIL