Continuing with the subject of Electrochemistry, a big topic is Electrochemical Cells.
Objectives:
1. Describe the production of electric current in an electrochemical (galvanic or voltaic) cell.
2. Identify the chemical reactions in an electrochemical (aka galvanic or voltaic) cell.
3. Define cell potential and describe how it is determined.
4. Define the standard electrode potential of an electrode.
5. Compute the standard emf of an electrochemical cell using standard electrode potentials.
Here is an overview of electrochemical cells:
Objectives:
1. Describe the production of electric current in an electrochemical (galvanic or voltaic) cell.
2. Identify the chemical reactions in an electrochemical (aka galvanic or voltaic) cell.
3. Define cell potential and describe how it is determined.
4. Define the standard electrode potential of an electrode.
5. Compute the standard emf of an electrochemical cell using standard electrode potentials.
Here is an overview of electrochemical cells:
I'm focusing on spontaneous reactions with a (+)Eo; this video does talk briefly about electroplating or a nonspontaneous reaction. I will examine nonspontaneous electrochemical reactions (electrolysis) more in depth during my next post.
Notice that a positive value for Eo is a spontaneous, versus a negative delta H being spontaneous. I usually point this out to my students.
Here's a brief video presenting an animation of a galvanic cell:
Notice that a positive value for Eo is a spontaneous, versus a negative delta H being spontaneous. I usually point this out to my students.
Here's a brief video presenting an animation of a galvanic cell:
I required my students to be responsible for several items of information when given an electrochemical cell combination.
For example, given a copper/copper nitrate and a zinc/zinc nitrate combination, identify the following:
1) What is the Eo of the cell? Show the calculation.
2) Which metal oxidizes (is the anode) ? What is the electric charge of this anode?
3) Which metal reduces (is the cathode)? What is the electric charge of this cathode?
4) Which electrode is gaining mass? Which electrode is losing mass?
5) What is the direction of the electrons through the voltmeter?
To answer these questions, a list of standard electrode potentials is necessary. Here's one example:
For example, given a copper/copper nitrate and a zinc/zinc nitrate combination, identify the following:
1) What is the Eo of the cell? Show the calculation.
2) Which metal oxidizes (is the anode) ? What is the electric charge of this anode?
3) Which metal reduces (is the cathode)? What is the electric charge of this cathode?
4) Which electrode is gaining mass? Which electrode is losing mass?
5) What is the direction of the electrons through the voltmeter?
To answer these questions, a list of standard electrode potentials is necessary. Here's one example:
With a table of Standard Electrode Potentials, you actually have twice as much information than the table shows directly. The reverse reaction has an equal value but opposite sign (+or -) on its electrode potential.
Of the two reactions, the reaction higher on the chart (having a higher Eo value) will be the reduction half-reaction.
The other (lower) reaction is reversed and the sign on the listed Eo value is reversed also, producing the oxidation half-reaction.
A pneumonic to remember what happens at which electrode is "an ox / red cat" where the "anode is where oxidation occurs" and the "cathode is where reduction occurs" .
In this copper & zinc combination, copper is a higher Eo value than zinc and is the cathode. The cathode has a positive charge and is +0.34 volts.
The zinc reaction is reversed to be the oxidation reaction, the anode with a negative charge and is+0.76 volts.
So, the total emf (electromotive force) -- also referred to as cell potential -- is +1.1 volts.
A typical question on quiz or test is to provide a combination and ask if the cell would generate electricity, including a combination that's not spontaneous (totals (-) Eo).
BTW, a student might ask why you don't multiply the total Eo by the coefficients.
One explanation is:
"It is important to note here that Eo's are intrinsic properties of reactions and, therefore, do not depend on the stoichiometry of the reaction. That means that you DO NOT multiply the Eo of a reaction by the coefficient used to balance the overall redox reaction. "
You can read more at the following page:
http://www.sparknotes.com/chemistry/electrochemistry/galvanic/section2.rhtml
I would photocopy a very minimalistic cell, such as that below and photocopy it as a handout for my students:
Of the two reactions, the reaction higher on the chart (having a higher Eo value) will be the reduction half-reaction.
The other (lower) reaction is reversed and the sign on the listed Eo value is reversed also, producing the oxidation half-reaction.
A pneumonic to remember what happens at which electrode is "an ox / red cat" where the "anode is where oxidation occurs" and the "cathode is where reduction occurs" .
In this copper & zinc combination, copper is a higher Eo value than zinc and is the cathode. The cathode has a positive charge and is +0.34 volts.
The zinc reaction is reversed to be the oxidation reaction, the anode with a negative charge and is+0.76 volts.
So, the total emf (electromotive force) -- also referred to as cell potential -- is +1.1 volts.
A typical question on quiz or test is to provide a combination and ask if the cell would generate electricity, including a combination that's not spontaneous (totals (-) Eo).
BTW, a student might ask why you don't multiply the total Eo by the coefficients.
One explanation is:
"It is important to note here that Eo's are intrinsic properties of reactions and, therefore, do not depend on the stoichiometry of the reaction. That means that you DO NOT multiply the Eo of a reaction by the coefficient used to balance the overall redox reaction. "
You can read more at the following page:
http://www.sparknotes.com/chemistry/electrochemistry/galvanic/section2.rhtml
I would photocopy a very minimalistic cell, such as that below and photocopy it as a handout for my students:
I would have this image on an overhead projector and would use it to determine the information to be determined about an electrochemical cell. The back of the page could have one or two more blank examples for students to practice with.
There is an abbreviated way to represent an electrochemical cell known as "cell notation" -- here's a discussion and a few examples:
There is an abbreviated way to represent an electrochemical cell known as "cell notation" -- here's a discussion and a few examples:
Here are some samples of worksheets about Electrochemical Cells:
These pages have answers:
http://chemistryconnections.com/electrochemistry/electrochem.pdf
http://kurtniedenzu.cmswiki.wikispaces.net/file/view/Wkst_echem_09.pdf/203482322/Wkst_echem_09.pdf
The following page could also be used as a quiz:
https://docs.google.com/document/d/11vmGv_OSmQNqF-N_ZDDiHw3x5HKua-G-I-oInGMnVlI/edit?pli=1
A simple lab or demonstration could be using voltmeters and alligator clips (if necessary, borrowed from the Physics department), filter paper cut into strips and saturated with a potassium nitrate solution for the salt bridge, and 100 mL beakers to contain the solution and electrode for each half-reaction.
Each half-reaction would contain an electrode submersed in its solution (e.g. Zn in ZnNO3) and a saturated piece filter paper to act as a salt bridge. Students could measure the emf of various combinations. They could also observe how the voltage disappears when the circuit is disconnected by removing the salt bridge.
Here's a lab based on this type of set-up:
http://webs.wofford.edu/splawnbg/Chem_124_files/11_Measuring%20Electrochemical%20Cells%202009.pdf
This lab is a little longer than I'd like, but it is basically the same overview:
http://www.doctortang.com/Honour%20Chemistry%20%28Old%29/Lab%2010%20Electrochemical%20Cells.pdf
Spot plates may not provide enough of the reactants for a good reading.
I appreciate your patience with my blog posting schedule. I should be able to post about Electrolytic cells this Sunday, 5/18/14.
Check out my lab book "Chemistry on a Budget" at amazon.com:
http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget
Each lab is presented with two possible report formats -- both labs use the same procedure but each has a different conclusion -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required. This gives the teacher the option of what type of report is desired, and each version is designed to be just two pages. This way the teacher can photocopy just one 2-sided page per student (saves paper).
*I'd love to hear from you! Your feedback would really help me to focus on your needs! There should be a "Contact" form below, or click on the "Contact" tab on the top right of this page.
Have a good end of the week!
These pages have answers:
http://chemistryconnections.com/electrochemistry/electrochem.pdf
http://kurtniedenzu.cmswiki.wikispaces.net/file/view/Wkst_echem_09.pdf/203482322/Wkst_echem_09.pdf
The following page could also be used as a quiz:
https://docs.google.com/document/d/11vmGv_OSmQNqF-N_ZDDiHw3x5HKua-G-I-oInGMnVlI/edit?pli=1
A simple lab or demonstration could be using voltmeters and alligator clips (if necessary, borrowed from the Physics department), filter paper cut into strips and saturated with a potassium nitrate solution for the salt bridge, and 100 mL beakers to contain the solution and electrode for each half-reaction.
Each half-reaction would contain an electrode submersed in its solution (e.g. Zn in ZnNO3) and a saturated piece filter paper to act as a salt bridge. Students could measure the emf of various combinations. They could also observe how the voltage disappears when the circuit is disconnected by removing the salt bridge.
Here's a lab based on this type of set-up:
http://webs.wofford.edu/splawnbg/Chem_124_files/11_Measuring%20Electrochemical%20Cells%202009.pdf
This lab is a little longer than I'd like, but it is basically the same overview:
http://www.doctortang.com/Honour%20Chemistry%20%28Old%29/Lab%2010%20Electrochemical%20Cells.pdf
Spot plates may not provide enough of the reactants for a good reading.
I appreciate your patience with my blog posting schedule. I should be able to post about Electrolytic cells this Sunday, 5/18/14.
Check out my lab book "Chemistry on a Budget" at amazon.com:
http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget
Each lab is presented with two possible report formats -- both labs use the same procedure but each has a different conclusion -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required. This gives the teacher the option of what type of report is desired, and each version is designed to be just two pages. This way the teacher can photocopy just one 2-sided page per student (saves paper).
*I'd love to hear from you! Your feedback would really help me to focus on your needs! There should be a "Contact" form below, or click on the "Contact" tab on the top right of this page.
Have a good end of the week!
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