Science Buddies: "Ask an Expert"

i need an expert to talk to about salt water cells or Electrolytic cell. My experiment question is: What effect does temperature have on the amount of electricity produced by a salt water battery? i need it by Friday January 7,2011.

Hi,

The temperature dependance of electrochemical cells is described by the Nernst equation. You need to investigate that, which can be done through these websites:

http://chemistry.about.com/od/electroch ... uation.htm

http://www.chem1.com/acad/webtext/elchem/ec4.html

http://www.ausetute.com.au/nernst.html

This site includes a calculator to compute the change of cell potential by changing various parameters like temperature or concentrations:

http://www.csupomona.edu/~seskandari/ph ... ntial.html

Here is a lab experiment and background info on electrochemical cells and the Nernst equation.

I hope this answers your questions - if not please post again with your specific questions.

Best regards,

Barrett L Tomlinson

If i could, i would like to interview you. if i can please answer the following:

1. When did you first learn about this topic?

2. What motivated you to study this topic?

3. What are some of the most important things you have learned about this topic?

4. Why do you think this topic is important, useful, and/or important?

5. What practical applications have you found for this topic?

6.What metals do you think would work the best for the cathode and anode and why?

7. What temperature do you think the cell will work best at? (i have heard that it works best at colder temperatures, but i am not sure)

8. Could this be a practical source of energy?

9. Does the cell produce more electricity if the water has more salt in it?

10. Will this cell produce more than it consumes?

Thank you for your time. If you dont want to answer these questions, thank you any way. You have been a big help.
(note that your name and the information you give will be writen in my project`s report)

Hi,

1. When did you first learn about this topic?
This a very important topic to chemists. I started to learn about it in a high school chemisty class, then studied different aspects of it again for three years of college.

2. What motivated you to study this topic?
I wanted to be a chemist very badly. This topic is highly related to how and why two chemicals react when they are brought together, the very essence of chemistry.

3. What are some of the most important things you have learned about this topic?
The voltage across the cell is a measure of how much the cell reactions want to occur.
Check out Lechatlier’s Principle , Gibbs Free Energy , Thermodynamics, Standard Half Cell Potentials, and on and on.

4. Why do you think this topic is important, useful, and/or important?
Have you ever had a car battery fail to start your car? It is usually on a very cold day.

5. What practical applications have you found for this topic?
One can figure out how much work or heat you can get out of a battery or reaction under various conditions, or what voltage it will produce. Also see answer to 4, and how you might get the car to start (by warming up the battery).

6.What metals do you think would work the best for the cathode and anode and why?
Zinc and copper are a practical choice. If you could avoid water and properly construct the cell, cesium and flourine would produce a lot more energy. Look into a table of standard half cell potentials to explore other choices. Notice that you could even get a voltage & current out of a cell with both anode and cathode made of the same metal as long as the salt concentrations were different at the two electrodes.


7. What temperature do you think the cell will work best at? (i have heard that it works best at colder temperatures, but i am not sure)
To develop a feel for this I recommend playing with a Nerst equation simulator here:
http://www.nernstgoldman.physiology.arizona.edu/

8. Could this be a practical source of energy?
See this for some history of practical uses:

http://en.wikipedia.org/wiki/Daniell_cell

When I toured the telephone company in grade school many years ago they had a room of Daniell cell batteries for emergency backup power for the city phone system.



9. Does the cell produce more electricity if the water has more salt in it?
These cells produce more electricity if the ion concentrations are very different at the two electrodes.

10. Will this cell produce more than it consumes?
More what? It typically consumes electrode material to produce electricity. Electrons are conserved. Mass of each element is conserved, though metals are usually converted to ions or ions converted to metal.

This is a extremely interesting subject, and the farther you burrow into it the more interesting it becomes.

I hope this helps.

Best regards,

Barrettt K Tomlinson

im sorry, i was reading some things and i got a few more questions. could you please answer them.

1.Would the half cell be a better choice for testing? or is it better one with one solution with bothe electrodes are in it?

2.Do you think this cell running on saltwater has the potential to produce the same amount of electricity as if it was working on a pure acid solution(like a car battery)?

3. woud a solar cell be able to be conected to the cell to provied the esential current that set off the reaction?

4. would this be concidered a "green machine" because it is running on saltwater?

5. would this be able to run off of sea water?

6. could it be used in any type of machinery, like cars, ships, and factories?

7. to clear up my one question before- does this cell consume more metal than it produces electricity?

i thank you for your time and attention. don't worry about the due date i can get a n extention.

Hi,

I have read your questions three times and unfortunately I do not understand what you are asking. Let me respond by giving you some general answers so that hopefully we will develop some common vocabulary and a shared concept base so we can communicate better.

First, the subject you are generally asking about is called “electrochemistry”. If you search google.com for that term you will get some links to general overviews of the subject that are at about the right level for you. I particularly like this article which describes some simple experiments of the type I think you are interested in:

http://www.funsci.com/fun3_en/electro/electro.htm

Second, to understand very much about electrochemistry you will need to know a little about thermodynamics, including what is meant by enthalpy, entropy and Gibbs free energy. Here is a pretty good introduction to these items:

http://www.chem1.com/acad/webtext/thermeq/index.html

Here is a high school level explanation of enthalpy:

http://www.ausetute.com.au/enthchan.html

http://chemistry.about.com/od/physicalc ... emlaws.htm

http://www.chem.tamu.edu/class/majors/t ... thalpy.htm

You asked whether you should focus on whole cells or half cells. Experimentally you cannot measure half cells, so you must work with whole cells.

You asked if you could use an external voltage source to start the reaction. I understand you to be asking whether you can charge an electrochemical cell from a discharged or equilibrium state to a charged state. The answer is yes you can in most instances. If a voltage larger than the open circuit voltage of the cell is applied to the cell (from a solar cell, battery or other source) and the half cell reactions are “reversible” you can make the current in the cell flow “backwards” which means the half cell reactions go in the opposite direction to what they naturally want to do. This means if metals disolve and become ions naturally, when the current flows backwards because a higher voltage than the cell voltage is applied in the reverse direction, then the ions deposit on the electrode and are reduced to the metallic state. Metal to ion half cell reactions are normally reversible. The problem comes when there are several possible reactions at the electrode, as you often cannot control which reaction happens. For example with zinc as an electroode if the electrode is normally losing electrons and shedding zinc ions, if the electron flow is reversed in aqueous solutions hydrogen ions can be reduced to molecular hydrogen or zinc can be reduced to metallic zinc. Under many conditions the hydrogen reduction wins the competition, so the charging direction for your battery won’t work quite the way you expected. If you removed the counter EMF and checked the voltage you might be able to see the EMF for the oxidation of the hydrogen you were forming in the “Charge” mode. If however you had charged so much that a bubble of gaseous hydrogen had formed and escaped from the metal surface the cell would not be able to reoxidize the hydrogen in the escaped bubble and the cell reaction would be irreversible. There are a number of cases where reactions are irreversible for various reasons.

You asked if all the metal dissolving from the electrodes generate electrons that flow in the external cell circuit, if I understood the question correctly. The answer is that no cell is 100% efficient, for a variety of reasons. For example in a Daniell cell where zinc dissolves into a solution of zinc chloride and is couple to a solution of copper sulfate depositing copper onto a copper electrode, if any copper ions manage to diffuse to the zinc electrode surface they are reduced to copper metal and the electrons used in this are taken from the zinc metal which dissolves to a zinc ion. The electrons involved in this never travel through the external wire circuit. People try to minimize this happening by making two half cells (for example a zinc electrode in a beaker of zinc chloride and a separate beaker of copper chloride with a copper electrode, and connecting the two beakers with a “salt bridge”, a tube filled with a salt solution - see the electrochemistry article link above to see an illustration). This helps but does not completely eliminate the problem because the metal ions migrate to the other electrode as the cell discharges/operates and this cannot be avoided.

This ion migration can be demonstrated and is an interesting phenomenon in itself. To check it out investigate “transference numbers” and “Hittorf experiment or method”:

http://books.google.com/books?id=4tw8AA ... ent&f=true

http://tsascience.com.au/images/pdf/LEC/LEC06_04.pdf

I hope this helps.

Best regards,

Barrett L Tomlinson

Thanks alot! you actually answered my questions verry well. thank you again!