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Electrochemical Reduction Potential Series

How tightly do atoms hold onto their electrons? Do some elements hold on more tightly than others? This is what this page is about.

Shown on the right is a highly sophisticated device that costs about $1 (battery included, of course!). In your textbooks you have seen that galvanic cells consist of two chambers separated by a semipermeable membrane. Each chamber consists of a metal electrode immersed in a solution of one of its salts. The "device" pictured here consists of just that: each metal electrode is immersed in a solution of citric acid, which reacts with the electrodes to make a little hydrogen and a surrounding solution of that metal's salt. The electrodes are separated by numerous semipermeable membranes. This galvanic cell will produce the voltages listed in chemical tables for "electrochemical reduction potentials." However, the output amperage of these "batteries" is miniscule because the electrodes are not optimized for surface area. To gain greater amperage, one would need to use vast corrogated arrays of plates rather than mere wires.

ITEMS NEEDED

• A collection of wires made of different metals - copper (electrical wire), aluminum (nails), iron (nails), zinc (galvanated steel nails), lead (solder), magnesium ribbon, carbon (automatic pencil lead [graphite]), tin ("safe solder" that doesn't contain lead).
• A citrus fruit - lemon, orange, grapefruit, etc. (bananas work almost as well, but all other fruits and even potatoes also work but with less voltage differentials)

  DO NOT EVEN CONSIDER EATING THE FRUIT after this exercise! Some of these toxic metals will have dissolved in the fruit.

• Voltmeter sensitive to at least 0.5 volts.

Your mission is to ascertain the relative positions of the metals on a series of electromotive force. Notice: you are making a "qualitative" study, rather than a "quantitative" study. While you are determining the relative positions, you are not being asked to record the actual voltages (unless your instructor possesses voltmeters of suffient sensitivity for you to use.

What to do:

1. Because many of the metals you are using quickly form impermeable oxidized coats, rub them briefly with fine emory paper to scratch through that coating. Do not do the carbon pencil lead! (Why not ordinary sandpaper? Two reasons: emory paper is made for wet work, and emory is the impure form of silicon carbide otherwise known as carborundum of which ruby or sapphire are other examples. Carborundum is the second hardest natural stuff known - good for sanding metals. Highly pure, and harder, synthetic SiC is called moissanite, and has many electronic uses as well as being a diamond-like gemstone.)
2. After poking the copper wire into the fruit at one place, and the magnesium ribbon into a premade hole about a centimeter away, pinch the probes from a voltmeter to these two metallic protrusions from the fruit. The needle or gauge on the meter will either go up or down. If it goes down, reverse the probes so that the reading goes up. Notice that the black probe is connected to the metal that is producing electrons - the one that is negative.* Thus the red voltmeter probe is in contact with the metal that is accepting electrons - the metal that is being reduced. Thus the red probe touches the metal that has the greater reduction potential.
3. Insert a second copper wire further away from the magnesium than the first one. Determine if there is an appreciable difference in voltage between the magnesium and the first and the second copper wire. If there is not, then distance isn't of much concern in this exercise. Also, as a control, see if there is a detectible voltage between the two copper wires. There shouldn't be.
4. Pull out one of the copper wires, and now insert the other wires that you have in a circle.

5. Two by two, determine which wires in all the possible pairs is the positive and which is the negative. In the following table, write the name of the metal that is the positive one in each of the pairs. For example, the boxes for Cu/Mg have already been filled in. (Teacher: an extra box has been added in event you have access to other metals. If you click , you will get a new page containing a handout upon which you can write your lab data.)

6. Constructing your list of relative reduction potentials: The more times the element appears in the table, the higher it is in the list (the higher its reduction potential) (the more it "wants" to be reduced relative to the others)

7. Compare your list to that in your textbook or, for the sophisticated student, with the table of reduction potentials published in the Handbook of Chemistry and Physics (published by the Chemical Rubber Publishing Company).

The last activity is to determine if the intact fruit was necessary, or would the juice alone work. See if you can design an experiment to test this.

QUESTIONS

1. Write a balanced redox reaction equation for any three of the pairs.
2. Did your zinc fit in correctly? If not, it might be that there was a gap in the zinc coating on your nail, and your answer was "corrupted" by the solution's contact with iron.

* The use of the terms cathode and anode are sometimes counterintuitively confusing. Thus those terms are not used here.

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