MadSci Network: Chemistry

Re: How does the copper anode work in a lemon without copper salts?

Date: Thu Mar 23 21:19:03 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 952537667.Ch

Greetings, Andrew:

Please bear with me as I work my way up to an Answer, from relatively basic beginnings.

There is a wide variety of devices that are able to create an electric current directly from chemical reactions. The generic label for any one such device is "electrochemical cell". When a group of such cells are connected together, we have a "battery". (The average 1.5-volt power source is in fact a single cell, and should never be called a battery. A 6-volt lantern battery, a 9-volt transistor battery, or a 12-volt automobile battery is indeed a groupings of cells in a single package, so they truly are batteries.) Cells and batteries are divided into two categories: primary and secondary. Secondary devices are recharge-able and are often referred to as "storage batteries"; primary devices are merely used up and then discarded.

Every electrochemical cell has just 3 critical components: an anode, a cathode, and an electrolyte (and packaging, of course). The electrolyte is an electrically conductive substance containing ions. Ions are atoms or molecules which have either gained or lost at least one electron each, and thereby are no longer electrically neutral particles. An electrolyte does not have to be a water-based solution; some cells use molten salts, for example. The anode and cathode must be two different substances, neither of which can too-easily react chemically with the electrolyte. (That is, you don't use something potent like a nitric-acid-and-water solution as your electrolyte, and expect a zinc electrode to do anything but directly be 'eaten up' via chemical reactions with the acid.) The substances used for anode and cathode don't have to be metals (like nickel and cadmium), and in fact commercially available cells almost always have an electrode made from a chemical compound: In a nickel-cadmium cell, the nickel electrode is actually nickel hydroxide; the lead-acid cells in an automobile battery contain one electrode of lead, and one of lead oxide; and in the ordinary carbon-zinc flashlight cell, the carbon electrode is actually a mixture of carbon and manganese dioxide -- and the manganese dioxide is the TRUE electrode material (the carbon is only mixed in to make the electrode a better electrical conductor).

When the electrodes are both in contact with the electrolyte, but not directly in contact with each other, the cell is ready to produce an electric current. If a connecting wire is attached to the two electrodes, electrons will begin to flow. The question that needs to be pursued for the moment is: Which way do the electrons flow?

Here is a diagram of a cell, with a partial answer to that question:

               electron flow
     (-)anode /--->--->--->-\ cathode(+)
         |    |             |    |
         |    |  positive   |    |
         |    | -ion flow-> |    |
         |    |    and/or   |    |
         |    |   negative  |    |
         |    | <-ion flow- |    |
         |    |             |    |
           electrolyte container
The anode is DEFINED as the electrode through which electrons emerge from an electrochemical cell; the cathode is defined as the electrode through which the electrons return to the cell. Note the peculiarity regarding what CAN go on inside the electrolyte: Depending on the particular chemical reactions that occur between it and the electrodes, there may be produced either positive ions or negative ions -- and often BOTH. They MUST flow in a manner consistent with a completed electrical circuit. Commercially available cells are usually designed so that both positive and negative ions flow, because the external electric current will equal the sum of the two ion flows.

Now I am almost ready to tackle your Question concerning a copper anode. As just described, it must be the point from which electrons electrons emerge from an electrochemical cell. How can we ensure this?

Time to diverge onto the topic of "electronegativity". This is a measure of the magnitude of the force by which an atom or molecule is able to acquire an extra electron, thereby becoming a negatively-charged ion. Differing electronegativies is a major reason why different substances must be used as the electrodes in an electrochemical cell. For example, consider Substance A, which has a higher electronegativity than Substance B: If these are used as electrode materials, then electrons will flow from B to A, because A has the greater greed for extra electrons. This means that B will be the anode and A will be the cathode of that particular cell.

From the preceding we may conclude that if we want to use copper as an electrode, and we want it to be the anode, then we must select a substance for the second electrode that has a higher electronegativity than copper. Now it is widely known that if copper and zinc are inserted into a lemon, the citric acid of the lemon will work as an electrolyte, and a small voltage and current can be produced for a short time. However, it happens that zinc has a lower electronegativity than copper; this means that in a 'lemon' cell the copper electrode is the cathode, and the zinc electrode is the anode. We shall return to this type of cell later, following some further pursuit of a copper anode:

After examining a list of the chemical elements and their electronegativities, we might conclude that a suitable candidate for the cathode could be lead, or tungsten. Lead fishing weights are easy to obtain, and the metal is soft enough to hammer into an electrode-shape. As for tungsten, a small amount in wire form is readily available as the filament in the average incandescent light bulb. (It would be a ninor matter to sacrifice a light bulb for the benefit of Mad Science; simply wrap one up in a rag, and tap it with a hammer...and then, of course, beware of broken glass as you snip the filament free. Other candidate metals are silver, gold, and platinum, but one would have to be both mad and rich to want to use those!)

OK, so now we have (in theory) a copper/lead or copper/tungsten 'lemon' cell, with the copper electrode being the anode. Whether or not we have such a cell in actuality depends on the ability of citric acid to react chemically with the lead or tungsten (remember that lead is non-reactive enough that pretty potent stuff, sulfuric acid, is used in automobile batteries). However, it is to be noted that only SOME of the driving force for such chemical reactions comes from the natural reactivity of an electrode with the electrolyte. Some of it also comes from the differing electronegativities of the two electrodes, and this could be enough to ensure that we do indeed have an electrochemical cell.

Well, folks, just to save a few million light bulbs, I obtained a lemon and tried the above. But my voltmeter didn't budge. I am willing to conclude that citric acid just isn't potent enough to be an adequate electrolyte for a copper/tungsten cell. And stronger acids are too risky for this experiment to be done at home. So it's back to theory...

In general, what happens at the anode inside an electochemical cell is something like this: There is a chemical reaction between the active substance in the electrolyte, and the atoms or molecules at the surface of the anode. Either or both of two events can/will happen:

  1. Atoms or molecules of the anode material can join the electrolyte as positive ions, naturally leaving electrons behind as they do so. The left-behind electrons are thereby made available for use outside the electrochemical cell.
  2. Negative ions in the electrolyte can attach themselves to the surface of the anode, giving off their electrons as they form a chemical compound, often with atoms or molecules comprising the anode. Those donated electrons are also available for use outside the cell.
Note that as time passes, the substance of the anode can gradually disappear into the electrolyte, or become 'blocked' by the buildup of nonreactive compound upon its surface -- and probably both. This is one reason why an electrochemical cell eventually stops working. (In the case of the hoped-for copper-anode-in-a-lemon, the most likely compound to build up would be copper citrate.)

Meanwhile, at the cathode, another and different chemical reaction takes place. We might say that the same thing happens as at the anode -- only backwards:

  1. Atoms or molecules of cathode material can join the electrolyte as negative ions, naturally taking electrons from the cathode as they do so. The left-behind SHORTAGE of electrons is a key reason why electrons flow (outside the cell) from the anode to the cathode.
  2. Positive ions in the electrolyte can attach themselves to the surface of the cathode, stealing electrons as they form a chemical compound, often with atoms or molecules comprising the cathode. This contributes to the shortage that attracts replacements from the anode.
As time passes, the substance of the cathode can gradually disappear into the electrolyte, or become 'blocked' by the buildup of nonreactive compound upon its surface, and probably both....

Finally we return to the particular case of the copper cathode in a 'lemon' cell, with a zinc anode. While it is true that copper has a greater greed for extra electrons than zinc, copper is not so greedy that its atoms will jump into the citric acid of the lemon, taking electrons along. And the citrate ions are all negative, so they will be busy moving towards the zinc elctrode, there to form zinc citrate. The positive ions in this electrolyte are hydrogen; THESE are what move towards the copper electrode, there to steal electrons. (And hydrogen does have a greater electronegativity than copper.)

Two things previously mentioned now need to be reiterated:

  1. If one attempted to use nitric acid as an electrolyte in a cell that included a zinc electrode, the acid would directly and vigorously attack the zinc (incidently producing hydrogen gas on the spot).
  2. If one attempted to use a very weak acid as an electrolyte, it is possible that almost nothing would happen chemically. (This is actually the desired state, if one wants a long shelf life for a cell.) Only when the two electrodes are connected, and differing electronegativities come into play, do any significant chemical changes occur.
It may seem peculiar that while a strong acid can directly trade hydrogen for zinc, a weak acid must do it in roundabout fashion. The reaction between citric acid and zinc yields hydrogen gas sure enough, but the gas emerges at the COPPER electrode! (And since the amount of current produced is small, one may have to look closely to see any bubbles.)

Hopefully, this has been a reasonably enlightening and acceptable Answer to your Question. But having written all this, and THEN having taken a second look at the list of electronegativities of the elements, I find I have a bit more to say:

It appears that a good reason why my copper/tungsten 'lemon' cell didn't work was the fact that tungsten has a higher electronegativity than hydrogen. Using a stronger acid might or might not make much difference because all acids contain hydrogen atoms available for exchange -- but if you expect such an exchange with a metal that wants electrons more than hydrogen, well, very little is likely to happen. (Gold and platinum are also metals with higher electronegativities than hydrogen, and we all know how inert to most acids those metals are!)

Nevertheless, there are plenty of elements -- and even more chemical compounds -- that also possess electronegativities, like nickel hydroxide, lead oxide, and manganese dioxide; I can't say anything about others because I've never encountered an actual list. If we sought an element with a higher electronegativity than copper, and a lower electronegativity than hydrogen, then perhaps we can make a working 'lemon' cell, after all, with the copper electrode being the anode!

Silver and tin are two such elements, though only slightly more electronegative than copper. Mercury should work better, if it wasn't for the difficulties in making an electrode from the stuff -- and BEWARE: Mercury is toxic! Arsenic might work, but it has nearly the same electronegativity as hydrogen -- and it's toxic, too. Rounding out this short list are a few elements that most people never hear about very often: molybdenum, rhenium, tellurium, antimony, germanium, and boron. Take your pick....

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