 |
Make A Battery and Discover The Reactions InvolvedMake A Battery and Discover The Reactions Involved To best understand how batteries work, it is best to make a battery cell out of several household materials and observe its operation. The idea of different metals rusting preferentially has been introduced in the 12+ kids page and was expanded into a home made galvanic cell in the 15+ kids' section. We are going to take that galvanic cell and look closely at what is happening to the different metals as they corrode. 
The setup shown to the right uses an iron nail and rolled up Aluminium foil as the two metals. It is possible to use wire that does not have clips on it; just strip the plastic cover off the end of the wire and twist it around the ends of the nail and foil that are not in the water. It is also possible to make a battery using any two metals, as long as they are different. Another good combination is copper and iron or copper and Aluminium. For the rest of this page we will assume the metals are as shown in the image.After the cell has been operating for a day or so, it is apparent that the iron nail is not rusting despite the salty water. The Aluminium though has become discoloured and may even have bubbles on its surface or froth around the top of the metal where it emerges from the water. Clearly the Aluminium is rusting and the Iron is not. From this we can see several things immediately. First, the Iron is more stable than the Aluminium. It must be; they are both in the same conditions and one is rusting in preference to the other. Secondly, the rusting of the Aluminium is protecting the iron nail in some way. This is easily proven by separating the two halves of the experiment and leaving them apart; both will rust. Third, the protection given to the iron from the decay of the Aluminium foil is being transferred through the wire. Again, if we remove the wire both metals rust. Fourth, the piece of tissue paper soaked in salt water is also involved in the operation of the cell since if it removed both metals will rust normally. We need to break this circuit open and look at the different components one at a time. Of course they are all dependent on each other but we have to start somewhere. Make a Battery: The Two Metals
The two metals in this cell we have constructed are the equivalent of the two ends of the battery, as shown to the right. We are familiar with the + and - on each battery we use. These symbols represent whether electrons are being released from that part of the battery or consumed by it. The - sign indicates electrons are coming from there (called an ANODE) and conversely the + means the electrons are going there (called A CATHODE).
The metal that is rusting is the one losing electrons which in our home made cell is the Aluminium and so we can label the Aluminium with a minus sign. Therefore the metal receiving those electrons must be the Iron, and so we can label it with a + sign, as you can see to the right. The red arrow shows the direction the electrons are moving. In an electrical device, the electrons are put to work during their journey from the - end of the cell to the + end.
Aluminium has three outer shell electrons and to form a stable ion it must lose all three of these, as shown to the right. As this happens the Aluminium metal crumbles and the unbalanced Al3+ ions break off the metal structure and are dissolved into the water. The water in the beaker is therefore gaining in positive charge. Make a Battery: What About the Iron? When the electrons get to the iron nail, what happens to them then? They cannot build up in the metal since the metal would then be negatively charged which would repel the electrons and cause the flow to cease. They cannot be being consumed in some way by the nail because it is not changing in any way and if we were to analyze it we would find it to be the same as before the experiment. Obviously the electrons are being consumed by something, but what? To find out we need to zoom in and have a closer look at what's in the beaker containing the iron nail.
From this it is clear that there are only three things that could be consuming the electrons. There is water, Sodium ions and Chloride ions. The Sodium and Chloride ions both have full outer shells of electrons and are therefore stable, meaning they are not interested in accepting and of the free electrons. That leaves us with water. Water is a covalently bonded molecule and while it is reasonably stable, the atoms involved do have some capacity to accept extra electrons. The actual reaction occurring is this:
If we look at the oxidation numbers of the components in this reaction we see that the Hydrogen has been reduced. This means that it is at a higher energy state than before. The Hydrogen gas (H2) bubbles to the surface of the beaker and the OH- (called hydroxide) ions remain in the solution. Therefore the water is becoming more negatively charged. If you want to rest whether this is Hydrogen gas, place another glass over the top of the iron nail beaker and leave it for a few hours. Then, put a lit match to the inside of that second glass; you should hear a loud popping sound as the Hydrogen ignites (be careful...). Make a Battery: The Reactions So Far The reactions involved so far need to be summarized with a diagram:
So now we have OH- building up in one glass and Al3+ building up in the other. This is not a good situation since if too much charge builds up in the water the electrons will either be repelled by it, in the case of the OH-, or pulled toward it in the case of the Al3+. So how is it that the current continues to flow? Make a Battery: Enter the Tissue Paper Finally we get to the tissue paper. The paper is soaked in salty water. We know that the salt that has dissolved in the water is made from Na+ Sodium ions and Cl- Chloride ions from the salt Sodium Chloride. Once dissolved these ions are free from each other and can move through the liquid, or indeed through the liquid soaked paper. The ions in the tissue paper migrate towards the area of opposite charge and neutralize it. IN this way the ions from the salt water in the tissue paper are canceling out the buildup of charges in each beaker:
For this reason the paper is called a Salt Bridge. While no electric current passes through the salt bridge, it does complete the electrical circuit by canceling out the charges building up in the beakers, thus allowing the reaction to go on. Make a Battery: In Summary This setup may not look anything like our Beauto-Cell AA battery from earlier, but the basic principles are the same. There is controlled corrosion at one end, absorption of the electrons at the other and the balancing out of the medium in which the reaction occurs, called the electrolyte, to allow the reaction to proceed. This is true of both single use and rechargeable batteries. The difference between the two is that in rechargeable batteries, the metals anode (the source of electrons) is regenerated when current is pushed back in to the battery. Make A Battery and Discover The Reactions Involved Return from Make A Battery to How Do Batteries Work or return to the Green Planet home page for more Solar Power Facts.
|
