Let’s Do It Creatively ... and Environmentally with Renewable Energy (CERE)”. Electrolysis, Electrochemical cells, Biological Batteries by Krzysztof Bahrynowski
The word “electrolysis” comes from the Greek ἤλεκτρον [ɛ̌ːlektron] which means “amber” and λύσις [lýsis] which means “dissolution”.
1) Explanation of the process:
Electrolysis is a process by which electric current passes through an ionic substance so as to induce a chemical change. It is a “redox” process (reduction-oxidation): the substance gains (reduction), or loses (oxidation) electrons. Oxidation and reduction reactions cannot be carried out separately. They have to appear together in a chemical reaction. In terms of redox reactions, a reducing agent and an oxidizing agent form a redox couple as they undergo the reaction:
Oxidant + n e- → Reductant Reductant → Oxidant + n e- Both members of the couple are the same element or compound, but of different oxidation state.
To perform this redox process during by electrolysis, an electrolytic cell is used (Figure 1); it is a system consisting of a current supply (direct current because the charge flows in only one direction), and 2 electrodes, one is charged positively (anode) and the other negatively (cathode). Both are held apart and dipped into a solution, which contains positively and negatively charged ions. The substance to be transformed can come from the electrodes themselves, or may be either molten or dissolved in a solution, and is called electrolyte. The most common electrodes used are made from metal, graphite or semiconductor materials. The choice depends on the chemical reactivity between the electrode and the electrolyte, and also of the cost of manufacture. The whole system can be likened to batteries, called like that because it is a “battery” of one or more electrolytic cells together. Each cell has a characteristic voltage range between charged and discharged that is set by the electrochemical nature of the metals used and the reactions that go on in the solution, gel, wet powder, etc. between the plates.
The current, and therefore the electrons, enters through the cathode, and the positively charged components (cations) of the solution are attracted by this negative pole; they combine with the electrons and are transformed to neutral elements or molecules (discharging). At the same time, the components in the solution which are negatively charged (anions) are attracted by the positive pole that forms the anode, and then they give up their electrons and are transformed into neutral elements or molecules.
The resultants atoms may be liberated as a gas, as a solid in the solution, or may be deposited as a solid on the electrode, in amounts that are proportional to the amount of current passed (first law of electrolysis, Michael Faraday, 1832). Quantity of electricity (charge) = current x time The units used are, coulombs for charge, amps for current and seconds for time. If the substance to be transformed is the electrode, the reaction is generally one in which the electrode dissolves by giving up electrons.
Figure 1: Schematic representation of electrolysis
Simple videos: http://www.bbc.co.uk/schools/gcsebitesize/science/videos/electrolysis_video1.shtml http://www.youtube.com/watch?v=i9xS9t-KMpc The basics of electrolysis (lessons): http://www.youtube.com/watch?v=LLc56HWugqQ http://www.youtube.com/watch?v=6XJTeR9pAr4&feature=related (first among several videos about electrolysis). 2) Uses: In the chloralkali industry, sodium hydroxide (caustic soda), hydrogen and chlorine gas are produced by the electrolysis of molten sodium chloride (brine); electrolysis of an aqueous solution of sodium chloride yields sodium hydroxide and chlorine gas. This is the largest application for electrolysis. The electrolytic cell used for this reaction is called the diaphragm cell: Figure 2: Electrolytic cell used in the chloralkali industry The diaphragm is made from a porous asbestos partition. It is used to prevent the sodium hydroxide making contact with the chlorine. The electrolyte is introduced on the side of the anode and the level is kept above that of the cathode. This allows the sodium chloride solution to seep into the cathode compartment and also prevents OH- ions migrating to the anode. Four ions are presents: Na+, Cl-, OH- and H+. At the cathode, the following reaction happens: 2H+(aq) + 2e- → H2(g) At the anode, this reaction happens: 2Cl-(aq) → Cl2(g) + 2e- The ions remaining in solution are OH- and Na+.
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is used extensively in metallurgical processes, such as in extraction (electrowinning) or purification (electrorefining) of metals from ores or compounds and in deposition of metals from solution (electroplating). Electroplating is used in layering metals to fortify them, and can be used for functional or decorative purposes, as in vehicles bodies and nickel coins. We can also use it for cleaning and preservation of old artifacts. Because the process separates the non-metallic particles from the metallic ones, it is very useful for cleaning old coins and even larger objects. Electrolysis is also used for coating metals for vehicles and ships (to save from corrosion by oxygen in the water), and forms the basis of a number of electrochemical analytical techniques, such as polarography. In the medical field, needle-type electrolysis is used for hair removal. The first one to use it was the American ophthalmologist Charles Michel in 1875. He used an electric current in order to remove ingrown eyelashes in patients with a condition called trichiasis. The root of the hair follicle is destroyed and the eyelash then ceases to grow. This technique is nowadays commonly used in the cosmetic field, in order to obtain a permanent removal of hairs on every part of the body. Finally, it may be considered using electrolysis of water in order to produce hydrogen for fuel, using a cheap source of electrical energy. Hydrogen can be use a fuel for powering internal combustion or electric motors via hydrogen fuel cells. Hydrogen fuel cell utilise differences in Standard electrode potential in order to generate an electrical potential from which useful power can be extracted. The energy efficiency of water electrolysis varies widely. Some reports quote efficiencies between 50% and 70%. About 4% of hydrogen gas produced in the world comes from electrolysis. NREL (National Renewable Energy Laboratory) estimated that a kilogram (equivalent to a gallon of gasoline) could be produced by wind powered electrolysis for between $5.55 in the near term and $2.27 in the long term. Electrolysis of rust: a testimony “It would be useful to give an example of the kind of results that can be obtained using the electrolytic process and to describe the conditions under which they were achieved. After much searching for a suitable subject, I finally decided on an old horseshoe I'd found some months earlier. This horseshoe was probably well over a hundred years old and in a particularly badly corroded condition, having spent much of its time buried in the ground where it had developed a thick layer of flaky rust which had obliterated any surface features. To the left is a photograph of the shoe and it is obviously in a very advanced state of corrosion and much damage had occurred to the underlying metal. Note that no surface detail can be distinguished, with no nails or their holes visible. Attempting to clean this would represent an extreme test of the electrolytic process, but I decided to give it a try just to see what could be salvaged. The shoe was initially prepared for treatment by using a small file to carefully remove a small area of rust on one edge in order to expose some metal, so that an electrical connection could be made using a crocodile clip. A fairly weak solution of twenty litres of washing soda was then made up at strength of one heaped dessert spoon to every two litres - fifteen heaped spoonfuls in total, and a method of suspending the shoe devised. Once ready, the shoe was connected as cathode and a current limited to one quarter of an amp was applied and everything left to run for forty-eight hours. Once the allotted time had elapsed, the treated shoe was removed from the tub. It should be mentioned here that the solution had remained quite clear throughout and no detectable corrosion of the anode plates had occurred, this being due to the low current applied, and therefore the low voltage, and the enormous area of the anode plates relative to the cathode.
Andrew Westcott
3) One example of a homemade battery: Lemon battery using zinc and copper http://www.youtube.com/watch?v=AY9qcDCFeVI Creating a battery from a lemon is a common project in many science textbooks. It is made by inserting two different metallic objects into an ion bridge made from one or more lemons (you can also use oranges, potatoes, or paper soaked in salt water or acid but lemons are more efficient because of their higher acidity). Material: · Fresh lemons: You have to choose large, fresh, and “juicy” lemon because the more juice the lemons contain, the more likely they will be able to reach the maximum possible electrical output, and the longer they will continue producing current. Squeeze the lemons gently to determine how thick the skin is and select those with the thinnest skins. Avoid lemons with green or pale yellow areas of skin, as riper lemons will contain more juice. The citric acid content of lemons will provide the acidic solution. · Pieces of metal: Copper and zinc work well as the metals. You can easily use a copper coin and a galvanised nail containing zinc for each lemon you use. However, the quality of the copper and zinc can be a problem for a battery like this. Pennies are rarely pure copper. If you can, try to substitute a copper wire of 14 gauge wire (common house) for the pennies. Other non-rusty metal combinations, for instance magnesium-copper, are more effective: indeed, using magnesium instead of zinc increases the voltage from 0.9 Volts to 1.3 Volts with one lemon. However, zinc and copper are usually preferred because they are reasonably safe and easy to obtain.
· A knife · A multimeter and/or a LED (Light-Emitting Diode) · Alligator clips Procedure:
How it works:
This zinc-copper lemon battery may be related to the original “simple voltaic pile” invented by Alessandro Volta (learn more here: http://www.wisegeek.com/what-is-a-voltaic-cell.htm). The copper coin serves as cathode and the zinc coated nail as the anode. These electrodes, in contact with the acidic solution contained in the fruit (the electrolyte), cause an electrochemical reaction which generates a small potential difference. This is a small amount of electricity that you can see or using the multimeter, or with the LED which is lighted by the current produced, but only when you use a few lemons connected in series. The energy for this battery comes from the chemical change in the metals when exposed to an acid. It does not come from the fruit. The lemon only provides an environment where the reaction happens, but they are not used up in the process. The acid in the lemon reacts differently with each of the two metals. Both oxidation and reduction occur. At the anode, zinc is oxidised inside the lemon, and enters the acidic solution as Zn2+ ions. At the same time, it exchanges some of its electrons with the acid so as to reach a lower energy state, and the energy released provides the power: Zn → Zn2+ + 2e- In current practice, zinc is produced by electrowinning of zinc sulfate or pyrometallurgic reduction of zinc with carbon, which requires an energy input. The energy produced in the lemon battery comes from reversing this reaction, recovering some of the energy input during the zinc production. At the copper cathode, hydrogen ions (solvated protons from the acidic solution) are reduced to form molecular hydrogen: 2H++ 2e- → H2
The electrons are driven through the wire by the potential difference. Potential difference can be positive or negative, likened to gravitational energy, moving up a hill or down a hill. In a battery the flow of electrons is downhill, electrons can flow uphill as in the case of a battery charger. References: http://www.wisegeek.com/science.htm S-cool revision website: http://www.s-cool.co.uk/a-level/chemistry/electrochemistry Rust removal using electrolysis by Andrew Westcott http://myweb.tiscali.co.uk/andyspatch/rust.htm |
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The shoe, once out, was then soaked in water and rinsed under a tap to remove remnants of the solution. It was discovered that the outer layers of rust could now simply be pushed off using mild finger pressure to reveal a solid black and grey metallic core. I initially scrubbed the core using a plastic scrubbing brush, but found that it wasn't entirely removing the remaining black deposits and there were small persistent areas of red rust still adhering. At this point things were looking very encouraging indeed despite the obvious damage and erosion that had occurred to the shoe, as the positions of the nail holes were now easily visible, and it was possible to see the grain of the iron where it had been etched by the rusting process. I finally decided to give the shoe a gentle scrub using a wire brush, and this proved to be the answer as the black deposits were now easily removed revealing a shiny grey metal base. The nail holes themselves proved to be a bit stubborn, but I discovered that all but one could be pushed through using a small screwdriver, the remaining hole having the stump of a nail still in it. The shoe was finally given a rinse in warm water in order to heat up the metal, and quickly dabbed dry using toilet tissue whilst the retained heat of the metal rapidly dried off any remaining damp areas, minimising the chances of too much re-rusting. Finally, to protect the shoe from further corrosion, it was given a coating of light oil.”