Kimberly H. Garske Chemistry 1046 Honors Project Chipola College Spring 2005 Electrolysis of Water For hundreds of years our main source of energy has come from burning carbonbased fossil fuels such as coal. This process has worked so well many people ask, Why change now? The answer is simple. Burning fossil fuels always results in by-products such as carbon monoxide and carbon dioxide not to mention it is increasingly more expensive. These compounds are considered pollutants because they help trap heat near the surface, which causes the Earth s surface temperature to rise (a.k.a. global warming). These compounds have been found to contribute to the greenhouse effect such as the one on the planet Venus. In an attempt to keep our planet healthy it has been proposed to find alternate energy sources. One such energy source uses the most abundant compound on Earth, water. In order to create energy from water a process known as electrolysis must be used which must be accompanied by the use of a hydrogen fuel cell. Electrolysis, first discovered in the 1820 s by Michael Faraday, is the first step in creating clean energy. In examining the word electrolysis one can infer its meaning. The word lysis means to break a substance apart; therefore, electrolysis means to break a substance apart using electricity. In this experiment water will be the substance being dissociated into its individual molecules. Electrolysis is a relatively simple process. It requires that electricity pass through some water using two electrodes (a cathode and an anode). The electricity can come from several different sources. A common source is a battery; however, using a battery will not entirely create clean
energy. Most likely the battery was charged with electricity by burning fossil fuels. Nevertheless, a completely clean way to dissociate water would be using the energy harnessed from a solar cell. In the following experiment, however, a battery will be used for convenience. Just knowing that water is able to be split using electricity is not enough to know in order to understand how hydrogen gas is formed by using electrolysis. When electricity is introduced to water the anode and cathode must be separate in order for electrolysis to occur. At the cathode, a negative charge which is created by the battery pushes electrons into the water. Likewise, at the anode a positive charge is present which absorbs the electrons. However, because water is a relatively poor conductor of electricity the electrons do not flow all of the way around the circuit and an electrolyte must be used such as Na 2 SO 4. Water molecules close to the cathode are split into a cation (H + ) and an anion (OH - ). The water splits into ions and not neutral atoms because the oxygen is more electronegative and attracts hydrogen s one electron. Now the cation is able to pick up an electron from the cathode. Hydrogen is stable as a diatomic molecule so the hydrogen bonds with another hydrogen and becomes hydrogen gas (H 2 ), which bubbles to the surface. At the same time, the anode has attracted the hydroxide ion (OH - ). At the anode the extra electron from the H + is removed from the hydroxide ion. The hydroxide ion then is able to combine with three other hydroxide ions to form one molecule of oxygen (O 2 ) and two molecules of water. The four electrons removed are then available to combine with the hydrogen which was described in the beginning. The oxygen created bubbles to the surface. The chemical reaction equations that correspond to electrolysis include:
2H 2 O -> O 2 + 4H + + 4e - The four electrons cancel when the equations are added. + 4H 2 O + 4e - -> 2H 2 + 4OH - 6H 2 O -> O 2 + 2H 2 + 4H + + 4OH - 4H 2 O 2H 2 O -> O 2 + 2H 2 The four hydrogen ions and hydroxide ions combine and yield four water molecules that cancel with the six reactant water molecules leaving two reactant molecules. In this way a closed circuit is created where the energy of the battery is not destroyed but transferred and stored in the hydrogen. The hydrogen gas can then be collected and used as an energy source in the hydrogen fuel cell. In this experiment, two small scale apparatus are created to demonstrate the electrolysis of water. In the first apparatus, the stem of a plastic pipet is cut so that 0.5 cm of the stem remains. Floral wire is inserted into the top of the pipet and pushed through the other side so that the wire is exposed. Be sure that if the floral wire is coated to remove the coating so that the experiment will work properly. The holes should be made as small as possible so that it is water and gas tight. However the pipet should not be sealed because the water is forced out when the gases are produced and the wires must not be in contact with each other. Next bend the wires 90 o above the bulb. Turn the pipet upside down and fill with saturated Na 2 SO 4 solution and an acid base indicator such as bromothymol blue. Place a petri dish under the open end of the pipet, into which the solution can run when the gases produced force the liquid out. Next connect the floral wires to a nine volt battery; electrolysis will begin immediately. The color change as a result of the ph changes at the electrodes and gas production can be observed. Electrolysis will continue until the solution is at the level where the wires exit the pipet. A drop of solution will remain sealing the gas inside. Next, hold the gas filled pipet over
an open flame. As the last of the solution drops out the gas will come into contact with the open flame. As soon as the gas molecules reach the flame an explosion is heard. This demonstrates the combustibility of the gas and proves that the products can be used as alternate sources of energy. With this rather loud explosion it is often wondered is hydrogen really safe? Hydrogen is no more dangerous than any other common gases that we use in everyday life. In fact, hydrogen has been used for many years in the production of some common household substances. Some of these substances include glass, margarine, soap, and even toothpaste. The second apparatus allows for the collection of hydrogen and oxygen separately. The stems of two pipets are cut so that 0.5 cm of each stem remains. Push floral wires through the pipets as before (one wire in each pipet) and bend to a 90 o angle just above the bulb. If the wire is blunt, cut a diagonal at one end with wire cutters before pushing it through the pipet. Gas will be produced at the protruding ends of the wires so keep the wires in the center of the stems, so that the gases bubble into the bulbs and not outside. Turn the pipets independently upside down and fill each with saturated Na 2 SO 4 solution and bromothymol blue. Turn the pipets so that the open ends are immersed into the same solution and hold them next to each other. Connect the floral wires directly to a nine volt battery. Electrolysis will start immediately and gas bubbles and color changes can be observed because of the change in ph. Other solutions can be used as electrolytes in electrolysis. One such solution is sodium iodide (NaI). In the electrolysis of this solution, gas is still created but a brown precipitate is formed. After collecting the precipitate in a test tube a small quantity of hexane was introduced to the precipitate. Hexane is nonpolar; if the solution was nonpolar, they would combine. However the
solution did not combine with the hexane indicating that it its polarity. After adding nitric acid (HNO 3 ) and finding that it combines with the precipitate because nitric acid is a strong oxidizing agent one can infer that the precipitate was iron hydroxide. This product is different from the others created from the sodium sulfate solution because the iron hydroxide precipitate was created from the iron electrodes. In certain solutions this occurs because the electrodes are more easily oxidized that the solution itself. And so, with the ever-increasing demand for alternative energy sources perhaps in the near future water will be used to power our automobiles, houses, and other requirements. The most abundant compound on earth has many advantages over fossil fuels among these include its abundancy and clean burning. A relatively simple process known as electrolysis is used to transform the compound into an energy source. The only technology left to perfect is the hydrogen fuel cell, which is used to harness the energy from electrolysis, and this perfection is probably not far off. Apparatus #1: Collection of oxygen and hydrogen in a single pipet (combustible).
Apparatus #2: Apparatus for the electrolysis of water with separate collection of oxygen and hydrogen. Apparatus #2: When the reaction takes place the ph indicator bromothymol blue indicates the change in ph between the two electrodes. Works Cited Hydrogen Safety. (2004). 2 Feb. 2005 <http://stuartenergy.com/h2_economy /hydrogen_safety.html> New Mexico Solar Energy Association. Electrolysis: Obtaining hydrogen from water: The Basis for a Solar-Hydrogen Economy (1999). 1 Feb. 2005 <http://nmsea.org/curriculum/7_12/electrolysis/electrolysis.htm> Harris, Harold H. A Small-Scale and Low-Cost Apparatus for the Electrolysis of Water. Journal of Chemical Education 81.9 (2004). 1 Feb. 2005 <www.jce.divched.org>