Reverse engineering of a dry cell
Andy Cherkas and Brian Dalziel
(Take note of an article related to this one on page 10. It goes carbonic acid.) into more detail about the chemistry of the dry cell.) Carbon + oxygen Æ carbon dioxide
Introduction C(s) + O2(g) Æ CO2(g) Carbon dioxide + water Æ carbonic acid It is often better to catch students’ attention by having them work + - CO2(g) + H2O(l) Æ H2CO3(aq) Æ H (aq) + HCO3 (aq) on materials that they are familiar with. Most students use dry cells, so they are familiar with them. How do they work? By Test for carbon dioxide using the standard test with limewater. taking one apart and analyzing the contents, one can study conductors, insulators, types of reactions, tests for materials, Carbon dioxide + calcium hydroxide Æ calcium carbonate + water catalysts, acids and bases, complex ions, activity series and CO2(g) + Ca(OH)2(aq) Æ CaCO3(s) + H2O(l) redox. Here is an outline of what can be done. (A second verification to show the presence of carbon dioxide
gas is to have the carbon dioxide further react with the To begin suspension of calcium carbonate in water to form the soluble
calcium hydrogen carbonate. But the only other common gas to Take one regular (non-alkaline) dry cell. Place the dry cell in a form a white precipitate with limewater is sulfur dioxide. This vice and cut through the top with a hacksaw. Cut just through gas would be immediately noted by its odour, so the second part the hard outer shell only, turning the dry cell as you cut through of the test is not really necessary.) to make a cut around the top edge. Separate the components as follows. The metallic case • Rotate the cut end around and pull out the centre post. This electrode post is a hard, brittle, black solid. Cut a piece of the metallic case and place it into 3 M HCl(aq). • Empty the black powder that is inside the cell into a 250-mL Then touch a piece of copper metal to it. What happens? What beaker. gas is produced? Collect some of the gas and insert a burning • The inside case is a shiny, silver coloured solid. splint. The gas explodes with a pale blue flame, indicating that hydrogen gas is produced. What are these materials? The pure zinc of the metal case reacts slowly with the acid. As the dry cell is taken apart you will also find an inner paper Copper will catalyse the reaction. A bit of copper(II) sulfate liner, a plastic liner, an outer case, and a metallic top and bottom added to the acid and zinc will speed the reaction to obtain the piece. Ask students for the purpose of these parts. hydrogen. When mossy zinc is used in the lab, impurities in the mossy zinc (including copper) result in the faster reaction of zinc The black centre post (electrode) with acid that is normally observed. Ask the students why pure zinc is desirable in the dry cell. copper [Teacher’s note: The electrode is made up of an epoxy carbon Zinc + hydrochloric acid ⎯⎯→⎯⎯ hydrogen + zinc chloride composite. When ignited it will decompose the epoxy, which Cu Zn(s) + 2 HCl(aq) ⎯⎯→⎯ H (g) + ZnCl (aq) liquefies. The liquid burns with an orange flame. This is not 2 2 indicative of carbon. What to do? I suggest that you take a Hydrogen + oxygen Æ water piece of charcoal or a charcoal bricket; break it up into small 2 H2(g) + O2(g) Æ 2 H2O(l) pieces, and use these to simulate the post. Tell the students that you have previously broken up the post material.] After the metal has dissolved in the acid, add 3 M NaOH(aq). A white gelatinous precipitate forms. This precipitate dissolves Take a piece of the “electrode”, heat it in a Bunsen burner flame. in aqueous ammonia solution indicating the metal case is zinc. Burn the material in a gas bottle filled with oxygen. Add The sodium hydroxide neutralizes the acid. limewater to the combustion product in the gas bottle. The limewater turns cloudy indicating that the gas produced is Sodium hydroxide + hydrochloric acid Æ sodium chloride + water carbon dioxide. Therefore, the electrode must be carbon. (A NaOH(aq) + HCl(aq) Æ NaCl(aq) + H2O(l) second piece could be burned in a bottle containing oxygen that Excess sodium hydroxide reacts with the zinc chloride to give has neutral [green] bromthymol blue solution in it. The solution zinc hydroxide. turns yellow showing the presence of an acid, in this case
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Sodium hydroxide + zinc chloride Æ sodium chloride + zinc hydroxide Take a carbon electrode and a zinc electrode and place them in a thick paste of ammonium chloride and manganese dioxide. 2 NaOH(aq) + ZnCl2(aq) Æ 2 NaCl + Zn(OH)2(s) What voltage is given? (1.5 V, the voltage of a dry cell.) The zinc hydroxide precipitate then forms the tetraamminezinc(II) ion with ammonia and dissolves in the The half cell reactions are: ammonia solution. 2+ Zn(s) Æ Zn (aq) + 2 e- 0.76 V (oxidation) Zinc hydroxide + ammonia Æ tetraamminezinc(II) hydroxide + - 2 MnO2(s) + 2 NH4 (aq) + 2 e Zn(OH) (s) + 4 NH (aq) Æ [Zn(NH ) ](OH) (aq) 2 3 3 4 2 Æ Mn2O3(s) + 2 NH3(aq) + H2O(l) 0.75 V (reduction)
The black powder The carbon electrode is an inert electrode for the reduction and conduction of electricity. Stir 20 mL of distilled water into the black powder. Stir, decant the liquid into a filter paper in a funnel and collect the filtrate in a How do we get 6 volt and 9 volt batteries? 125-mL flask. Repeat two more times with 20-mL aliquots of water. Dry the remaining black solid, which is insoluble in water. Cut one open and you will find four or six cells connected in series. Check the clear, colourless filtrate with litmus paper to show it is acidic. Add a few drops of the liquid to silver nitrate solution to Caution! obtain a white precipitate, indicating the presence of chloride ions. Evaporate the liquid to obtain a white solid. Mix some of Alkaline dry cells are very caustic when opened. The centre this white solid with solid calcium hydroxide and heat the mixture post is a brass pin surrounded by cotton soaked in a very gently. Hold moist red litmus in the gas produced to show it is alkaline solution. Instead of ammonium chloride mixed with the basic. Note the odour of ammonia. The white solid is manganese(IV) oxide, potassium hydroxide is used. These dry ammonium chloride. cells can damage skin — and the powder, if inhaled, can Silver ions + chloride ions Æ silver chloride solid damage mucous membranes. These alkaline dry cells should not be opened. g Ag+(aq) + Cl-(aq) Æ AgCl(s)
Calcium hydroxide + ammonium chloride Æ ammonia + calcium chloride + water Jarape’s lab
Ca(OH)2(s) + 2 NH4Cl(s) Æ 2 NH3(g) + CaCl2(s) + 2 H2O(g)
The black solid looks like carbon powder. Heat some to see if it burns. It does not. Add some to hydrogen peroxide solution. Test the gas produced with a glowing splint. Note the production of oxygen gas. What other black solid acts as a catalyst for the decomposition of hydrogen peroxide? Manganese dioxide does, which is the identity of the black solid. manganese dioxide Hydrogen peroxide ⎯⎯→⎯⎯⎯⎯⎯⎯⎯ oxygen + water MnO2 2 H2O2(aq) ⎯⎯→⎯⎯ O2(g) + 2 H2O(l)
If you obtain the mass of the original mixture, and the mass of the manganese(IV) oxide recovered, a quantitative determination of the proportions of ammonium chloride to manganese dioxide can be made. Dry the powdered mixture before measuring the mass of the powder, since some water is added to make a thick paste in the dry cell.
Putting it all back together
At one time dry cell kits were available so students could build one. The source of these for me has disappeared. The building of a dry cell and its use is very instructive. If anyone finds a (May be copied for non-profit, classroom use. source for the kits, please get them for your students. You can contact JARAPE at: [email protected].)
January 2006/CHEM 13 NEWS 9