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ExposeExciteIgniteMay2012

7. Only approach the

7. Only approach the flask once the reaction subsides. When all has cooled the residue of oxides and hydroxides can be shown to all. What a remarkable chemical change (Figue 7.10)! Mg(OH)2 MgO Al2O3 Figure 7.10 Disposal Allow the solids produced to cool to room temperature. The glass flask sometimes cracks and should then not be re-used. Discard all solids in the waste bin. C. Drawing electricity from a pencil sharpener There are two other interesting non-combustion demonstrations that can be done with a magnesium pencil sharpener. 1. Cathodic protection [] When the sharpener comes in contact with an electrolyte (eg. table salt solution), the electrolyte will act as a salt bridge and an electrochemical cell will be established. This electrochemical cell is made up of the sharpener body (Mg) as the anode and the steel (Fe) blade as the cathode: Anode (oxidation): Mg ! Mg 2+ + 2e - Cathode (reduction): 2H2O + 2e - ! H2 + 2OH - Bubbles of hydrogen will stream off the steel blade and the salt solution will become more alkaline due to the formation of OH - . Figure 9 What you will need • Magnesium pencil sharpener • Table salt (NaCl) solution (any strength) • Small glass beaker • Universal Indicator 81

Here’s how 1. Make up a table salt solution (any strength) in a small glass beaker. Add two drops of universal indicator - you’ll have a neutral green solution. 2. Drop the sharpener in the solution. Hydrogen gas will immediately start bubbling off the steel blade and the metal will turn black. The solution will turn purple ! basic. 3. After a few minutes a white gel precipitate (Mg(OH)2, insoluble) will form. 4. Over the next three days the sharpener body will be sacrificed (corroded) and only the blade will be left (Figure 7.11)! The magnesium sacrifice after 48 hours. Figure 7.11 The magnesium acts as a sacrificial anode and protects the iron blade cathode from corroding. The process is known as cathodic protection and is used to protect metal structures such as bridge foundations, pipes, storage tanks, ships, oil well casings and many more. Teaching Extension Questions ! If you started with a sharpener body with mass of 4.0 g (0.17 moles of Mg), calculate the volume of hydrogen to be formed for the complete dissolution of the magnesium. The answer is 3.8 liters. I have tried this reaction in a soft drink bottle joined to a balloon to collect the H2 gas but unfortunately the effusion rate through the balloon is faster than the production rate. ! Why didn’t the steel blade ‘rust’? The steel blade acts as the cathode and is protected from corrosion (oxidation) by the much easier oxidized magnesium - the process of cathodic protection. The blade did not give up any of its atoms as only water was reduced to hydrogen and the hydroxide ion on its surface. ! Explain the solution colour change from green to purple. The OH - ions formed in the reduction half cell are basic causing the 82