Aim: What are electrochemical cells?
Electrochemistry Electrochemistry- involves a redox reaction and a flow of electrons
TWO TYPES of ELECTROCHEMICAL CELLS 1.Voltaic (similar to a battery) 2.Electrolytic (similar to alternator in cars)
Electrochemical Cells Every electrochemical cell contains two surfaces called electrodes that can conduct electricity. An electrode is the site at which oxidation or reduction occurs. The electrode at which reduction occurs is called cathode. The electrode at which oxidation occurs is called anode. Remember: RED CAT and AN OX.
Electrochemical Cells RED CAT AN OX Reduction ALWAYS occurs Oxidation ALWAYS occurs at the cathode at the anode (ions gain e-) (metal loses e-) Electrons ALWAYS flow from the ANODE to the CATHODE.
Electrochemical Cells If you have two electrodes made of different metals connected, how can you tell which one is oxidized and which one is reduced? Table J! The metal that is higher on Table J will be oxidized, and is thus the anode. (more active = easier to lose electrons) The metal that is lower on Table J will be reduced, and is thus the cathode. (less active = harder to lose electrons)
Practice: Between each of the following electrodes: 1. State which one will oxidize and which one will reduce. 2. State which one is the anode and which one is the cathode a. Calcium and iron b. silver and nickel c.magnesium and Lead d. Copper and silver
Voltaic Cell http://www.mhhe.com/physsci/ chemistry/essentialchemistry/fla sh/galvan5.swf An electrochemical cell in which spontaneous chemical reaction produces a flow of electrons. In other words, chemical energy is converted to electrical energy.
Voltaic Cell: How it works 1. Electrons are produced at the zinc rod according to the oxidation half reaction Zn (s) Zn 2+ (aq) + 2e- Because it is oxidized, the zinc rod is the anode. (AN OX) 2. The electrons leave the zinc anode and pass through the wire to the copper rod, thus creating an electrical current. The voltmeter measure the voltage. 3. Electrons enter the copper rod and interact with copper ions in solution. There the following reductions half reaction occurs. Cu 2+ (aq) + 2e- Cu(s) Because copper ions are reduced at the copper rod, the copper rod is the cathode. (RED CAT) 4. The salt bridge connects the two containers. It provides a path for a flow of ions between the two beakers in order to maintain neutrality in the solutions. The two half reactions can be summed to show the overall reactions: Zn (s) Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- Cu(s) Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s)
Practice
1. Use Table J to predict the direction that electrons will spontaneously flow. Draw arrows to indicate the direction on the wire. 2. Based on your answer above, which would be the negative electrode and which would be the positive electrode? _ the anode is the negative end and the cathode is the positive end 3. Explain your answer to #2. electrons move from the positive end to the negative end; therefore, the anode is the negative end and the cathode is the positive end. 4. At which electrode or in which half-cell does reduction occur?half cell 2 5. At which electrode or in which half-cell does oxidation occur?_half cell 1 6. Which electrode is the cathode? Ag 7. Which electrode is the anode? Pb
Electrolytic Cell Reaction cannot occur spontaneously, so electricity is used to force the reaction to occur. In other words, electrical energy is converted to chemical energy.(opposite of voltaic cell) When electricity is used to force a chemical reaction to occur, the process is called electrolysis.
Electrolytic Cell CATHODE electrode where ELECTRONS are SENT the NEGATIVE electrode (opposite of voltaic cell) electrode where REDUCTION occurs (RED CAT) ANODE electrode where ELECTRONS are DRAWN AWAY FROM the POSITIVE electrode (opposite of voltaic cell) electrode where OXIDATION occurs (AN OX)
Uses: 1. To obtain pure elements such as sodium and chlorine by the electrolysis of molten salts. Ex: 2 NaCl(l) 2Na(s) + Cl 2 (g)
Uses: 2. To electroplate metals onto a surface. The material to be plated with a metal is the cathode. The anode is made of the metal used for the plating. Ag Ag + + e- Anode Oxidation Positive ions go over and plate spoon Ag + + e- Ag Cathode Reduction
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