Chemistry_Grade_10-12 (1).pdf

In 1780 galvani discovered that when two different

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In 1780, Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this ”animal electricity”. While many scientists accepted his ideas, another scientist, Alessandro Volta, did not. In 1800, because of his professional disagreement over the galvanic response that had been suggested by Luigi Galvani, Volta developed the voltaic pile , which was very similar to the galvanic cell. It was the work of these two men that paved the way for all electrical batteries. 323
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17.2 CHAPTER 17. ELECTROCHEMICAL REACTIONS - GRADE 12 Worked Example 83: Understanding galvanic cells Question: For the following cell: Zn | Zn 2+ || Ag + | Ag 1. Give the anode and cathode half-reactions. 2. Write the overall equation for the chemical reaction. 3. Give the direction of the current in the external circuit. Answer Step 1 : Identify the oxidation and reduction reactions In the standard notation format, the oxidation reaction is written on the left and the reduction reaction on the right. So, in this cell, zinc is oxidised and silver ions are reduced. Step 2 : Write the two half reactions Oxidation half-reaction: Zn Zn 2+ + 2 e Reduction half-reaction: Ag + + e Ag Step 3 : Combine the half-reactions to get the overall equation. When you combine the two half-reactions, all the reactants must go on the left side of the equation and the products must go on the right side of the equation. The overall equation therefore becomes: Zn + Ag + + e Zn 2+ + 2e + Ag Note that this equation is not balanced. This will be discussed later in the chapter. Step 4 : Determine the direction of current flow A build up of electrons occurs where oxidation takes place. This is at the zinc electrode. Current will therefore flow from the zinc electrode to the silver electrode. 17.2.4 Uses and applications of the galvanic cell The principles of the galvanic cell are used to make electrical batteries . In science and tech- nology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries are made of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. Batteries have many uses including in torches, electrical appliances (long-life alkaline batteries), digital cameras (lithium battery), hearing aids (silver-oxide battery), digital watches (mercury battery) and military applications (thermal battery). Refer to chapter 23 for more information on batteries. The galvanic cell can also be used for electroplating . Electroplating occurs when an electrically conductive object is coated with a layer of metal using electrical current. Sometimes, electroplat- ing is used to give a metal particular properties such as corrosion protection or wear resistance.
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