Chater 26 – Electricity from Chemical Reactions26章从–电化学反应.pptVIP

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Chater 26 – Electricity from Chemical Reactions26章从–电化学反应.ppt

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Chater 26 – Electricity from Chemical Reactions26章从–电化学反应

Chapter 26 – Electricity from Chemical Reactions Galvanic Cells An electric current flows through the wire and light globe. This part of the cell is called the external circuit. The current flows because a chemical reaction is taking place in the cell. Initially, there is little indication that a reaction is taking place in the beakers. However, it it left for several hours: the zinc metal corrodes The copper metal is covered with a furry deposit The blue copper sulfate solution loses some colour. Galvanic Cells cont… If the light globe is replaced with a galvanometer, the galvanometer will indicate that electrons flow from the zinc electrode, through the wire and to the copper electrode. Current flows only if a salt bridge is present. These observation provide information on what is happening in the cell: The reaction is a redox reaction The zinc electrode is eating away, forming zinc ions in solution The oxidation of the zinc metal releases electrons, which flow to the copper electrode Electrons are accepted by the copper ions in solution when they collide with the copper electrode. Copper acts as the oxidant and zinc as the reductant. Galvanic Cells cont… Why is electrical energy released? A galvanic cell is designed so that the two half cells are totally separated the each other. Because the oxidant and reductant do not come into contact, electrons can only be transferred through an external circuit. This flow creates an electrical current. Chemical energy of the reactants is transferred into electrical energy. Half Cells The species in each half cell form a conjugate redox pair. In general, if one member of the conjugate pair in a half cell is a metal, it is usually used as the electrode. If not metal is present, than an inert electrode such as platinum is used. If one of the conjugate pairs is a gas, a special gas electrode is used. Half cells also usually contain spectator ions and that particles that make up the solvent. The electrode where oxidation occur