Chemical reactions light bulbs

Let us examine a chemical reaction to see if these same conditions apply. Suppose we fill two identical bulbs to equal pressures of nitrogen dioxide. Now immerse the first bulb (bulb A) in an ice bath and the second bulb (bulb B) in boiling water, as in Figure 9-4. The gas in bulb A at 0°C is almost colorless the gas in bulb B at 100°C is reddish-brown. The predominant molecular species in the cold bulb must be different from that in the hot bulb. A variety of experiments shows that the cold bulb contains mostly N204 molecules. These same experiments show that the hot bulb contains mostly NOa molecules. The N20 molecules absorb no visible light, so... 

Even though electrons had not yet been discovered, Daniell had the insight to know that he could harness the reaction to do work by separating the oxidation and reduction half-reactions in his cell. He set up the arrangement shown in Fig. 12.4. The chemical reaction is the same, but the reactants are separated by a porous cup. For the electrons to travel from Zn atoms to Cu2+ ions, they must pass through the external circuit (the wire and the light bulb) and as they travel from one electrode to the other, they can be used to do work by lighting the bulb. The Cu2+ ions are converted into Cu atoms in one compartment by the reduction halfreaction... 

The salt bridge contains an electrolyte such as KCI in an aqueous gelatinous medium to control the flow of K (aq) and Cr(aq) ions. When current is drawn from the cell, the electrodes are linked by an external wire to a load (examples include a light bulb or motor). Current (electrons) flows through the wire from anode to cathode as indicated and the spontaneous chemical cell reaction Zn(s) + Cu (aq) — Cu(s) + Zn (aq) occurs. 

A fuel cell (Fig. 9.7) can be described as a kind of accumulator in which hydrogen is introduced at one electrode, the anode, and oxygen at the other, the cathode. By a chain of chemical reactions, differing from case to case, water is eventually formed. This process is accompanied by the creation of a voltage difference between the anode and the cathode. When the electrodes are connected, for example by a light bulb or an electric appliance, an electric current is generated. The nature of the reactions depends on the choice of the electrolyte (the medium in which the electrochemical reactions take place). 

Thomson (15) states that most of the electricity which passes through an ordinary tungsten lamp is converted to heat (98% for a 100 watt lamp) not light . Because chemical reactions are accelerated in the presence of increased heat, the conditions are prime for chemical and physical deterioration of the artifact. If a fluorescent bulb were substituted for an incandescent bulb, the heat emission would be diminished, provided that the element were separated from the display case. However, extremely energetic UV radiation would be introduced (15). 

In hghting the light bulb or running a small motor, our cell in Figure 12.2 is doing work. We have already seen, in Chapter 4, how much work can be done by a chemical reaction, but you may have forgotten this because we have put so much emphasis on reactions that do no work, other than the minimum necessary PAV work. But here we have a chemical reaction that is certainly... 

For an electrical current to flow and light up the bulb shown in Figure 8.5, there must be a voltage difference (difference in electrical potential) between the iron and copper bar. These bars are used to transfer electrons between a wire and solution—and in this case, they actually participate in the oxidation-reduction reaction that occurs. The metal bars are called electrodes. Because this cell is used to generate a voltage and to extract electricity from a chemical reaction, it is called a voltaic cell. 

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