Chemical reactions nonspontaneous change

Electrochemical cells produce electrical energy from a spontaneous chemical reaction. In electrolysis, the process is reversed so that electrical energy is used to carry out a nonspontaneous chemical change. A cell arranged to do this is called an electrolytic cell. An electrolytic cell is similar to an electrochemical cell except that an electrolytic cell s circuit includes a power source, for example, a battery. The same electrochemical cell terminology applies to electrolytic cells. Reduction occurs at the cathode and oxidation at the anode. 

From a study of the examples mentioned and many more cases, we come to the following conclusion Exothermicity favors the spontaneity of a reaction bnt does not guarantee it. Just as it is possible for an endothermic reaction to be spontaneous, it is possible for an exothermic reaction to be nonspontaneous. In other words, we cannot decide whether or not a chemical reaction will occur spontaneously solely on the basis of energy changes in the system. To make this kind of prediction we need another thermodynamic qnantity, which turns ont to be entropy. 

If the change in free energy of a chemical reaction is positive, then represents the minimum amount of energy required to make the reaction occur. Again, AG n represents a theoretical limit. Making a real nonspontaneous reaction occur always requires more energy than the theoretical limit. 

The electrochemical cells of Figures 19-3 and 19-4 produce elecfricity as a resulf of spontaneous chemical reactions as such, they are called voltaic, or galvanic, cells. In Section 19-7 we will consider electrolytic cells—electrochemical cells in which electricity is used to accomplish a nonspontaneous chemical change. 

SPONTANEOUS PROCESSES (SECTION 19.1) Most reactions and chemical processes have an inherent directionality They are spontaneous in one direction and nonspontaneous in the reverse direction. The spontaneity of a process is related to the thermodynamic path the system takes from the initial state to the final state. In a reversible process, both the system and its surroundings can be restored to their original state by exactly reversing the change. In an irreversible process the system cannot return to its original state without a permanent change in the sturoimdings. Any spontaneous process is irreversible. A process that occtu s at a constant temperatiu e is said to be isothermal. 

An electrolytic cell, you may recall, is an electrochemical cell in which an electric current drives an otherwise nonspontaneous reaction. The process of producing a chemical change in an electrolytic cell is called electrolysis. Many important substances, including aluminum and chlorine, are produced commercially by electrolysis. We will begin by looking at the electrolysis of molten salts. 

Until now, the emphasis has been on voltaic (galvanic) cells, electrochemical cells in which chemical change is used to produce electricity. Another type of electrochemical cell—the electrolytic cell—uses electricity to produce a nm-spontaneous reaction. The process in which a nonspontaneous reaction is driven by the application of electric energy is called electrolysis. 

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