Nonspontaneous process made spontaneous

Years ago, tourists in Salt Lake City visited gravity hill, where an optical illusion made a stream of water appear to flow uphill. The fascination with the scene came from the apparent violation of natural laws—everyone knew that water does not flow uphill. Processes that take place naturally with no apparent cause or stimulus are called spontaneous processes. Nonspontaneous processes take place only as the result of some cause or stimulus. For example, imagine you are in the positions depicted in P Figure 8.1 and want to move a boulder. 

Study the speed of the reaction—how fast a reaction takes place (Figure 17.4 ). A reaction may be thermodynamically spontaneous but kinetically slow at a given temperature. For example, the conversion of diamond to graphite is thermodynamically spontaneous. But your diamonds will not become worthless anytime soon, because the process is extremely slow kinetically. Although the rate of a spontaneous process can be increased by the use of a catalyst, a nonspontaneous process cannot be made spontaneous by the use of a catalyst. Catalysts affect only the rate of a reaction, not the spontaneity. 

One last word about nonspontaneity— nonspontaneous process is not impossible. For example, the extraction of iron metal from iron ore is a nonspontaneous process it does not happen if the iron ore is left to itself, but that does not mean it is impossible. As we will see later in this chapter, a nonspontaneous process can be made spontaneous by coupling it to another process that is spontaneous or by supplying energy from an external source. Iron can be separated from its ore if external energy is supplied, usually by means of another reaction (that is itself highly spontaneous). 

Coupled Reactions—Nonspontaneous processes can be made spontaneous by coupling them with spontaneous reactions and by taking advantage of the state function property of G. Coupled reactions, that is, paired reactions that yield a spontaneous overall reaction, occur in metallurgical processes and in biochemical transformations. 

Voltage, a measure of the strength of an electric current, represents the force that moves electrons from the anode to the cathode in a voltaic cell. When a greater force (voltage) is applied in the opposite direction, electrons can he pushed from what would normally be the cathode toward the voltaic cell s anode. This process is called electrolysis. In a broader sense, electrolysis is the process by which a redox reaction is made to occur in the nonspontaneous direction. For example, sodium metal reacts readily with chlorine gas to form sodium chloride, but we do not expect sodium chloride, as it sits in our saltshakers, to decompose into sodium metal and chlorine gas. We say the forward reaction below is spontaneous, and the reverse reaction is nonspontaneous. 

---