Nonspontaneous reactions

The entropy of the system decreases because 4 mol of gaseous molecules react and only 2 mol of gaseous molecules are produced. Therefore, AS°systemis negative. A decrease in the entropy of the system tends to make the reaction nonspontaneous, but the reaction is exothermic (AHsystem is negative), which tends to make the reaction spontaneous. 

A value of IQ. much greater than 1 indicates that the numerator concentrations (products) would be much greater than the denominator concentrations (reactants) this means that at equilibrium most of the reactants would be converted into products. For example, the reaction CITfg) + Chfg) CH3Cl(g) -I- HCl(g) shown earlier goes nearly to completion earlier in this chapter and in Chapter 15, we called such a reaction spontaneous or product-favored. On the other hand, if IQ is quite small, equilibrium is established when most of the reactants remain unreacted and only small amounts of products are formed.The reaction N2(g) + 02(g) k 2NO(g) shown earlier reaches equilibrium with only a tiny amount of NO present earlier in this chapter and in Chapter 15, we called such a reaction nonspontaneous or reactant-favored. ... 

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. 

Figure 23-8 shows that at low temperatures, A Gq,) is much more negative than Ai.G( ). This makes Aj.G° for the overall reaction positive and the reaction nonspontaneous. At high temperatures, the situation is reversed A,.G(°a)is more negative than A.G(°b), and the overall reaction is spontaneous. The switchover from nonspontaneous to spontaneous occurs at the point of intersection of the blue line and the red line— about 950 °C. There, AjG for the overall reaction is zero. 

If a reaction is spontaneous under a given set of conditions, the reverse reaction must be nonspontaneous. For example, water does not spontaneously decompose to the elements by the reverse of the reaction referred to above. 

However, it is often possible to bring about a nonspontaneous reaction by supplying energy in the form of work. Electrolysis can be used to decompose water to the elements. To do this, electrical energy must be furnished, perhaps from a storage battery. 

Perhaps the simplest way to define spontaneity is to say that a spontaneous process is one that moves the reaction system toward equilibrium. A nonspontaneous process moves the system away from equilibrium. 

There is still another basic objection to using the sign of AH as a general criterion fir spontaneity. Endothermic reactions that are nonspontaneous at room temperature often become spontaneous when the temperature is raised. Consider far example, the decomposition of limestone ... 

FIGURE 7.28 The effect of an increase in temperature on the spontaneity of a reaction under standard conditions. In each case, "spontaneous" is taken to mean AC0 < 0 and "nonspontaneous" is taken to mean AC° > 0. (a) An exothermic reaction with negative reaction entropy becomes spontaneous below the temperature marked by the dotted line, (b) An endothermic reaction with a positive reaction entropy becomes spontaneous above the temperature marked by the dotted line, (c) An endothermic reaction with negative reaction entropy is not spontaneous at any temperature, (d) An exothermic reaction with positive reaction entropy is spontaneous at all temperatures. 

Reactions that are nonspontaneous can be made to take place by coupling them with spontaneous reactions. This coupling is used extensively in biological systems. 

In this part of Chapter 12, we study electrolysis, the process of driving a reaction in a nonspontaneous direction by using an electric current. First, we see how electrochemical cells are constructed for electrolysis and how to predict the potential needed to bring electrolysis about. Then, we examine the products of electrolysis and see how to predict the amount of products to expect for a given flow ot electric current. 

To drive a reaction in a nonspontaneous direction, the external supply must generate a potential difference greater than the potential difference that would be produced by the reverse reaction. For example,... 

By the same reasoning, a negative AS ° and a positive AH ° oppose spontaneity, so a reaction in which the system becomes constrained and energy is absorbed is nonspontaneous regardless of temperature. The system and its surroundings both would experience decreases in entropy if such a process were to occur, and this would violate the second law of thermod3mamics. 

A reaction that has the same sign for A S ° and A H ° will be spontaneous at some temperatures but nonspontaneous at others. At low temperature, A iS ° is multiplied by a small value for T, so at sufficiently low temperature, AH ° contributes more to A G ° than T AS ° does. At high temperature, A S ° is multiplied by a large value for T, so at sufficiently high temperature, AS° contributes more to A G ° than does AH°. 

Reactions that have positive AH° and positive A S ° are favored by entropy but dis-favored by enthalpy. Such reactions are spontaneous at high temperature, where the T AS° term dominates A G °, because matter becomes dispersed during the reaction. A reaction is entropy-driven under these conditions. These reactions are nonspontaneous at low temperature, where the A iiT ° term dominates A G °. 

For a spontaneous reaction to drive one that is nonspontaneous, the two sets of reactants must interact chemicaiiy. A coupied biochemicai reaction occurs on the surface of a protein. The protein has a particuiar shape that accommodates the moiecuie that participates in the coupied reaction. In this way, the protein acts as an enabier for a particuiar biochemicai reaction. 

The synthesis of ATP from ADP and phosphoric acid is nonspontaneous. Consequently, ATP synthesis must be coupied to some more spontaneous reaction. Example 14-10 describes one of these reactions. 

A coupled process links a spontaneous reaction with a nonspontaneous one. In this case, the negative free energy change of the acetyl phosphate reaction drives the conversion of ADP to ATP. 

C14-0080. Two children on opposite ends of a seesaw can be used as an analogy for a coupled reaction. Describe the coupling of spontaneous and nonspontaneous processes during the actions of a seesaw. 

C14-0117. The notion of thermodynamic coupling of a nonspontaneous process with a spontaneous process is not restricted to chemical reactions. Identify the spontaneous and nonspontaneous portions of the following coupled processes (a) Water behind a dam passes through a turbine and generates electricity, (b) A gasoline engine pumps water from a valley to the top of a hill. 

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