Spontaneity coupled reactions

We can predict whether pairs of coupled reactions will proceed spontaneously by simply summing the free energy changes for each reaction. For example, consider the reaction from glycolysis (discussed in Chapter 19)... 

However the formation ofXY will not proceed spontaneously because the free energy of the product PCY) exceeds the free energy of the substrates (X and Y). We refer to the formation of XV as being an unfavorable process because, for Equation (4), AG > 0. Cells can form the XY they need only by coupling its formation to a reaction, such as the breakdown of ATP, that provides the energy required to build the chemical bonds that hold X and Y together. This process is shown in the coupled reaction below ... 

The energy released from the breakdown of ATP has been used to drive an unfavorable process. A reaction (the formation of XY) that would not have occurred spontaneously has taken place. Of course, the amount of energy required for the formation of one molecule of XY must be less than the ainount released when one ATP is broken down, otherwise the system would have gained total energy during the coupled reaction, and violated the first law of thermodynamics. 

Coupled reactions are common in human metabolism. Spontaneous processes, such as the oxidation of glucose,... 

Which is the more powerful oxidizing agent under standard conditions, an acidified aqueous permanganate solution or an acidified aqueous dichromate solution Specify the cell for the spontaneous reaction of the two couples by writing a cell diagram that under standard conditions has a positive emf. Determine the standard emf of the cell and write the net ionic equation for the spontaneous cell reaction. 

STRATEGY Find the standard potentials of the two reduction half-reactions in Appendix 2B. The couple with the more positive potential will act as an oxidizing agent (and be the site of reduction). That couple will be the right-hand electrode in the cell diagram corresponding to the spontaneous cell reaction. To calculate the standard emf of the cell, subtract the standard potential of the oxidation half-reaction (the one with the less-positive standard potential) from that of the reduction half-reaction. To write the cell reaction, follow the procedure in Toolbox 12.2. 

Couple this reaction with the ADP-ATP reaction, balance the net equation, and show that the coupled reaction is spontaneous. 

C14-0025. The coupled reaction that adds phosphate to glucose is spontaneous ... 

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. 

C14-0083. Although the ATP-ADP reaction is the principal energy shuttle in metabolic pathways, many other examples of coupled reactions exist. For example, the glutamic acid-glutamine reaction discussed in the text can couple with the acetyl phosphate reaction shown in Example 14-10. Write the balanced equation for the coupled reaction operating in the direction of overall spontaneity and calculate A G ° for the overall process. 

Corrosion (from Latin corrodere, gnaw to pieces ) of metals is the spontaneous chemical (oxidative) destruction of metals under the elfect of their environment. Most often it follows an electrochemical mechanism, where anodic dissolution (oxidation) of the metal and cathodic reduction of an oxidizing agent occur as coupled reactions. Sometimes a chemical mechanism is observed. 

A potential undesirable effect of DCC coupling reactions is the spontaneous rearrangement of the o-acylisourea to an inactive N-acylurea (Stewart and Young, 1984) (Figure 3.6). The rate of this rearrangement is dramatically increased in aprotic organic solvents, such as DMF. 

Consider a cell made up of two half cells, where one contains the Fe , Fe couple and the other the Cu, Cu couple. By looking up the respective values of the standard electrode potentials E given in Appendix 3, deduce the spontaneous cell reaction that would occur if the leads connecting the two half cells were allowed to short by touching. 

Enzymes can couple reactions that would not occur spontaneously to an energy-releasing reaction, such as ATP hydrolysis, that makes the overall reaction favorable. 

Kim, A., Filler, M. A., Kim, S. and Bent, S. F. Layer-by-layer growth on Ge(100) via spontaneous urea coupling reactions. Journal of the American Chemical Society 127, 6123-6132... 

In this chapter we try to classify the more important types of reactions encountered in inorganic chemistry, and describe some of their mechanisms. The emphasis is placed upon the principles which determine the stability or instability, existence and nonexistence of inorganic substances from the viewpoint of the ease or otherwise of preparing a compound, and the tendency a compound - once prepared - may have to react spontaneously to give other products. Both thermodynamic and kinetic considerations are obviously involved here. The division of material between this chapter and the next has not been easy, and there is inevitably a good deal of overlap. Coupling reactions, which might have deserved a section in this chapter, are discussed in Sections 10.5 and 10.6. 

Fig. 2.3 Orthogonal synthesis (C=coupling group P= protected group). On in-situ activation, P is temporarily transformed into F (= functional group), which undergoes spontaneous further reaction...

The lesson to be learned from this rather coarse and simplified analysis is that spontaneous reactions are costly and energy-inefficient. The spontaneous combustion of fossil fuels for example costs about 30% of the work available in the original fuel [4]. Instead, one should aim for bridging the distance in affinity by a limited number of coupled reactions, which are sequential and share a common intermediate. 

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