Spontaneous reactions predicting, 718 table

According to the activity series, a metal will oxidize spontaneously when it is combined with the reverse of the half-reaction for any metal below it on the list. We use the activity series to predict the direction of the spontaneous reaction. Suppose we have two beakers. In one, we place a Mg strip in a solution containing Ni ions. In the other, we place a Ni strip in a solution containing Mg ions. Looking at the activity series we see that the half-reaction for the oxidation of Mg is listed above that for Ni, which means that Mg is the more active metal and loses electrons more easily than Ni. Using the activity series table, we write these two half-reactions as follows ... 

Another way to predict the spontaneity of a redox reaction is to note the relative positions of the two half-reactions in Table 18.1. Since the table lists half-reactions in order of decreasing electrode potential, the half-reactions near the top of the table— those having large positive electrode potentials— attract electrons and therefore tend to occur in the forward direction. Half-reactions near the bottom of the table— those having large negative electrode potentials—repel electrons and therefore tend to occur in the reverse direction. In other words, as you move down Table 18.1, the half-reactions become less likely to occur in the forward direction and more likely to occur in the reverse direction. As a result, any reduction half-reaction listed is spontaneous when paired with the reverse of a half-reaction that appears below it in Table 18.1. 

The positive value of the standard voltage obtained in the example indicates that the cell reaction shown is spontaneous. Thus, the standard potentials in Table 6.11 can be used to predict whether a particular reaction will occur, or not. The advantage of Table 6.11 is that it provides quantitative as well as qualitative information. It not only conveys that nickel is a stronger oxidizing agent than silver (because nickel is positioned below silver in the electrochemical series), but it also conveys how much stronger, in terms of the cell emf of+1.05 V. 

You can easily predict what will happen in a number of physical and chemical processes. What will happen if you let go of a pencil that you are holding tip-down on a table What will you observe if you add a few drops of food colouring to some water in a glass If a piece of paper starts to burn in a plentiful supply of air, what are the products of the reaction How will an iron nail change if it is left outside These are all examples of favourable (or spontaneous] changes. A favourable change is a change that has a natural tendency to happen under certain conditions. 

When predicting whether a / V / reaction is spontaneous using Table 18.1, some students find it useful to circle the two potential reactants and connect them with a line. If the line has a negative slope, the reaction is spontaneous if the line has a positive slope, the reaction is nonspontaneous. If both potential reactants are on the same side of the arrows, no reaction can occur because an oxidation requires a simultaneous reduction. 

To predict whether a redox reaction is spontaneous, remember that an oxidizing agent can oxidize any reducing agent that lies below it in the table but can t oxidize one that lies above it. To calculate E° for a redox reaction, sum the E° values for the reduction and oxidation half-reactions. 

The half-cell potentials listed in Table lenable us to attach numbers to our predictions. As was implied in the preceding problem example, if the potential of a cell made up of the two half-cell reactions is positive, then the reaction will proceed spontaneously to the right if it is negative, the reverse reaction will be spontaneous. 

Another noticeable characteristic of captodative olefins is the influence of the reaction medium. The stabilizing effect of solvent on the persistency of a captodatively radical has been reported experimentally for the bond homolysis of bis(3,5,5-trimethyl-2-oxomorpholin-3-yl) [111], but was not found for the 2,3-diphenyl-2,3-dimethoxysuccinonitrile homolysis [112]. Theoretically the solvent-assisted stabilization las been predicted for the captodative substituted nitriles in solvent with large dielectric constants [113-114], Table 16 illustrates the solvent effect on the spontaneous thermal polymerizations [115]. The polymer yields are... 

Table 20.1 Predicting Whether a Reaction Is Spontaneous in the Forward Direction Ji ...

We can make a table of standard reduction potentials, and use its values to calculate the potential of any cell consisting of two of its half-cells. See Table 9-1. The table of standard reduction potentials is a quantitative measure of the "activity series" learned earlier to enable us to predict whether a substitution reaction would proceed spontaneously. However, not only reductions of cations to metals, but any reduction half-reaction can be included in the table of standard reduction potentials. 

The Example Problems showed you how to use the data from Table 20.1 to calculate the standard potential (voltage) of voltaic cells. Another important use of standard reduction potentials is to determine if a proposed reaction under standard conditions will be spontaneous. How can standard reduction potentials indicate spontaneity Electrons in a voltaic cell always flow from the half-cell with the lower standard reduction potential to the half-cell with the higher reduction potential, giving a positive cell voltage. To predict whether any proposed redox reaction will occur spontaneously, simply write the process in the form of half-reactions and look up the reduction potential of each. Use the values to calculate the potential of a voltaic cell operating with these two half-cell reactions. If the calculated potential is positive, the reaction is spontaneous. If the value is negative, the reaction is not spontaneous. However, the reverse of a nonspontaneous reaction will occur because it will have a positive cell voltage, which means that the reverse reaction is spontaneous. 

Figure 172 Predicting acid-base reactions from positions in Table 17.1. The spontaneous chemical change always transfers a proton from the stronger acid to the stronger base, both shown in pink. The products of the reaction, the weaker acid and the weaker base, are shown in blue. The favored direction, forward or reverse, is the one that has the weaker acid and base as products. 

An imdeistanding of thermodynamics enables us to predict whether or not a reaction will occur when reactants are combined. This is important in the synthesis of new compounds in the laboratory, the manufacturing of chemicals on an industrial scale, and the understanding of natural processes such as cell function. A process that does occur under a specific set of conditions is called a spontaneous process. One that does not occur under a specific set of conditions is called nonspontaneous. Table 18.1 lists examples of familiar spontaneous processes and their nonspon-taneous counterparts. These examples illustrate what we know intuitively Under a given set of conditions, a process that occurs spontaneously in one direction does not also occur spontaneously in the opposite direction. 

Use the activity series in Table 15.3 to predict whether each of the following reactions will occur spontaneously ... 

Equation (13.35) is the key to predicting the direction of spontaneous change, so we must be certain to interpret it properly. It shows clearly that if Aj.G < 0, then we must have reaction occurring in the forward direction d > 0) to ensure that G decreases dG < 0), as required by the second law. On the other hand, if Aj.G > 0, then we must have reaction occurring in the reverse direction d < 0) to ensure that G decreases. Table 13.9 summarizes the various possibilities. 

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