Reaction spontaneity

The absorbed light may act as calalv. i for a spontaneous reaction, but in other cases it may supply energy to make possible a reaction which, without light, would be thermodynamically impossible. In some cases, such a reaction reverses itselfby thermal reaction (e.g. if left in the dark) and, hence, during irradiation a phoiostationary state is reached. 

The most abundant literature is that bearing on solar eneigy conversion, mainly centered on the use of Ru(2,2 -bipyridine)3 and its analogues. The excited state of the parent compound was found some years ago to be a powerful reducing agent [212], allowing the following spontaneous reactions to be written ... 

Electrons are shared when nitrogen monoxide combines with oxygen, a spontaneous reaction, to give nitrogen dioxide... 

B) Methiodi s. Members of Classes (i), (ii) and (iv) combine wdth methyl iodide (some very vigorously) to form quaternary methiodides. It is best to add the amine to an excess of methyl iodide dissolved in about twice its volume of methanol, allow any spontaneous reaction to subside, and then boil under reflux for 30 minutes (extend to 1 hour for Class (iv) except pyridine and quinoline). The methiodide may crystallise when the reaction-mixture cools if not, evaporate the latter to small bulk or to dryness, and recrystallise, (M.ps., pp. 553-554 )... 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Tantalum Nitrides. Tantalum nitride [12033-62-4] TaN, is produced by direct synthesis of the elements at 1100°C. Very pure TaN has been produced by spontaneous reaction of lithium amide, L1NH2, and TaCl ( )- The compound is often added to cermets in 3—18 wt %. Ta N [12033-94-2] is used as a red pigment in plastics and paints (78). 

In addition to the health hazards mentioned above, it is important be aware of the potential for explosions due to the Cannizzarro reaction ([77], pp. 36-37). When strong alkali is mixed with formaldehyde solutions, the Cannizzarro reaction will result in a rapid and spontaneous reaction even at relatively low temperatures. Depending on conditions, an induction period may be seen. The main organic products of this reaction are methanol and formic acid (salt form). In addition, significant amounts of hydrogen are evolved. The potential for explosions in closed containers is high, and even open containers will often erupt. 

In a spontaneous reaction, electrons are donated by (flow away from) the half-reaction with the more negative reduction potential and are accepted by (flow toward) the half-reaction with the more positive reduction potential. Thus, in the present case, isocitrate donates electrons and NAD accepts electrons. The convention defines as... 

Note that a reaction with a net positive A, yields a negative AG°, indicating a spontaneous reaction. 

In thermodynamic terms, a spontaneous reaction AG < 0) may proceed only slowly without enzymes because of a large activation energy (EJ. Adding enzymes to the system does not change the free energy of either the substrates or products (and thus does not alter the AG of the reaction) but it does lower the activation energy and increase the rate of the reaction. 

It is apparent from this that since the rates of the cathodic and anodic processes at each electrode are equal, there will be no net transfer of charge in fact, with this particular cell, consisting of two identical electrodes in the same electrolyte solution, a similar situation would prevail even if the electrodes were short-circuited, since there is no tendency for a spontaneous reaction to occur, i.e. the system is at equilibrium and AG = 0. 

The oxidation of hydrogen to water (Hj -t- i Oj -> HjO) is thermodynamically spontaneous and the energy released as a result of the chemical reaction appears as heat energy, but the decomposition of water into its elements is a non-spontaneous process and can be achieved only by supplying energy from an external source, e.g. a source of e.m.f. that decomposes the water electrolytically. Furthermore, although the heat produced by the spontaneous reaction could be converted into electrical energy, the electrical... 

Some spontaneous reactions, notably the rusting of iron, are quite slow. Often a reaction that energy isn t,... 

For both of these spontaneous reactions, AH is a negative quantity. 

On the other hand, this simple rule fails for many familiar phase changes. An example of a spontaneous reaction that is not exothermic is the melting of ice. This takes place spontaneously at 1 atm above 0°C, even though it is endothermic... 

A Zn-Cu + voltaic cell. In this voltaic cell, a voltmeter (left) is connected to a half-cell consisting of a Cu cathode in a solution of blue Cu2+ ions and a half-cell consisting of a Zn anode in a solution of colorless Zn2+ ions. The following spontaneous reaction takes place in this cell Zn(s) + Ctf+lag) — M+(atfl + Cu(s). 

When chlorine gas is bubbled through an aqueous solution of NaBr, chloride ions and liquid bromine are the products of the spontaneous reaction. For this cell,... 

The driving force behind the spontaneous reaction in a voltaic cell is measured by the cell voltage, which is an intensive property, independent of the number of electrons passing through the cell. Cell voltage depends on the nature of the redox reaction and the concentrations of the species involved for the moment, we ll concentrate on the first of these factors. 

Notice from this equation that AG° and E° have opposite signs. This is reasonable a spontaneous reaction is one for which AG° is negative but E° is positive. 

Use the following half-equations to write three spontaneous reactions. Justify your answers by calculating ° for the cells. 

Voltaic cell A device in which a spontaneous reaction produces electrical energy,... 

Another difficulty is that spontaneous chemical reactions do not go to completion. Even if a spontaneous reaction is exothermic, it proceeds only till it reaches equilibrium. But in our golf ball analogy, equilibrium is reached when all of the golf balls are on the lower level. Oui analogy would lead us to expect that an exothermic reaction would proceed until all of the reactants are converted to products, not to a dynamic equilibrium. 

First of all, let us define what we mean by stability. Consider an initially pure sample of 2 U. Regardless of the physical or chemical state in which we find the uranium atoms, some of them will decay each instant to become thorium atoms by the spontaneous reaction ... 

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