Spontaneous reactions, definition

By definition,. E°ell voltages that are positive indicate a spontaneous reaction. 

The method of experiment was to pass hydrogen at a pressure of 5-15 mm. over the heated metal, whereby a definite small amount of the vapour was taken up, and then to introduce small amounts of chlorine. For each molecule of sodium chloride many molecules of hydrogen chloride are formed. The experiments are carried out at a temperature about 50° lower than that where there is an appreciable spontaneous reaction between the hydrogen and chlorine. 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

If the /ill potential of the reaction is known, it can always be estimated if the work produced by it is sufficient for performing another, non-spontaneous reaction, i.e. reaching a definite final state of the system. However, the /ill potential may not relate to the realization of a particular chemical mechanism and the notion of chemical intermediate associated with it. Therefore, identification of the /ill potential with the notion of intermediate is, to choose our words carefully, incorrect and, in this connection, we may scarcely agree with the experts in bioenergetics that the /2H potential is the conjugation intermediate. 

Spontaneous reaction Among several definitions, the one most relevant to the material in this text defines a spontaneous reaction as one that proceeds with a decrease in free energy (AG < 0). The official definition is that a spontaneous process is one in which the entropy of the universe increases. 

Although self-assembly and self-construction are spontaneous by definition, the amoimt of human input that goes into them is usually enormous, and takes the form of instructions that are built into the structures of the starting modules and into the reaction conditions. These then determine the course of the reactions and the structure of the final product. 

In fact, we require heating to produce ammonia by the Haber process, so the reaction is definitely not spontaneous. 

Definition of enthalpy and entropy Definition of free energy Enthalpy (a measure of the change in heat content of the reactants and products) and entropy (a measure of the change in the randomness or disorder of reactants and products) determine the direction and extent to which a chemical reaction will proceed. When combined mathematically, they can be used to define a third quantity, free energy, which predicts the direction in which a reaction will spontaneously proceed. 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

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