Reversible and Irreversible Reactions

Every chemical reaction is in theory reversible. Reactants are converted to products (we will call this a "forward reaction"), and products are converted to reactants (a "reverse reaction"). The overall reaction is a reversible reaction:

forward

Reactants K Products

As a reaction progresses, the rate of the forward reaction will decrease as the concentration of reactants decreases. Simultaneously the rate of the reverse reaction will increase as the concentration of the product molecules increases. Eventually the reaction will reach a state of chemical equilibrium in which the forward and reverse reaction rates are equal. At this point there will be no further change in the concentrations of re-actants or products even though reactants will continue to be converted into products and products converted to reactants.

Consider our previous example in which carbonic acid breaks down into carbon dioxide and water. The products of this reaction, carbon dioxide and water, can also recombine to form carbonic acid:

H2CO3 155 kcal/mol

Since carbonic acid has a greater energy content than the sum of the energies contained in carbon dioxide and water, energy must be added to the latter molecules in order to form carbonic acid. (This 4 kcal of energy is not activation energy but is an integral part of the energy balance.) This energy can be obtained, along with the activation energy, through collisions with other molecules.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

TABLE 4-3 Characteristics of Reversible and Irreversible Chemical Reactions

Reversible Reactions

At chemical equilibrium, product concentrations are only slightly higher than reactant concentrations.

Irreversible Reactions

At chemical equilibrium, almost all reactant molecules have been converted to product.

When chemical equilibrium has been reached, the concentration of products need not be equal to the concentration of reactants even though the forward and reverse reaction rates are equal. The ratio of product concentration to reactant concentration at equilibrium depends upon the amount of energy released (or added) during the reaction. The greater the energy released, the smaller the probability that the product molecules will be able to obtain this energy and undergo the reverse reaction to reform reactants. Therefore, in such a case, the ratio of product to reactant concentration at chemical equilibrium will be large. For example, when carbonic acid breaks down to form carbon dioxide and water, the amount of energy released is 4 kcal per mol, and the ratio of product to reactant molecules at equilibrium is about 1000 to 1. If there is no difference in the energy contents of reactants and products, their concentrations will be equal at equilibrium.

Thus, although all chemical reactions are reversible to some extent, reactions that release large quantities of energy are said to be irreversible reactions in the sense that almost all of the reactant molecules have been converted to product molecules when chemical equilibrium is reached. It must be emphasized that the energy released in a reaction determines the degree to which the reaction is reversible or irreversible. This energy is not the activation energy and it does not determine the reaction rate, which is governed by the four factors discussed earlier. The characteristics of reversible and irreversible reactions are summarized in Table 4-3.

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  • nasih
    Can a reversible chemical reaction convert reactants to products?
    8 years ago
  • tiblets tesfalem
    How reaction will reversible if the reaction is reversible or irreversible?
    8 years ago
  • Winta
    What reactions in the human body are irreversible?
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  • Massimiliano
    What are reverse reactions in the body?
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    What are the characteristics of reversible and irreversible reactions?
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  • Marina
    Where in the body the forward reaction and reverse reaction are most likely to occur?
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    Which chemical reactions in the body are irreversible?
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