What is the Relation of Energy in Chemical Reactions?



Energy Exchanges:

All chemical reactions involve the exchange of energy. You will recall that compounds are made up of atoms which are held together by energy forces called chemical bonds. The amount of residual energy in compounds is variable, of course, but we must conceive of a compound as a reservoir of energy.

You will also recall that the second law of thermodynamics implies that a closed system is a losing proposition, from an energy viewpoint. In other words, there is a decrease in total energy content and organization, or we sometimes say, that there is an increase in randomness.

Since chemical reactions involve the exchange of energy, a closed chemical reaction displays this tendency. We might say that it exemplified the second law. In any given chemical reaction, the state of the participating compounds which represents the lowest enthalpy and the highest entropy is the stables form of that reaction. In a closed system the reaction gets it will proceed in the direction of greatest stability.

Thus, chemical reactions may be predicted on the basis of energy relationships. In actual practice, reactions are characterized by the changes in H and S.

These changes in total energy and in organization are symbolized by Hand S, respectively. More precisely, chemical reactions are usually characterised in relation to energy changes by considering the amount of free energy which results from the reaction.

By free energy, we mean energy available for doing useful work, and we symbolize it as F. The change in free energy resulting from a chemical reaction would then be symbolized as delta F. This amount of free energy can be determined by considering the delta H and delta S that were introduced above.

For example, consider the following generalized reaction: A + B -» C + D. If //represented by A and B on the left side of the equation is greater than H represented by C and D on the right side, then the reaction will proceed readily from left to right, inasmuch as C and D represent a more stable condition.

In this case, energy will be lost to the environment so that the total energy change is negative, that is, it is lost from the system, in consideration of the energetic involved, the reaction may be written as: A + B -» C+ D -» AH If the converse were true, then energy would have to supplied from some external source and the reaction would be A + B-»C+D - AH

Now consider S and AS. If delta His zero in the reaction immediately above, which means that there is no change in total energy in the reaction, and if energy is equally distributed between C and D on the right but unequally distributed between A and B on the left, there is a difference in S.

The left side of the equation would thus represent more organization, or less entropy, and the right side would represent a higher S. Left to itself, the reaction would tend to go from left to right, since a high S represents greater stability.

In this situation, of course, energy would have to be supplied in order for the reaction to occur, since reactions tend toward randomness if they are completely closed. To view the matter from a different standpoint, entropy is a function of temperature.

To illustrate, let us consider the physical states of water, that is, its existence as a solid, a liquid, or a gas. Steam is the least organized state of the three, whereas ice represents the greatest state of organization. Since the physical states of water are temperature-dependent, then temperature must be considered in determining changes in entropy from one state to another. The mathematical expression used is a product of temperature and entropy change, or Tdelta S.

Stability changes in a chemical reaction are also dependent upon Hand TS, which are variables. As enthalpy decreases, stability increases. However, the very reverse is true of entropy. As entropy decreases, stability decreases. In other words, changes in stability are inversely proportional to changes in enthalpy and directly proportional to changes in entropy.

It should be obvious from the discussion presented above that reaction exhibiting a negative free energy change result in a more stable condition, and those reactions exhibiting a positive free energy change result in a less stable condition.

Now let us consider the following generalized reaction: A + B-> C+F

This means that in the reaction free energy is lost from the system to the environment. Consequently, the reaction will proceed in the direction of C, which represents the greatest stability with reference to this system. Such a reaction is called an exergonic reaction.

Now consider this reaction: A + B-> C+ F

In order for this reaction to take place, energy will have to be supplied from the environment to the system. In this reaction C represents a less stable condition than A + B. Such a reaction is termed an energetic reaction.

It is generally true that in a living system decomposition reaction are exergonic, characterized by -AF, while synthetic reactions are endergonic, characterized by + AF. In a living system, metabolism is so ordered that the exergonic reactions are coupled to the endergonic reactions, thus supplying the necessary energy for uphill synthetic processes.

Perhaps this explanation of energy relationship in chemical reactions seems unduly complex, and indeed, it is not an easy subject. We shall find these concepts extremely useful, however, in understanding many of the life processes.