First, we must consider more carefully the term "energetically favorable," which we have so far used loosely without giving it a definition. As explained earlier, a chemical reaction can proceed spontaneously only if it results in a net increase in the disorder of the universe. Disorder increases when useful energy energy that could be harnessed to do work is dissipated as heat; and the criterion for an increase of disorder can be expressed conveniently in terms of a quantity called the free energy, G.
This is defined in such a way that changes in its value, denoted by delta- G , measure the amount of disorder created in the universe when a reaction takes place. Energetically favorable reactions , by definition, are those that release a large quantity of free energy, or, in other words, have a large negative delta- G and create much disorder.
A familiar example on a macroscopic scale would be the "reaction" by which a compressed spring relaxes to an expanded state, releasing its stored elastic energy as heat to its surroundings. Conversely, energetically unfavorable reactions , with a positive delta- G, such as those in which two amino acids are joined together to form a peptide bond, by themselves create order in the universe and therefore do not occur spontaneously.
Reactions of this kind can take place only if they are coupled to a second reaction with a negative delta- G so large that the delta- G of the entire process is negative.
To be precise, it depends on the environmental parameters. What you said is true for constant pressure and constant temperature. But the case you describe, the open beaker, is the standard case and your description is correct. If a reaction is "spontaneous", it does not necessarily mean that the reaction actually runs at an observable speed, which is a totally different question.
Spontaneity is only concerned with thermodynamics. One example is the decomposition of diamond to graphite. Some compounds, when dissolved in water, cause it to cool down e. These are examples of spontaneous but energetically unfavorable reactions. Many biological reactions are energetically favorable but not spontaneous and need to be pushed or pulled by other reactions.
So the cold denaturation of proteins is one example. Another example would be saturated solution of gypsum with a small amount of undissolved gypsum. No more gypsum is dissolved so the reaction is at the given conditions not spontaneous any more but if we would cool down the solution more gypsum would dissolve, indicating that this reaction is exothermal that is energetically favorable. So the reaction of solvation in a stable gypsum solution with remaining solid gypsum is an example for a non spontaneous energetically favorable reaction.
Note that the last example is exactly the reverse of my first spontaneous but energetically not favorable example. Of course the reverse reaction of any reaction that is energetically not favorable but spontaneous is an example of a reaction that is energetically favorable but not spontaneous.
Therefore, the reaction would not occur without some outside influence such as persistent heating. However, endothermic reactions do occur spontaneously, or naturally. There must be another driving force besides enthalpy change which helps promote spontaneous chemical reaction. A very simple endothermic process is that of a melting ice cube. Energy is transferred from the room to the ice cube, causing it to change from the solid to the liquid state. The solid state of water, ice, is highly ordered because its molecules are fixed in place.
The melting process frees the water molecules from their hydrogen-bonded network and allows them a greater degree of movement. Water is more disordered than ice. The change from the solid to the liquid state of any substance corresponds to an increase in the disorder of the system. There is a tendency in nature for systems to proceed toward a state of greater disorder or randomness. Entropy is a measure of the degree of randomness or disorder of a system. Entropy is an easy concept to understand when thinking about everyday situations.
When the pieces of a jigsaw puzzle are dumped from the box, the pieces naturally hit the table in a very random state. In order to put the puzzle together, a great deal of work must be dome in order to overcome the natural entropy of the pieces.
The entropy of a room that has been recently cleaned and organized is low. As time goes by, it likely will become more disordered, and thus its entropy will increase see figure below. The natural tendency of a system is for its entropy to increase. Chemical reactions also tend to proceed in such a way as to increase the total entropy of the system. How can you tell if a certain reaction shows an increase or a decrease in entropy?
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