A reaction that is thermodynamically possible but for which no reasonably rapid mechanism is available is said to be kinetically limited.
Conversely, one that occurs rapidly but only to a small extent is thermodynamically limited. As you will see later, there are often ways of getting around both kinds of limitations, and their discovery and practical applications constitute an important area of industrial chemistry.
Basically, the term refers to what we might call a "balance of forces". In the case of mechanical equilibrium, this is its literal definition. A book sitting on a table top remains at rest because the downward force exerted by the earth's gravity acting on the book's mass this is what is meant by the "weight" of the book is exactly balanced by the repulsive force between atoms that prevents two objects from simultaneously occupying the same space, acting in this case between the table surface and the book.
If you pick up the book and raise it above the table top, the additional upward force exerted by your arm destroys the state of equilibrium as the book moves upward. If you wish to hold the book at rest above the table, you adjust the upward force to exactly balance the weight of the book, thus restoring equilibrium. An object is in a state of mechanical equilibrium when it is either static motionless or in a state of unchanging motion.
Another kind of equilibrium we all experience is thermal equilibrium. When two objects are brought into contact, heat will flow from the warmer object to the cooler one until their temperatures become identical. Thermal equilibrium arises from the tendency of thermal energy to become as dispersed or "diluted" as possible.
A metallic object at room temperature will feel cool to your hand when you first pick it up because the thermal sensors in your skin detect a flow of heat from your hand into the metal, but as the metal approaches the temperature of your hand, this sensation diminishes.
The time it takes to achieve thermal equilibrium depends on how readily heat is conducted within and between the objects; thus a wooden object will feel warmer than a metallic object even if both are at room temperature because wood is a relatively poor thermal conductor and will therefore remove heat from your hand more slowly.
Thermal equilibrium is something we often want to avoid, or at least postpone; this is why we insulate buildings, perspire in the summer and wear heavier clothing in the winter. When a chemical reaction takes place in a container which prevents the entry or escape of any of the substances involved in the reaction, the quantities of these components change as some are consumed and others are formed.
Eventually this change will come to an end, after which the composition will remain unchanged as long as the system remains undisturbed. The system is then said to be in its equilibrium state , or more simply, "at equilibrium". What is the nature of the "balance of forces" that drives a reaction toward chemical equilibrium?
It is essentially the balance struck between the tendency of energy to reside within the chemical bonds of stable molecules, and its tendency to become dispersed and diluted. Exothermic reactions are particularly effective in this, because the heat released gets dispersed in the infinitely wider world of the surroundings. Once this equilibrium state has been reached, no further net change will occur.
The only spontaneous changes that are allowed follow the arrows pointing toward maximum dispersal of energy. Chemical equilibrium is something you definitely want to avoid for yourself as long as possible.
The myriad chemical reactions in living organisms are constantly moving toward equilibrium, but are prevented from getting there by input of reactants and removal of products. So rather than being in equilibrium, we try to maintain a "steady-state" condition which physiologists call homeostasis — maintenance of a constant internal environment.
Equilibrium is death! The direction in which a chemical reaction is written and thus which components are considered reactants and which are products is arbitrary. Consider the following two reactions:. This is central to the concept of chemical equilibrium. It makes no difference whether we start with two moles of HI or one mole each of H 2 and I 2 ; once the reaction has run to completion, the quantities of these two components will be the same.
In general, then, we can say that the composition of a chemical reaction system will tend to change in a direction that brings it closer to its equilibrium composition.
Once this equilibrium composition has been attained, no further change in the quantities of the components will occur as long as the system remains undisturbed. The composition of a chemical reaction system will tend to change in a direction that brings it closer to its equilibrium composition. The two diagrams below show how the concentrations of the three components of this chemical reaction change with time. Examine the two sets of plots carefully, noting which substances have zero initial concentrations, and are thus "products" of the reaction equations shown.
Satisfy yourself that these two sets represent the same chemical reaction system , but with the reactions occurring in opposite directions. Most importantly, note how the final equilibrium concentrations of the components are the same in the two cases. Whether we start with an equimolar mixture of H 2 and I 2 left or a pure sample of hydrogen iodide shown on the right, using twice the initial concentration of HI to keep the number of atoms the same , the composition after equilibrium is attained shaded regions on the right will be the same.
The equilibrium composition is independent of the direction from which it is approached i. In general, both processes forward and reverse can be expected to occur, resulting in an equilibrium mixture containing finite amounts of all of the components of the reaction system. We use the word components when we do not wish to distinguish between reactants and products. If the equilibrium state is one in which significant quantities of both reactants and products are present as in the hydrogen iodide example given above , then the reaction is said to incomplete or reversible.
The latter term is preferable because it avoids confusion with "complete" in its other sense of being completed or finished, implying that the reaction has run its course and is now at equilibrium.
In principle, all chemical reactions are reversible, but this reversibility may not be observable if the fraction of products in the equilibrium mixture is very small, or if the reverse reaction is very slow the chemist's term is " kinetically inhibited ". We can thank Napoleon for bringing the concept of reaction reversibility to Chemistry.
Napoleon recruited the eminent French chemist Claude Louis Berthollet to accompany him as scientific advisor on the most far-flung of his campaigns, the expedition in Egypt in Once in Egypt, Berthollet noticed deposits of sodium carbonate around the edges of some the salt lakes found there. He was already familiar with the reaction. He immediately realized that the Na 2 CO 3 must have been formed by the reverse of this process brought about by the very high concentration of salt in the slowly-evaporating waters.
This led Berthollet to question the belief of the time that a reaction could only proceed in a single direction. His famous textbook Essai de statique chimique presented his speculations on chemical affinity and his discovery that an excess of the product of a reaction could drive it in the reverse direction. Unfortunately, Berthollet got a bit carried away by the idea that a reaction could be influenced by the amounts of substances present, and maintained that the same should be true for the compositions of individual compounds.
This brought him into conflict with the recently accepted Law of Definite Proportions that a compound is made up of fixed numbers of its constituent atoms , so his ideas the good along with the bad were promptly discredited and remained largely forgotten for 50 years.
Ironically, it is now known that certain classes of compounds do in fact exhibit variable composition of the kind that Berthollet envisioned. In irreversible reactions, once the reactants are converted to products, they cannot be regenerate again from the products. In a reversible reaction when reactants are going to products it is called the forward reaction and when products are going to reactants, it is called the backward reaction.
When the rate of forward and backward reactions is equal, then the reaction is said to be at equilibrium.
So over a period of time the amount of reactants and products are not changing. Reversible reactions always tend to come to equilibrium and maintain that equilibrium. When the system is at equilibrium, the amount of products and the reactants have not to be necessarily equal.
There can be a higher amount of reactants than the products or vice versa. The only requirement in an equilibrium equation is to maintain a constant amount from both over time. An equilibrium constant can be defined for a reaction in equilibrium; the equilibrium constant is equal to the ratio between concentration of products and concentration of reactions. For an equilibrium reaction, if the forward reaction is exothermic then the backward reaction is endothermic and vice versa.
Normally, all the other parameters for forward and backward reactions are opposite to each other like this. And some reactions 3 may start but not go to completion, that is, the reaction might start but not go completely to products. In this last case, the reaction vessel would contain some reactants and some products.
In this section, we are going to take a closer look at the third type of reaction. This is what we call an "irreversible reaction" or a "reaction that goes to completion". What if the two reactions, the forward reaction and the reverse reaction, were occurring at the same time?
What would this look like? If the forward and reverse reactions are happening at the same rate, the reaction is said to be at equilibrium or dynamic equilibrium. It is important to point out that having constant amounts of reactants and products does NOT mean that the concentration of the reactants is equal to the concentration of the products. It means they are not changing. These reactions appear to have stopped before one of the reactants has run out. We would write the example reaction as:.
Another way to think about reversible and irreversible reactions is to compare them to two types of games of tag.
Reversible reactions are in many ways like a traditional game of tag: the "it" person can become "not it" and somebody who is "not it" is tagged and becomes "it". In this way, it is a reversible change.
It is also like a reaction at equilibrium, because overall no change is occurring. There is always a constant number of "it" people and "not it" people in the game. Also, having constant numbers of "it" and "not it" people in our game does not mean that the number of "it" people reactants is equal to the number of "not it" people products.
Furthermore, this is similar to equilibrium in that this game never truly ends unless everybody gets tired of playing. The game could go on forever. We could write this as the following reversible reaction:. Irreversible reactions those that only go in one direction from reactants to products and cannot reach a state of equilibrium are more like a game of sharks and minnows.
In sharks and minnows, almost everybody starts out as a minnow. Once tagged, they become a shark. However, the difference here is that once you are a shark you are always a shark; there is no way to go back to becoming a minnow.
The game continues until everybody has been tagged and becomes a shark. This is similar to irreversible reactions in that the reactants turn into products, but can't change back.
Furthermore, the reaction will proceed until the reactants have been used up and there are not any more left. We could write the reaction as:. Here's another example of a reversible reaction — dissolving salt in a beaker of water, described by the following reaction:. If you keep adding more and more solid salt, eventually you'll reach the point where no more salt dissolves, and the excess sits at the bottom of the beaker.
At this point we have a saturated solution. Has the dissolving reaction stopped?
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