por A reversible reaction It is one that at some point in its course reaches a state of equilibrium in which the concentrations of reactants and products remain constant; that is to say, they do not vary, since the speed with which one is consumed is the same with which the other appears. It is also said that such a state corresponds to a dynamic equilibrium.
However, equilibrium could be seen as a consequence of the reversibility of a chemical reaction; since in irreversible reactions it is impossible to establish any equilibrium. For this to happen, the products must be able to react with each other, under specific conditions of pressure and temperature, giving rise to the reactants.
The above is oversimplified by the use of the double arrow symbol (with two antiparallel heads). When we see it in a chemical equation, it means that the reaction proceeds in both directions: from left to right (formation of products), and from right to left (formation of reactants or reactants).
The minority of chemical reactions is reversible, and they are found mostly in organic and inorganic synthesis. In these, it is extremely important to know what conditions the equilibrium favors in order to estimate the quantities of product that can be obtained.
[toc]
Characteristics of reversible reactions
General equation and equilibrium
A reversible reaction has the following general equation, considering that there are only two reactants, A and B:
A + B ⇌ C + D
The double arrow indicates that A and B react to produce C and D, but also C and D can react with each other to regenerate the reactants; that is, the reaction in the opposite direction, from right to left, takes place.
The direct reaction produces products, while the reverse produces reactants. If one is exothermic, the other logically must be endothermic, and both occur spontaneously; but not necessarily at the same speed.
For example, A and B may be smaller or more unstable than C and D; and that therefore, they are consumed faster than C and D can regenerate them.
If products C and D hardly react with each other, then there will be a greater accumulation of products than of reactants. This means that when chemical equilibrium is reached, we will have higher concentrations of C and D than of A or B, regardless of whether their concentrations do not change.
The equilibrium is then said to be shifted to the left, where there will be more products than reactants.
Le Châtelier’s principle
A reversible reaction is characterized by taking place in both directions in a chemical equation, reaching an equilibrium point, and responding to changes or external influences following le Châtelier’s principle.
In fact, thanks to this principle, Berthollet’s observations in 1803 could be explained, when he recognized Na2CO3 crystals in a sandy lake located in Egypt. The double displacement reaction would be:
Na2CO3(aq) + CaCl2(aq) ⇌ NaCl(aq) + CaCO3(aq)
For the reverse reaction to take place, there has to be an excess of NaCl, and thus the equilibrium would shift to the right: towards the formation of Na2CO3.
This characteristic is of great importance because in the same way pressures or temperatures are manipulated to favor the direction of the reaction generated by the species of interest.
Chemical changes
The chemical changes of reversible reactions tend to be less obvious than those observed for irreversible reactions. However, there are reactions, especially those involving metal complexes, in which we see temperature-dependent color changes.
chemical species
Any type of compound can be involved in a reversible reaction. It was seen that two salts are capable of establishing an equilibrium, Na2CO3 and CaCl2. The same thing happens between metal complexes or molecules. In fact, much of the reversible reaction is due to molecules with specific bonds breaking and regenerating over and over again.
Examples of Reversible Reactions
Cobalt Chloride Solution
A solution of cobalt chloride, CoCl2, in water stains it pink, due to the formation of an aqueous complex. When this solution is heated, the color changes to blue, giving the following reversible reaction:
[Co(H2O)6]2+(aq) (pink) + 4Cl–(aq) + Q ⇌ CoCl42-(aq) (blue) + 6H2O(l)
where Q is the heat supplied. This heat dehydrates the complex, but as the solution cools, or if water is added, it will return to its original pink color.
hydrogen iodide
The following reversible reaction is perhaps the most classic in introducing the concept of chemical equilibrium:
H2(g) + I2(s) ⇌ 2HI(g)
Note that the reaction manages to establish equilibrium even when the iodine is in the solid state. All species are molecular: HH, II and HI.
Hydrolysis
Hydrolysis are very representative examples of reversible reactions. Among the simplest we have the one that suffers a conjugate acid or base. The hydrolysis of the ammonium ion, NH4+, and the carbonate ion, CO32-, are as follows:
NH4+(aq) + H2O(l) ⇌ NH3(g) + OH–
CO32-(aq) + H2O(l) ⇌ HCO3–(aq) + OH–
If we add a base that contributes OH– ions to the medium, we will shift both equilibria to the left.
Chromate-dichromate solution
Very similar to the first example, a chromate solution undergoes a color change, but due to variations in temperature but not in pH. The reversible reaction becomes:
2CrO42-(aq) (yellow) + 2H3O+(aq) ⇌ Cr2O72-(aq) (orange) + 3H2O(l)
So, if a yellow CrO42- solution is acidified with any acid, its color will immediately turn orange. And if it is later made alkaline or abundant water is added, the equilibrium will shift to the right, the yellow color reappearing and the Cr2O72- being consumed.
Ammonia
The synthesis of ammonia, NH3, involves a reversible reaction adjusted so that gaseous nitrogen, a very inert species, reacts:
N2(g) + 3H2(s) ⇌ 2NH3(g)
esterification
And finally, an example of organic chemistry is mentioned: esterification. This consists of obtaining an ester from a carboxylic acid and an alcohol in a strong acid medium. The reversible reaction becomes:
RCOOH + R’OH ⇌ RCOOR’ + H2O
References
Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
Walter J. Moore. (1963). Physical Chemistry. In Chemical kinetics. Fourth edition, Longmans.
Ira N. Levine. (2009). Principles of physical chemistry. Sixth edition, pages 479-540. Mc Graw Hill.
Wikipedia. (2020). Reversible reaction. Retrieved from: en.wikipedia.org
Helmenstine, Anne Marie, Ph.D. (August 19, 2019). Reversible Reaction Definition and Examples. Retrieved from: thoughtco.com
Binod Shrestha. (June 5, 2019). Reversible and irreversible reactions. Chemistry LibreTexts. Retrieved from: chem.libretexts.org
David Wood. (2020). Reversible Chemical Reactions: Definition & Examples. Study. Retrieved from: study.com