Reversible reaction characteristics and examples

A reversible reaction It is one that at some point in its course reaches a state of equilibrium in which the concentrations of the reactants and products remain constant; that is, 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, causing the reactants to return..

The above is oversimplified by using 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 are reversible, and they are found mostly in organic and inorganic syntheses. In these, it is extremely important to know what conditions favor equilibrium in order to estimate the quantities of product that can be obtained..

Article index

• 1 Characteristics of reversible reactions
  • 1.1 General equation and equilibrium
  • 1.2 Le Châtelier principle
  • 1.3 Chemical changes
  • 1.4 Chemical species
• 2 Examples of reversible reactions
  • 2.1 Cobalt chloride solution
  • 2.2 Hydrogen iodide
  • 2.3 Hydrolysis
  • 2.4 Chromate-dichromate solution
  • 2.5 Ammonia
  • 2.6 Esterification
• 3 References

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, reactive. If one is exothermic, the other must logically be endothermic, and both occur spontaneously; but not necessarily at the same speed.

For example, A and B may be smaller or 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 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 vary..

Equilibrium is then said to be shifted to the left, where there will be more products than reactants.

Le Châtelier principle

A reversible reaction is characterized by taking place in both directions in a chemical equation, reaching a point of equilibrium, and responding to external changes or influences following the principle of le Châtelier.

In fact, thanks to this principle, Berthollet's observations in 1803, when he recognized Na crystals, could be explained._twoCO₃ in a sandy lake located in Egypt. The double displacement reaction would be:

Na_twoCO₃(ac) + CaCl_two(ac)  ⇌ NaCl (aq) + CaCO₃(ac)

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 Na_twoCO₃.

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

Chemical changes for reversible reactions tend to be less obvious than those seen 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, Na_twoCO₃ and CaCl_two. The same happens between metal complexes or molecules. In fact, much of the reversible reactions are due to molecules with specific bonds that break and regenerate over and over again..

Examples of reversible reactions

Cobalt chloride solution

A solution of cobalt chloride, CoCl_two, in water it stains it pink, due to the formation of a complex aqueous. When this solution is heated, the color changes to blue, giving the following reversible reaction:

[Co (H_twoOR)₆]^two+(ac) (pink) + 4Cl^-(ac) + Q ⇌ CoCl₄^two-(ac) (blue) + 6H_twoO (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:

H_two(g) + I_two(s) ⇌ 2HI (g)

Note that the reaction manages to establish an equilibrium even when the iodine is in the solid state. All species are molecular: H-H, I-I and H-I.

Hydrolysis

Hydrolysis are very representative examples of reversible reactions. Among the simplest we have the one that undergoes a conjugated acid or base. Hydrolysis of the ammonium ion, NH₄⁺, and the carbonate ion, CO₃^two-, are as follows:

NH₄⁺(ac) + H_twoO (l) ⇌ NH₃(g) + OH^-

CO₃^two-(ac) + H_twoO (l) ⇌ HCO₃^-(ac) + OH^-

If we add a base that contributes OH ions^- to the middle, we will shift both balances to the left.

Chromate-dichromate solution

Very similar as in the first example, a chromate solution undergoes a color change but due to variations in temperature but not in pH. The reversible reaction is:

2CrO₄^two-(ac) (yellow) + 2H₃OR⁺(ac) ⇌ Cr_twoOR₇^two-(ac) (orange) + 3H_twoO (l)

So if to a yellow CrO solution₄^two- acidified with any acid, its color will immediately turn orange. And if it is then alkalized or abundant water is added, the balance will shift to the right, the yellow color reappearing and the Cr_twoOR₇^two-.

Ammonia

The synthesis of ammonia, NH₃, It involves a reversible reaction adjusted so that the gaseous nitrogen, a very inert species, reacts:

N_two(g) + 3H_two(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 is:

RCOOH + R'OH ⇌ RCOOR '+ H_twoOR

References

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2. Walter J. Moore. (1963). Physical Chemistry. In Chemical kinetics. Fourth edition, Longmans.
3. Ira N. Levine. (2009). Principles of physicochemistry. Sixth edition, pp. 479-540. Mc Graw Hill.
4. Wikipedia. (2020). Reversible reaction. Recovered from: en.wikipedia.org
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