IRREVERSIBLE REACTION: CHARACTERISTICS AND EXAMPLES - CHEMISTRY - 2025

An irreversible reaction is one that virtually does not reach equilibrium and that, therefore, all reactants have been transformed into products. It is said to occur in only one direction: from left to right, since the products cannot recombine to re-originate the reactants.

Today there are several theories and angles from which the irreversibility of a chemical reaction can be explained. The simplest view is taking into account how unstable the reactants are, the stability of the products, or if gases or solids are formed that escape or separate from the reaction medium.

Irreversible reactions are very common on a day-to-day basis. If we see changes in our environment in which to reverse their effects it is necessary to go back in time, then surely it will be these types of chemical reactions. For example, a cake by itself will not return to its initial state: the ingredients.

However, the products of an irreversible reaction can undergo reactions that make them reactive. This is the case of rust, which when treated with strong reducing agents, we can recover the metallic iron contained in them.

Characteristics of irreversible reactions

The preparation of a cake or tart comprises a large number of irreversible reactions that take place simultaneously. Source: Pxhere.

General chemical equation

Any irreversible reaction can be represented by a simple chemical equation, assuming that two reactive species, A and B, participate:

A + B => C + D

A and B irreversibly react to become C and D. There is no room for an equilibrium to be established. What reacted is not regenerated, and what has not, will remain as excess due to the performance of the reaction itself, or because one of the reagents has been consumed.

Note that the state of aggregation in each reactant or product (solid, gas or liquid) is not specified.

There are reactions where a negligible amount of C and D, due to their chemical nature, recombine to regenerate A and B. If this happens at equilibrium, it is said that it is very far to the right; that is, towards the formation of products.

Only in these cases is there no certainty to assert that a supposed reaction is undoubtedly irreversible. However, such a situation does not usually occur regularly in reactions that exhibit too marked changes.

Chemical changes

It is not a general nor definitive rule, but several of the irreversible reactions generate remarkable chemical changes. For example, highly exothermic reactions are considered essentially irreversible, due to the amount of energy in the form of heat and light that is released.

The same reasoning applies when we observe the appearance of a gas, either bubbling within the liquid, or oozing from the pores of a solid. If this gas escapes from the reaction medium, it will be impossible for it to participate in the establishment of equilibrium; that is, it will not react to regenerate any of the reagents.

Likewise, the formation of a solid or precipitate immediately means that the reaction is irreversible, since only its surface will still have contact with the reaction medium. And if this solid has a very stable structure, much less will it participate in an equilibrium (other than its solubility), since its particles will be confined.

On the other hand, you can't always rely on color changes. Many reactions where these are seen are actually reversible, and sooner or later the change is reversed.

Stable products and unstable reactants

A more general characteristic of irreversible reactions is that the products formed are much more stable than the reactants participating in the reaction. This explains why C and D no longer "want" to recombine to regenerate A and B, since the latter are more unstable species.

Said stability can be predicted by knowing the structures of the products, how strong and stable the new bonds are, or even by means of a molecular mechanism that shows the consecutive steps of a reaction.

Apparent reversibility

There are irreversible reactions that would practically require time to turn back for the reactants to re-form. Or more than reactions, they would be changes or processes that involve a series of them. This is because it is not about reversing one reaction, but many and instantly. For example: the decomposition of fruits.

Other irreversible reactions, on the other hand, can be reversed if their products are made to react with other species. Likewise, there are reactions that occur in a "reverse version" through different processes or mechanisms. The best known example is that of cellular respiration and photosynthesis, the difference being that the latter takes advantage of solar energy.

Examples

Some representative examples of irreversible reactions will be mentioned below.

Oxidations

Matter when oxidizes does so irreversibly unless it comes into contact with reducing agents. When metals oxidize, an oxide layer appears on their surface, which when moisture and carbon dioxide are deposited, in addition to salts, begins a corrosion process.

The metal oxide will not disintegrate out of nowhere to re-originate the metal and let the oxygen gas escape.

Combustion

All organic matter that reacts vigorously with oxygen will undergo combustion and gaseous oxides and ashes will be released from it. These oxides, essentially CO ₂, H ₂ O, NO _2, and SO ₂, will never recombine to form the initial molecules. This is the case of plastics, hydrocarbons, wood, vegetable and animal matter.

Strong acid base neutralization

When a strong acid and base react or are neutralized, the generated species do not recombine again to generate them. For example, NaOH and HCl react to produce NaCl and H ₂ O, both very stable species:

NaOH + HCl => NaCl + H ₂ O

This reaction is irreversible, there is no point where it has been verified that part of the NaOH or HCl has been regenerated. The same is true for other pairs of strong acids and bases.

Dissolution of metals

When metals dissolve in strong acids, they form a salt, water and a gas. For example, copper is attacked by nitric acid to give copper nitrate, water, and nitrogen dioxide:

Cu + 4HNO ₃ => Cu (NO ₃) ₂ + 2H ₂ O + 2NO ₂

The resulting solution is bluish in color, and coppery particles will never magically reappear, indicating the formation of metallic copper.

Saponification

Finally, we have the saponification reaction, which is irreversible; although several of its internal steps are reversible. The soaps that originate will not be converted back into the fats from which they came; not even potassium hydroxide, KOH, such a strong base, can be regenerated by any equilibrium action.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. BBC. (2020). What are irreversible changes? Recovered from: bbc.co.uk
3. Khan Academy. (2020). Chemical reactions. Recovered from: khanacademy.org
4. Fact Monster. (2020). DK Science: Chemical Reactions. Recovered from: factmonster.com
5. Ginesa Blanco. (October 16, 2019). Is it true that no chemical reaction is irreversible? The country. Recovered from: elpais.com