Reversible Reaction: Definition and Examples

A reversible reaction can proceed in either direction.
A reversible reaction can proceed in either direction. Lumina Imaging, Getty Images

A reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. Reversible reactions will reach an equilibrium point where the concentrations of the reactants and products will no longer change.

A reversible reaction is denoted by a double arrow pointing both directions in a chemical equation. For example, a two reagent, two product equation would be written as

A + B ⇆ C + D

A Note on Notation

Bidirectional harpoons or double arrows (⇆) should be used to indicate reversible reactions, with the double-sided arrow (↔) reserved for resonance structures, but online you'll most likely encounter arrows in equations, simply because it's easier to code. When you write on paper, the proper form is to use the harpoon or double arrow notation.

Example of a Reversible Reaction

Weak acids and bases may undergo reversible reactions. For example, carbonic acid and water react this way:

H2CO3 (l) + H2O(l) ⇌ HCO3 (aq) + H3O+(aq)

Another example of a reversible reaction is:

N2O4 ⇆ 2 NO2

Two chemical reactions occur simultaneously:

N2O4 → 2 NO2

2 NO2 → N2O4

Reversible reactions do not necessarily occur at the same rate in both directions, but they do lead to an equilibrium condition. If dynamic equilibrium occurs, the product of one reaction is forming at the same rate as it is used up for the reverse reaction.

Equilibrium constants are calculated or provided to help determine how much reactant and product is formed.

The equilibrium of a reversible reaction depends on the initial concentrations of the reactants and products and the equilibrium constant, K.

How a Reversible Reaction Works

Most reactions encountered in chemistry are irreversible reactions (or reversible, but with very little product converting back into reactant).

For example, if you burn a piece of wood using the combustion reaction, you never see the ash spontaneously make new wood, do you? Yet, some reactions do reverse. How does this work?

The answer has to do with the energy output of each reaction and that required for it to occur. In a reversible reaction, reacting molecules in a closed system collide with each other and use the energy to break chemical bonds and form new products. Enough energy is present in the system for the same process to occur with the products. Bonds are broken and new ones formed, that happen to result in the initial reactants.

Fun Fact: At one time, scientists believed all chemical reactions were irreversible reactions. In 1803, Berthollet proposed the idea of a reversible reaction after observing the formation of sodium carbonate crystals on the edge of a salt lake in Egypt. Berthollet believed excess salt in the lake pushed the formation of sodium carbonate, which could then react again to form sodium chloride and calcium carbonate:

2NaCl + CaCO3 ⇆ Na2CO3 + CaCl2

Waage and Guldberg quantified Berthollet's observation with the law of mass action that they proposed in 1864.

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Helmenstine, Anne Marie, Ph.D. "Reversible Reaction: Definition and Examples." ThoughtCo, Mar. 7, 2017, thoughtco.com/definition-of-reversible-reaction-and-examples-605617. Helmenstine, Anne Marie, Ph.D. (2017, March 7). Reversible Reaction: Definition and Examples. Retrieved from https://www.thoughtco.com/definition-of-reversible-reaction-and-examples-605617 Helmenstine, Anne Marie, Ph.D. "Reversible Reaction: Definition and Examples." ThoughtCo. https://www.thoughtco.com/definition-of-reversible-reaction-and-examples-605617 (accessed November 24, 2017).