Reversible Reactions

Reversible reaction

A reversible reaction involves the reactants forming products, which can again react to form the reactants. Two separate reactions are going on simultaneously: the forward reaction, where the reactants become products, and vice versa. In such a reaction, the reactants and products continuously react with each other, both in forward and backward reactions.

The reactants A and B combine to form product C in the forward reaction.

Forward reaction: A + B → C

C can then break down into A and B again in the backward reaction.

Backward reaction: C → A + B

We can combine both reactions into one to create a reversible reaction:

Reversible reaction: A + B ⇌ C

This equation demonstrates that the reaction can proceed in both directions: the reactants can combine to form the product, and the product can decompose back into the reactants.

In simple terms, the collision of molecules breaks down the reactant bonds, and this broken bond generates enough energy to create new molecules of the product. Reversible reactions will reach an equilibrium point where the concentrations of the reactants and products will no longer change.

The general form of reversible reaction is:

A + B ⇆ C + D

The double arrow ⇆ symbol is used for reversible reactions.

An example of a reversible process is melting ice into water and freezing water into ice.

Discovery Of Reversible Reactions

Berthollet, a French chemist, first proposed the idea of a reversible reaction in 1803 after observing the formation of sodium carbonate crystals at the edge of a salt lake (one of the natron lakes in Egypt, in limestone):

2 NaCl + CaCO3 → Na2CO3 + CaCl2

He recognized this as the reverse of the already-known reaction.

Na2CO3 + CaCl2 → NaCl + CaCO3

It was previously believed that chemical reactions always proceeded in a single direction. The excess salt in the lake, according to Berthollet, pushed the “reverse” reaction toward the formation of sodium carbonate.

Waage and Guldberg developed their law of mass action in 1864, quantifying Berthollet’s observation. Between 1884 and 1888, Le Chatelier and Braun formulated Le Chatelier’s principle, which extended the same concept to a more general statement on the effects of factors other than concentration on the equilibrium position.

Conditions For Reversible Reactions

Conditions for a reversible reaction are:

  • It should take place in a container that is tightly closed.
  • No precipitation may form from the product.
  • According to Le Chatelier’s principle, a reversible reaction is self-correcting. Consider a situation in which the concentration, temperature, or pressure changes. In such a case, the system will adjust toward equilibrium naturally.

Reversible Reactions And Equilibrium

A reversible reaction will eventually attain dynamic equilibrium in a closed system. Starting with the number of reactants or products, the forward and reverse reaction rates will ultimately reach equilibrium. This will result in no net change in the concentration of reactants or products.

Under specific conditions, a dynamic equilibrium will have a certain ratio of reactants to products. This ratio is expressed with the constant of equilibrium, Keq. There are various equilibrium constants, such as Kc, which measures the equilibrium concentration rate, and Kp, which measures the partial pressure.

Examples Of Reversible Reactions

The reaction between hydrogen (H2) and iodine (I2) produces hydrogen iodide (HI).

H2 (g) + I2 (g) ⇆ 2 HI (g)

 When sulfur dioxide (SO2) reacts with oxygen (O2) to make sulfur trioxide (SO3).

2 SO2 (g) + O2 (g) ⇆ 2 SO3 (g)

When nitrogen (N2) reacts with hydrogen (H2) to produce ammonia (NH3).

N2 (g) + 3 H2 (g) ⇋ 2 NH3 (g)

The forward reaction is exothermic whereas the reverse is endothermic.

When heated, blue-hydrated copper sulfate (CuSO4.5H2O) converts into white anhydrous copper sulfate (CuSO4).

CuSO4.5H2O (s) ⇋ CuSO4 (s) + 5 H2O (g)

When heated, the white solid of ammonium chloride (NH4Cl) breaks down into ammonia (NH3) and hydrogen chloride (HCl).

NH4Cl (s) ⇋ NH(g) + HCl (g)

Dynamic Equilibrium

When a reversible reaction occurs in a closed container, dynamic equilibrium is reached. At equilibrium:

  • The forward and reverse reactions continue to occur.
  • Both forward and reverse reactions have the same reaction rate.
  • Constant concentrations of all reacting substances exist.

Energy Changes and Reversible Chemical Reactions

If a reversible reaction is exothermic in one direction, it must be endothermic in the opposite direction, and vice versa.

Copper sulfate blue is hydrated, meaning that the copper and sulfate ions in its crystal structure are enveloped by water molecules. When heated, the water is driven off, leaving behind a white solid copper sulfate that is anhydrous. As soon as water is present, anhydrous copper sulfate will convert back into the blue hydrated form. The forward reaction is endothermic, so the reverse reaction must be exothermic in order to absorb thermal energy.

Predicting the direction of a reversible chemical reaction

  • The direction of a reversible reaction and its impact on the ratio of reactants to products can also be predicted using our understanding of reversible reactions and equilibria.
  • When the rate of the forward reaction exceeds that of the backward reaction, a greater proportion of the reactants will be converted into products, resulting in a net production of products.
  • In the event that the rate of the reverse reaction surpasses that of the forward reaction, a greater proportion of the products will undergo conversion into reactants, resulting in a net generation of reactants.
  • When the rates of the forward and backward reactions are equal, the system reaches a state of dynamic equilibrium, where there is no net production of either reactants or products.
  • It is imperative that one possesses the ability to elucidate the process by which a reversible chemical reaction attains a state of equilibrium.

References

  • https://www.bbc.co.uk/bitesize/guides/z32bpbk/revision/2
  • https://www.thoughtco.com/definition-of-reversible-reaction-and-examples-605617
  • Helmenstine, Anne Marie, Ph.D. “Reversible Reaction Definition and Examples.” ThoughtCo, Apr. 5, 2023, thoughtco.com/definition-of-reversible-reaction-and-examples-605617.
  • https://www.chemistrylearner.com/chemical-reactions/reversible-reaction
  • https://www.studysmarter.co.uk/explanations/chemistry/physical-chemistry/reversible-reaction/
  • https://www.thesciencehive.co.uk/reversible-reactions-aqa

About Author

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Kabita Sharma

Kabita Sharma is a graduate student from the central department of chemistry, Tribhuvan University. She has been actively involved in research related to natural products, computational chemistry, and nanochemistry. She is currently working on enzyme assay, molecular docking, and molecular dynamic simulation.

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