Difference Between Hydrolysis and Hydration

Main Difference – Hydrolysis vs Hydration

Hydrolysis is the addition of a water molecule to a compound by splitting the water molecule and breaking a chemical bond in the compound. Therefore, hydrolysis is considered as a double decomposition reaction. The term hydration has two different applications in organic chemistry and inorganic chemistry. In organic chemistry, it is the addition of a water molecule to an alkene or alkyne. But in inorganic chemistry, hydration is the association or the combination of water molecules without cleaving the water molecule. Therefore the main difference between hydrolysis and hydration is that hydrolysis includes the split of a water molecule whereas hydration does not always include the split of a water molecule.

Key Areas Covered

1. What is Hydrolysis
      – Definition, Different Types
2. What is Hydration
     – Definition, Applications in Organic and Inorganic Chemistry
3. What is the Difference Between Hydrolysis and Hydration
     – Comparison of Key Differences

Key Terms: Alkene, Alkyne, Brønsted-Lowry Acid-Base Theory, Decomposition Reaction, Hydration, Hydrolysis, Solubility Constant

What is Hydrolysis

Hydrolysis is a double decomposition reaction with water as one of the reactants. In other words, it is the breaking of a bond in a molecule using a water molecule. Hydrolysis reactions are often reversible. There are three main types of hydrolysis reactions:

1. Acid hydrolysis
2. Base hydrolysis
3. Salt hydrolysis

Water can act as an acid or a base according to the Brønsted-Lowry acid theory (according to this theory, when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton). If water acts as a Brønsted-Lowry acid, then water molecule donates a proton. If it acts as a Brønsted-Lowry base, then the water molecule can accept a proton, forming hydronium ion (H₃O⁺). Acid hydrolysis resembles an acid dissociation reaction.

Figure 1: Hydrolysis of Succinic Anhydride

Base hydrolysis resembles a base dissociation reaction. Here, water donates a proton, producing a hydroxide anion (OH^–). Therefore, water acts as a Brønsted-Lowry acid.

When a salt is dissolved in water, the salt dissociates into its ion. This dissociation can be either a complete dissociation or an incomplete dissociation based on the solubility constant. But when a salt of a weak acid or a weak base is dissolved in water, water can ionize the salt and form hydroxide anions and hydronium cations. The salt also separates into its cations and anions. This is known as salt hydrolysis.

What is Hydration

Hydration is a chemical process in which water molecules combine with a substance. In organic chemistry, hydration refers to the addition of a water molecule to an unsaturated compound, an alkene or alkyne. But in inorganic chemistry, hydration refers to the association of water molecules with compounds.

In organic chemistry, the water molecule is added to the point where an unsaturation is present. Here, the water molecule cleaves into a proton and a hydroxide anion. The hydroxide anion forms a bond with the carbon atom that has more substituents. The proton will combine with the less substituted carbon, following the Markovnikov’s rule. Any unsaturated organic molecule is susceptible to hydration.

In inorganic chemistry, hydration refers to the association of water molecules with an inorganic compound. For example, in the sulphate process for the production of TiO₂ pigments from Ilmenite sand (FeTiO₃), FeSO4 is formed as a byproduct. This byproduct through crystallization via hydration. Here, FeSO₄.7H₂O is formed by the reaction of FeSO₄ with water followed by cooling to 10^oC. Then FeSO₄.7H₂O crystals, which can easily be removed, are formed. The crystals are ferrous sulphate heptahydrate.

Figure 2: Hydrated Sodium Cation and Chloride Anion

Hydration is the chemical process that occurs with desiccants. A desiccant is any compound that can absorb water vapor. Furthermore, the term hydration is also used to explain the dissolution of salt ions such as sodium ion. When a salt is dissolved in water, salt dissociates into its cations and anions. These cations and anions are separated from each other due to the hydration of the ions by water molecules. Here, the water molecules will surround the salt ion, which is called hydration.

Difference Between Hydrolysis and Hydration

Definition

Hydrolysis: Hydrolysis is a double decomposition reaction with water as one of the reactants.

Hydration:  Hydration is a chemical process in which water molecules combine with a substance.

Chemical Bonds

Hydrolysis: Hydrolysis occurs with bond cleavage in the water molecule.

Hydration: Hydration of organic molecules occurs with bond cleavage, but in inorganic compounds, no cleavage takes place.

Nature

Hydrolysis:  Hydrolysis forms saturated compounds from unsaturated compounds.

Hydration: Hydration forms hydrated compounds from dehydrated compounds.

Conclusion

Hydrolysis and hydration are two different terms students often confuse. The main difference between hydrolysis and hydration is that hydrolysis includes the split of a water molecule whereas hydration does not always include the split of a water molecule.

Reference:

1. “Hydrolysis.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 16 Nov. 2016, Available here.
2. “Hydrolysis.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here.
3. “Hydrolysis.” Wikipedia, Wikimedia Foundation, 24 Jan. 2018, Available here.

Image Courtesy:

1. “Hydrolysis of Succinic Anhydride” By –Drdoht (talk) 00:26, 21 February 2016 (UTC) – Own work (Public Domain) via Commons Wikimedia
2. “Hydration” By Kkeyshar – Own work (Public Domain) via Commons Wikimedia