Difference Between Lattice Energy and Hydration Energy

Key Difference – Lattice Energy vs Hydration Energy
 

Lattice energy and hydration energy are two related terms in thermodynamics. Lattice energy is the amount of energy that is released when a lattice is formed. Hydration energy is the energy that is released when the lattice is dissolved in water. Both formation and hydration of a lattice release energy because both processes involve chemical bond (or chemical interaction) formation. The key difference between lattice energy and hydration energy is that lattice energy is the amount of energy released when a mole of the lattice is formed from infinitely separated ions whereas hydration energy is the amount of energy released when a lattice is separated into ions by solvation in water.

CONTENTS

1. Overview and Key Difference
2. What is Lattice Energy
3. What is Hydration Energy
4. Relationship Between Lattice Energy and Hydration Energy
5. Side by Side Comparison – Lattice Energy vs Hydration Energy in Tabular Form
6. Summary

What is Lattice Energy?

Lattice energy is a measure of the energy contained in the crystal lattice of a compound, which is equal to the energy that would be released if the component ions were brought together from infinity. In other words, lattice energy is the energy required in the formation of a crystal from completely separated ions. It is very difficult to measure the lattice energy experimentally. Hence, it is derived theoretically.

Figure 01: Lattice Energy

The value of the lattice energy is always a negative value. That is because the formation of lattice involves the formation of chemical bonds. The formation of chemical bonds is exothermic chemical reactions, which release energy. The theoretical value for lattice energy is determined as follows.

Δ_GU     =     Δ_GH    –    p.ΔV_m

In which, Δ_GU is the molar lattice energy, Δ_GH molar lattice enthalpy and ΔV_m is the change of volume per mole. P is the external pressure. Therefore, lattice energy can also be defined as the work that has to be done against the external pressure, p.

What is Hydration Energy?

Hydration energy (or enthalpy of hydration) is the amount of energy released when one mole of ions undergoes hydration. Hydration is a special type of dissolution of ions in water. Ions can be either positively charged or negatively charged chemical species. When a solid ionic compound is dissolved in water, the outermost ions of that solid move away from the solid and are dissolved in water. There, the released ions are covered with neighbouring water molecules.

The hydration of an ionic compound includes intramolecular interactions. These are ion-dipole interactions. Enthalpy of hydration or the hydration energy is the energy that is released when ions are dissolved in water. The hydration is hence an exothermic reaction. That is because the dissolution of ions creates interactions between ions and water molecules. Formation of interactions releases energy because the hydration stabilizes the ions in an aqueous solution.

Figure 02: Hydration of Na+ and Cl- ions

Hydration energy is denoted as H_hyd. When the hydration energies of different ions are considered, the value of hydration energy decreases with the increase of ionic size. That is because, when the ionic size is increased, the electron density of the ion is decreased. Then the interaction between the ion and water molecules is also decreased resulting in a decreased hydration energy.

What is the Relationship Between Lattice Energy and Hydration Energy?

• The energy of dissolution is equal to the sum of lattice energy and hydration energy. That is because, in order to dissolve a lattice in water, the lattice should undergo dissociation and hydration. The lattice should be provided with an amount of energy that can dissociate the lattice into ions. This is equal to the lattice energy.

What is the Difference Between Lattice Energy and Hydration Energy?

Summary – Lattice Energy vs Hydration Energy

Lattice energy relates to the formation of a lattice whereas hydration energy relates to the destruction of a lattice. The difference between lattice energy and hydration energy is that lattice energy is the amount of energy released when a mole of the lattice is formed from infinitely separated ions whereas hydration energy is the amount of energy released when a lattice is separated into ions by solvation in water.

Reference:

1.“Lattice energy.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here  
2.“Lattice Energy: The Born-Haber cycle.” Chemistry LibreTexts, Libretexts, 15 Feb. 2017. Available here    

Image Courtesy:

1.’Lattice-enthalpy-NaCl-3D-ionic’By Benjah-bmm27 – Own work,  (Public Domain) via Commons Wikimedia
2.’OSC Microbio 00 AA hydration’By CNX OpenStax  (CC BY 4.0) via Commons Wikimedia  

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