Lattice Energy And Hydration Energy, Definition, Important Factors

Hydration energy  : Hydration energy is the energy released or absorbed when ions are surrounded by water molecules in an aqueous solution. The process involves the breaking of ionic bonds in the crystal lattice and the subsequent formation of ion-dipole interactions with water molecules. Hydration energy is integral in explaining the solubility of salts in water and the dissolution of ionic compounds. The strength of hydration energy is influenced by factors such as ion size & ion charge.

Lattice Energy

Lattice Energy : Lattice energy is a measure of the strength of the ionic bonds in a crystal lattice structure. When ionic compounds are formed, cations and anions come together due to electrostatic forces of attraction. The lattice energy is the energy change associated with the formation of a crystal lattice from its constituent ions. It is essentially the energy released when oppositely charged ions pack together.

Lattice energy is simply defined as,

“Amount of energy released when a solid ionic compound is formed from gaseous cation and gaseous anion”.

A ⁺ _(g) + B ^– _(g) → AB _(s) ; Lattice Energy = Negative

In other words, Energy required to completely separate one mole of solid ionic compound into gaseous constituent ions.

AB _(s) → A ⁺ _(g) + B ^– _(g) ; Lattice Energy = Positive

Exp : - NaCl _(s) → Na ⁺ _(g) + Cl ^– _(g)

• Unit of lattice energy is KJ/mole

• There is no direct method to determine lattice energy.

• We can determine the lattice energy with the help of Born Haber Cycle.

Heat of formation (Enthalpy of Formation) ∆H :

Heat of formation (Enthalpy of Formation) ∆H : Amount of energy released when 1 mole of compound is formed from its elements in pure and stable state.

Na _(s) + 1/2Cl _2(g) → NaCl _(s)

As Sodium is found in the solid state and Chlorine is found in the gaseous state, hence elements are getting combined in their pure and stable elemental state, now from these constituents, a solid compound is formed, Energy released during this process is called Heat of formation (H _f ).

∆H _f = S.E. + I.E. + D.E./2 - E.A. - L.E.

Important Factors on which Lattice Energy Depends: Lattice energy is directly related to the force of attraction.

• Lattice energy Magnitude of Charge

• Lattice energy increases

• Charge of cation increases.

• Lattice cation

• Size of cation increasing

• Size of anion is constant.

• Lattice energy decreases.

Solved Example Related To Lattice Energy And  Hydration energy :

Q1: What will be the order of lattice energy in the following?

NaF, MgF ₂ , AlF ₃

Ans. After ionisation, we will get: Na ⁺ , Mg ²⁺ , Al ³⁺ And Lattice Energy Charge.

Hence, the Order of lattice energy will be: NaF < MgF ₂ < AlF _3.

Q2: What will be the order of lattice energy in the following? NaF & MgCl ₂

Ans. Here if we consider charge factor, then Lattice energy of MgCl ₂ will be higher. On the other hand, size factor is considered, Lattice energy of NaF will be higher. In this situation, Charge will be the dominating factor. Hence, The order of Lattice energy will be: NaF < MgCl ₂

Hydration Energy :

Hydration Energy : It is the energy released when 1 mol of gaseous ions are hydrated in water. It is directly proportional to nuclear charge and inversely proportional to size. It always decreases down the group.

It is the Energy released when one mole of a gaseous ion is hydrated in large amount of water to form an aqueousion.

When a salt dissolves in water, the outermost ions situated at the lattice's edge disperse from the structure and become enveloped by adjacent water molecules. If the hydration energy is equal to or greater than the lattice energy, the salt is soluble in water. In the case of salts where the hydration energy surpasses the lattice energy, solvation takes place with the release of energy in the form of heat.

• As the charge density of an ion increases, the hydrated size (or aqueous radius) also increases.

Size :

• The increase in the hydrated size of an ion leads to a decrease in ionic mobility, consequently reducing the conductivity of ions.

Mobility :

Conductivity :

• The solubility of ionic compounds is influenced by hydration energy. When hydration energy surpasses lattice energy, the ionic compound tends to be soluble in water. Greater hydration energy corresponds to increased solubility. Conversely, a decrease in lattice energy enhances the solubility of the ionic compound.

Energy released when one mole of ions undergoes hydration (in water H ₂ O)