Enthalpy of Dilution

What is Enthalpy of Dilution?

Enthalpy of dilution, also known as the heat of dilution, can be defined as the change in enthalpy that is associated with the dilution of a specific component of a solution when the pressure is kept constant. Usually, the enthalpy of dilution of a component in a solution is expressed in terms of energy per amount of substance. However, this quantity can also be expressed in terms of energy per unit mass. The most common units used to express the enthalpy of dilution are joules per mole (J/mol) and kilojoules per mole (kJ/mol).

Given that a solution exists in the liquid phase if a pure liquid component is dissolved into the solution, the enthalpy of dilution will be the same as the enthalpy of dissolution (also known as the enthalpy of solution). This is because the dilution process in this scenario is the same as the dissolution process for the component being dissolved.

Understanding Enthalpy of Dilution – Differential and Integral Perspectives

Enthalpy of dilution (or heat of dilution) can be defined in two different ways – in terms of integral heat and in terms of differential heat. When it comes to differential heat of dilution, the enthalpy is considered at a very small scale. Here, the primary focus is the change in enthalpy associated with the addition of a very small quantity of the solvent to a relatively huge quantity of the solution. Therefore, from the differential perspective, the molar differential enthalpy of dilution can be defined as the change in enthalpy associated with the addition of one mole of the solvent to a comparatively large amount of solution when the pressure and the temperature of the environment is kept constant. It is important to note that the overall change in the concentration of the solution in this scenario is negligible because a very small quantity of solvent is added to the solution.

When viewed from the integral perspective, the enthalpy of dilution is calculated by considering the solution at the macroscopic scale. Here, a dilution process is considered where the solution is diluted from a specific initial concentration to a specific final concentration. Therefore, the molar integral enthalpy of dilution (also known as the molar integral heat of dilution) can be evaluated by calculating the total enthalpy change associated with the change in the dilution of the solution (from its initial concentration to its final concentration) and normalizing this value by the number of moles of the solute.

It is important to note that if an infinite quantity of the solvent is added to a given solution in which the concentration of the solute is known, the resulting change in enthalpy is called the integral enthalpy of dilution to infinite dilution. It can also be noted that the dilution that occurs between any two given values of solute concentration can be associated with the intermediary enthalpy of dilution (per mole of solute).

Distinguishing between Dissolution and Dilution

Due to the fact that their processes are quite similar to each other, dissolution and dilution are closely related. However, it is important to note that dissolution and dilution are not the same. Despite the fact that both processes have similar final stages, they differ in their initial stages. To elaborate, the solute initially exists in a pure phase (be it solid, liquid, or gaseous) initially before getting converted to the solution phase in a dissolution process. However, when it comes to dilution, the solution as a whole experience a shift or a change in concentration.

However, it can be observed that the key similarity between dissolution and dilution is that they both occur in three different phases:

• In the first phase, the forces of attraction that hold the solute particles together are broken. This is often referred to as the lattice energy of the solute.
• In the second phase, the attractive forces between solvent molecules are also broken.
• Finally, in the third phase, attractive forces arise between the solute molecules and the solvent molecules.

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