which of the following steps in solution formation is exothermic

2 min read 21-02-2025

which of the following steps in solution formation is exothermic

Which Steps in Solution Formation Are Exothermic?

The formation of a solution isn't a single step, but rather a series of processes. Understanding which of these steps are exothermic (release heat) and which are endothermic (absorb heat) is crucial to predicting the overall enthalpy change of solution formation. Let's break it down.

The Three Key Steps in Solution Formation

Separation of solute particles: This involves overcoming the attractive forces holding the solute molecules or ions together in their solid or liquid state. This always requires energy input, making it an endothermic process. Think of it like breaking apart a crystal lattice – you need to put in energy to disrupt those bonds.
Separation of solvent particles: Similarly, separating solvent molecules from each other to create space for the solute requires overcoming intermolecular forces (like hydrogen bonds in water). This is also an endothermic process. The solvent molecules need to be "pushed apart" to accommodate the solute.
Interaction of solute and solvent particles (solvation): This is where the solute particles are surrounded by solvent molecules. The attractive forces between the solute and solvent molecules are formed, releasing energy. This step is often exothermic. This is the "mixing" part where new attractions are formed, releasing energy as heat.

Which Step is Exothermic? The Overall Picture

While steps 1 and 2 are always endothermic, step 3 (solvation) can be either endothermic or exothermic depending on the specific solute and solvent. The overall enthalpy change of solution formation (ΔHsoln) is the sum of the enthalpy changes for these three steps:

ΔHsoln = ΔHseparation of solute + ΔHseparation of solvent + ΔHsolvation

If the energy released during solvation (ΔHsolvation) is greater than the energy required for separating the solute and solvent (ΔHseparation of solute + ΔHseparation of solvent), then the overall process is exothermic (ΔHsoln < 0). Conversely, if the energy required for separation is greater, the process is endothermic (ΔHsoln > 0).

Therefore, the exothermic step is the interaction (solvation) of solute and solvent particles. However, the overall solution formation process can be either exothermic or endothermic, depending on the relative magnitudes of the energy changes in each step.

Examples of Exothermic Solution Formation

Many common solutions form exothermically. Dissolving sodium hydroxide (NaOH) in water is a classic example. You'll feel the solution get noticeably warmer. This indicates a large exothermic solvation process, where the energy released from the solute-solvent interactions outweighs the energy needed for separation.

Examples of Endothermic Solution Formation

Dissolving ammonium nitrate (NH4NO3) in water is an example of an endothermic solution formation. You'll feel the solution become cooler. In this case, the energy required to separate the ions and water molecules is greater than the energy released during solvation.

Factors Affecting Exothermicity/Endothermicity

Several factors influence whether the overall process will be exothermic or endothermic:

Solute-solvent interactions: Strong solute-solvent interactions (like those between polar solutes and polar solvents) favor exothermic solutions.
Solute-solute and solvent-solvent interactions: Strong solute-solute and solvent-solvent interactions (like those in ionic crystals or highly structured liquids) require more energy to break apart, making endothermic solutions more likely.
Temperature: Temperature affects the kinetic energy of particles, influencing the likelihood of successful interactions.

In conclusion, while the solvation step is often exothermic, the overall process of solution formation depends on the balance between the endothermic steps of separating solute and solvent and the exothermic step of solute-solvent interaction. Careful consideration of these steps is crucial for understanding the thermodynamics of solution formation.