which of the following steps in solution formation is exothermic

Smons

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27-02-2025

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The process of solution formation involves several steps, and understanding which are exothermic (release heat) and which are endothermic (absorb heat) is crucial to predicting the overall enthalpy change. Let's break down the steps and determine which is exothermic.

The Three Steps of Solution Formation

Creating a solution from a solute and solvent involves three key steps:

1. Separation of solute particles: Breaking the intermolecular forces holding the solute molecules or ions together requires energy input. This is always an endothermic process. Think of it like breaking apart a crystal lattice – you need energy to overcome the attractive forces.

2. Separation of solvent particles: Similarly, overcoming the intermolecular forces between solvent molecules necessitates energy. This step is also endothermic. Imagine making space between water molecules to accommodate the solute.

3. Mixing of solute and solvent particles: Once the solute and solvent particles are separated, they interact and form new attractive forces. This process often releases energy, making it exothermic. The attractive forces between solute and solvent molecules (solvation) are responsible for this.

The Exothermic Step: Solvation

The key to understanding the overall enthalpy change of solution formation lies in comparing the energy required for steps 1 and 2 (endothermic) with the energy released in step 3 (exothermic). Step 3, the mixing of solute and solvent particles (solvation), is the exothermic step.

The overall process can be either exothermic or endothermic, depending on the relative magnitudes of the enthalpy changes in each step. If the energy released during solvation (step 3) is greater than the energy required for separating the solute and solvent (steps 1 and 2), then the overall process is exothermic. Conversely, if the energy required for separation exceeds the energy released during solvation, the overall process is endothermic.

Example: Dissolving Sodium Chloride in Water

When table salt (NaCl) dissolves in water, the crystal lattice of NaCl must be broken (endothermic). The hydrogen bonds between water molecules must be disrupted (endothermic). However, the strong ion-dipole interactions between the Na⁺ and Cl⁻ ions and water molecules release a significant amount of energy (exothermic). In this case, the energy released in solvation outweighs the energy required for separation, resulting in an overall exothermic process. The solution gets warmer.

Example: Dissolving Ammonium Nitrate in Water

In contrast, dissolving ammonium nitrate (NH₄NO₃) in water is endothermic. While solvation still releases energy, the energy required to break apart the ionic bonds and disrupt the water structure is greater. The solution becomes cooler.

Why Understanding Exothermic/Endothermic is Important

Knowing whether solution formation is exothermic or endothermic is essential in various applications:

• Chemical Engineering: Predicting the heat released or absorbed is critical for designing efficient reactors and processes.
• Chemistry: Understanding the thermodynamics of solution formation allows us to predict solubility and understand reaction mechanisms.
• Environmental Science: Understanding heat transfer during dissolution can have implications for environmental impact assessments.

In summary, while the separation steps of solute and solvent particles are always endothermic, the solvation (mixing) step is the exothermic step in solution formation. The overall process's exothermic or endothermic nature depends on the balance between these energy changes.