Question

When \(\mathrm{NaOH}\) dissolves in water, the solution feels warm. What can we say about the relative strengths of the forces between solute and solvent? Between the particles in the pure substances?

Short answer

The forces between the solute NaOH and the solvent water are stronger than those present between the NaOH molecules themselves in the pure substance. This conclusion is based on the exothermic nature of dissolution, where the release of heat indicates the formation of stronger, more stable bonds between solute and solvent than those in the solute alone.

Step-by-step solution

Interpret the Information

An exothermic process gives off heat, which means energy is being released. The heat released indicates that the process of breaking the ionic bonds in NaOH and forming new bonds with water molecules is occurring. From this, deduce that the energy released when new bonds form is greater than the energy absorbed to break the existing bonds.

Compare the Forces

The overall process is exothermic which means forces between the NaOH and water (the solute and solvent respectively) are stronger than those between the NaOH molecules themselves. This can be inferred because when NaOH dissolves, energy is released as the bonds between NaOH and water molecules are stronger consequently more stable; this releases energy (as warmth in this case).

Conclude the Reasoning

Considering the entire process, we can say that the forces between the NaOH (solute) and water (solvent) are stronger than the forces among the NaOH molecules themselves in their pure state. The process is exothermic, implying the formation of stronger bonds than those that were broken. In a nutshell, the forces between solute and solvent have higher strengths in comparison to those between particles in the pure substances.

Key concepts

Chemical Bonds in Dissolution

Understanding the nature of chemical bonds during dissolution is crucial to comprehend why substances like NaOH dissolve in water. Dissolution is a chemical process in which a solute (e.g., NaOH) integrates into a solvent (e.g., water) to form a solution. This process involves breaking and forming of chemical bonds.

When NaOH dissolves in water, the ionic bonds between the sodium (Na+) and hydroxide (OH-) ions in the solid NaOH are broken. At the same time, new bonds between these ions and the polar water molecules are formed. These new interactions are hydrogen bonds and ion-dipole interactions, which are quite strong due to the polarity of water molecules. If this dissolution releases heat, as is the case with NaOH, it's an indication that the energy released when new bonds are formed is greater than the energy needed to break the original ionic bonds.

The specific interactions with water molecules, such as the hydrogen bonding with OH- and the ion-dipole interactions with Na+, are the major contributors to the dissolution of NaOH in water.

NaOH Solubility

NaOH, commonly known as sodium hydroxide or caustic soda, is highly soluble in water. Solubility refers to the capability of a substance to dissolve in a solvent, and in the context of NaOH, its solubility is attributed to the favorable interactions between NaOH and water. Upon dissolution, NaOH dissociates into sodium ions (Na+) and hydroxide ions (OH-) which are then stabilized by the surrounding water molecules.

The polar nature of water molecules allows them to surround the ions effectively, reducing the electrostatic attraction between Na+ and OH- ions and promoting their separation into the solvent. This is also facilitated by the high dielectric constant of water, which has the ability to diminish the forces holding the ions together. The result is a homogeneous aqueous solution of NaOH, which can be used in various applications, from soap-making to chemical reactions where a strong base is required.

Exothermic Process

An exothermic process is a chemical reaction or physical change that releases heat to its surroundings. In the context of NaOH dissolving in water, the heat felt is a direct consequence of this exothermic nature.

When the chemical bonds in NaOH solid are broken, energy is absorbed, but the formation of new bonds between the resultant ions and water molecules releases more energy than was consumed to break the original bonds. It is this net release of energy that makes the dissolution of NaOH an exothermic process. As it happens, the surroundings, including the solution itself, become warmer, which is why you can feel the warmth when touching the container where NaOH was dissolved. This exothermic property of NaOH dissolution is significant in industrial processes, where the heat released can influence reaction conditions and should be carefully managed.

Solute-Solvent Interactions

Solute-solvent interactions are the forces at play between the dissolved substance (solute) and the substance in which it dissolves (solvent). These interactions determine the extent to which a substance will dissolve. When NaOH dissolves in water, it's the strong attraction between the Na+ and OH- ions and the polar water molecules that dominate the process.

In the case of NaOH and water, the solute-solvent interactions are stronger than the interactions between NaOH molecules in the solid state. This strength disparity explains why NaOH is readily soluble and why heat is released during the process. Water's ability to strongly interact with the ions is due to its polar nature, which allows it to attract and stabilize both positive (cations) and negative (anions) charges.

Consequently, these interactions not only dictate the solubility but also affect the physical properties of the resulting solution, such as boiling point, vapor pressure, and viscosity. Understanding these interactions provides significant insight into mixing behaviors, reaction mechanisms, and the underlying principles of solubility.