Problem 48 - Chemistry The Molecular Science

Question

When table salt is dissolved in water, the temperature drops slightly. Write a chemical equation for this process and indicate if it is exothermic or endothermic.

Step-by-Step Solution

Answer

The dissolution of NaCl in water is endothermic.

Step 1: Understand the Dissolution Process

When table salt, or sodium chloride (NaCl), is dissolved in water, it separates into sodium ions (Na⁺) and chloride ions (Cl⁻). The chemical equation for this process can be written as: \[ \text{NaCl}_{(s)} \rightarrow \text{Na}^+_{(aq)} + \text{Cl}^-_{(aq)} \] Here, "(s)" denotes solid state, and "(aq)" denotes aqueous or dissolved in water.

Step 2: Recognize the Temperature Change

Observe that the temperature of the solution drops when NaCl is dissolved. This implies that the process absorbs heat from the surroundings.

Step 3: Classify the Reaction

A process that absorbs heat from the surroundings and leads to a drop in temperature is known as "endothermic." As the dissolution of NaCl in water causes a temperature decrease, it is classified as endothermic.

Key Concepts

Dissolution Process

The dissolution process is an important concept in chemistry that occurs when a solute (such as table salt) interacts with a solvent (like water) to form a solution. In this process, the solid structure of the solute breaks down, and individual particles become surrounded by solvent molecules.
When sodium chloride ( NaCl ) is placed in water, the water molecules begin to interact with the NaCl crystals. The polar nature of water molecules helps them to pull apart the ions in the salt. This separates sodium ions ( Na^+ ) and chloride ions ( Cl^- ), dispersing them evenly throughout the water.
Here’s a breakdown of the dissolution process:

• Ionic Interaction: Water molecules have a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. These charges allow water to attract the positive Na^+ ions and the negative Cl^- ions, leading to the formation of an aqueous solution.
• Hydration: Each of the ions becomes surrounded by water molecules. This is referred to as hydration, as the ions are now hydrated by water.

This process continues until the entire salt is dissolved, provided there is sufficient solvent available.

Chemical Equations

Chemical equations provide a concise representation of chemical reactions, illustrating the transformation of reactants into products. For the dissolution of sodium chloride in water, the chemical equation succinctly captures this change.
The chemical equation for this process is:\[ \text{NaCl}_{(s)} \rightarrow \text{Na}^+_{(aq)} + \text{Cl}^-_{(aq)} \]
Let's break down this equation:

• Initial State: NaCl_{(s)} denotes solid sodium chloride.
• Dissociation: The arrow ( \rightarrow ) indicates the process of dissolution, where NaCl separates into ions.
• Product Ions: Na^+_{(aq)} and Cl^-_{(aq)} are the resulting ions, with "(aq)" specifying they are in an aqueous, or water-based, solution.

Such equations help chemists communicate the specifics of chemical processes in a clear and standard format, enhancing understanding of the underlying changes.

Temperature Change

A fascinating aspect of the dissolution process is the temperature change which occurs. This can tell us a lot about the nature of the reaction. When sodium chloride dissolves in water, the temperature of the solution drops slightly. This phenomenon is a clear indicator that the process is endothermic.
An endothermic reaction is one that absorbs heat from its surroundings. Let’s dive deeper into what this means:

• Energy Absorption: During the dissolution of NaCl , energy is required to break the ionic bonds in the salt and to overcome forces between water molecules. This energy absorption results in a temperature drop.
• Surrounding Temperature: The fact that the temperature decreases means that energy from the surrounding environment is consumed, cooling the solution.

Observing such temperature changes not only helps identify the nature of the reaction but also provides insight into energy transformations occurring at a molecular level during the dissolution process.