Question

Solid ammonium chloride is added to water in a beaker and dissolves. The beaker becomes cold to the touch. (a) Make an appropriate choice of system and surroundings and describe it unambiguously. (b) Explain why you chose the system and surroundings you did. (c) Identify transfers of energy and material into and out of the system that would be important for you to monitor in your study. (d) Is the process of dissolving \(\mathrm{NH}_{4} \mathrm{Cl}(\mathrm{s})\) in water exothermic or endothermic? Explain your answer.

Short answer

(d) The process is endothermic; energy is absorbed into the system.

Step-by-step solution

Define the System

The system is the mixture of ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{s})\)) and the water in the beaker as it dissolves.

Identify the Surroundings

The surroundings are everything outside the mixture, including the beaker itself and the room's air.

Reason for System Choice

This choice is made because we want to observe the process of the solid dissolving in the liquid, which is the primary area where changes and interactions occur.

Monitor Energy Transfer

As the beaker becomes cold, it signifies that thermal energy is absorbed from the surroundings into the system. No significant material crosses the boundary—only energy transfer is relevant here.

Determine Process Type

Since thermal energy is absorbed from the surroundings, the process is endothermic. An endothermic reaction absorbs energy, causing the surroundings (like the beaker) to feel cold.

Key concepts

Endothermic Processes

Endothermic processes are fascinating phenomena in thermodynamics. They occur when a system absorbs energy from its surroundings in the form of heat. For example, when ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}\)) dissolves in water, the beaker holding the mixture becomes cold. This sensation happens because the dissolution process requires energy to break the ionic bonds in the solid and interact with the water molecules. Instead of releasing heat, the system draws it from its surroundings, making the beaker's feel cold on touch.
This feature is characteristic of endothermic reactions. You can think of it as a two-step process:

• Breaking bonds in the solid requires energy input.
• Forming new interactions between dissolved ions and water molecules may release less energy than what was absorbed, leading to a net energy intake.

Generally, endothermic reactions include melting ice, evaporating liquid, or chemical reactions where products have higher energy than reactants.

Energy Transfer

Energy transfer is a crucial concept in understanding thermodynamics. In the described experiment, the energy transfer is evident since the beaker feels cold during the dissolution. This coldness is due to the absorption of thermal energy from the surroundings into the system. Energy can be transferred through different means: heating, working (doing work), and radiation.
In our context, heating is the form of energy transfer occurring. When thermal energy is absorbed from the surroundings, the temperature of the surroundings decreases. This is because energy moves from a higher temperature region (the surroundings) to a lower temperature region (the system). While discussing energy transfer in an endothermic process, remember:

• The direction of energy flow is from surroundings to system.
• No work is done on or by the system in this scenario.
• The temperature change in the surroundings can indicate how much energy is transferred.

System and Surroundings

Defining a system and its surroundings is fundamental in thermodynamics. The system refers to the area or substance of interest, while the surroundings include everything else. This division helps in analyzing energy and material flows in and out of the system. In our studied case, the system consists of ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}\)) and water in the beaker.
The surroundings comprise the beaker and air around it. This setup is chosen to focus on direct interactions influencing the reaction—specifically, the dissolution process. Important points to remember about systems and surroundings:

• The system boundaries are crucial for identifying energy exchanges.
• Material transfer is typically negligible unless specifically mentioned.
• This separation allows for precise monitoring of energy changes occurring during reactions.

Dissolution

Dissolution in thermodynamics refers to the process in which a solute dissolves in a solvent, forming a solution. In the example given, when ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}\)) dissolves in water, the process is endothermic, highlighting that energy is absorbed during dissolution.
Dissolution often involves:

• Breaking ionic bonds in the solute molecules.
• Disrupting interactions in the solvent (e.g., hydrogen bonds in water).
• Forming new interactions between solute ions and solvent molecules.

Overall, the energy absorbed or released during dissolution depends on whether breaking solute bonds and solute-solvent interactions costs more energy than is released when new interactions form. In endothermic dissolutions like in our case, more energy is absorbed than released.