Problem 36 - Chemistry: An Atoms-Focused Approach

Question

Breaking the small pouch of water inside a chemical cold pack containing ammonium nitrate activates the pack, which is used by sports trainers for injured athletes. What is the sign of \(\Delta H\) for the process taking place in the cold pack?

Step-by-Step Solution

Answer

Answer: The sign of ΔH for the process taking place in the chemical cold pack containing ammonium nitrate is positive.

Step 1: Understand the properties of a chemical cold pack.

A chemical cold pack is a device that is used to provide instant cold therapy to injured athletes. When the small water pouch inside the pack is broken, it dissolves the ammonium nitrate, which results in a cooling effect. In this situation, the cooling effect indicates that the reaction is endothermic since it absorbs heat from the surroundings, causing a decrease in temperature.

Step 2: Determine the sign of ΔH for an endothermic reaction.

In an endothermic reaction, heat is absorbed by the system from the surroundings. Consequently, the enthalpy of the system increases. The sign of \(\Delta H\) for an endothermic reaction is positive.

Step 3: Conclude the sign of ΔH for the process in the cold pack.

Given that the reaction taking place in the chemical cold pack is endothermic, the sign of \(\Delta H\) will be positive. So, the sign of \(\Delta H\) for the process taking place in the chemical cold pack containing ammonium nitrate is positive.

Key Concepts

Ammonium Nitrate

Ammonium nitrate is a chemical compound that is often used in chemical cold packs to provide instant cooling. This white crystalline solid is highly soluble in water, and when it dissolves, it undergoes an endothermic process. An endothermic reaction is one where the system absorbs heat from the surrounding environment.

• In a chemical cold pack, the ammonium nitrate is typically kept separate from water until the moment it's needed.
• When the user breaks a smaller internal water bag, the water mixes with the ammonium nitrate.
• This mixing starts the dissolution reaction with the ammonium nitrate, which absorbs heat, causing the temperature to drop quickly.

This rapid cooling effect is why ammonium nitrate is favored in cold packs, especially useful in first-aid situations for sports injuries.

Chemical Cold Pack

A chemical cold pack is a convenient device designed to deliver immediate cold therapy. Simply put, it's a portable way to cool down affected areas without the need for a refrigerator. Inside a chemical cold pack, you typically find two separate compartments: one for water and one for chemicals like ammonium nitrate that remain inactive until needed.

• The water and the chemicals are isolated from each other until the pack needs to be activated.
• By pressing or squeezing the pack, you break an inner pouch, allowing water to meet ammonium nitrate.
• This interaction leads to an endothermic reaction, where the ammonium nitrate dissolves in the water.

This process absorbs heat from the surrounding environment, or in this case, from the injury site, providing effective cold therapy. This method is quick and doesn't rely on external cooling sources, making it a popular choice for emergencies and outdoor activities.

Enthalpy Change

Enthalpy change, symbolized as \(\Delta H\), is a crucial concept to understand when dealing with chemical reactions like the one in a chemical cold pack. Enthalpy refers to the total heat content of a system, and a change in enthalpy indicates energy absorbed or released during a reaction.

• An endothermic reaction is characterized by a positive \(\Delta H\) because it absorbs heat from its surroundings.
• In the case of a cold pack, when ammonium nitrate dissolves in water, the system absorbs heat, making the enthalpy change positive.
• This absorbed heat is what results in the cooling effect, as the surrounding area, such as an injured body part, loses heat to the system.

Understanding the sign of \(\Delta H\) as positive in such reactions helps explain why these devices effectively cool and soothe injuries.