Question

Hot packs used by skiers produce heat via the crystallization of sodium acetate from a concentrated solution. What is the sign of \(\Delta H\) for this crystallization? Is the reaction exothermic or endothermic?

Short answer

\(\Delta H\) is negative and the reaction is exothermic.

Step-by-step solution

Understand Crystallization Process

Crystallization is the process where a substance transitions from a liquid or gas phase into a solid phase.

Analyze Heat Production

If a reaction produces heat, it releases energy into its surroundings. This type of reaction is known as exothermic.

Determine \(\Delta H\)

The enthalpy change (\(\Delta H\)) of an exothermic reaction is negative because energy is released.

Conclusion

Since heat is produced in the crystallization of sodium acetate, \(\Delta H\) is negative and the reaction is exothermic.

Key concepts

Crystallization Process

Crystallization is a fundamental process where a substance moves from a fluid or gas state to a solid state. It is commonly used in various fields, such as chemistry and pharmaceuticals. The process involves the formation of solid crystals from a homogeneous solution.
Crystallization typically happens through nucleation, where a seed crystal forms, and then growth, where additional molecules adhere to it. This phase change is essential:

• It often purifies the substance, as only the compound of interest crystallizes.
• It is essential in manufacturing, especially for substances like sodium acetate.
  In the case of hot packs, sodium acetate is dissolved at high concentrations. When activated, it rapidly crystallizes, releasing heat.

Enthalpy Change

Enthalpy change, represented by \(abla H\), is a measure of the heat energy released or absorbed during a chemical process at constant pressure. It is a critical concept in thermodynamics.
Enthalpy change can be positive or negative:

• Positive \(abla H\): Indicates endothermic reactions where the system absorbs heat.
• Negative \(abla H\): Indicates exothermic reactions where the system releases heat.
  For the crystallization of sodium acetate, the process is exothermic. Heat production signifies energy release, making \(abla H\) negative. You can think of it like a cozy blanket on a cold day - the blanket is the solidifying sodium acetate releasing warm energy around it.

Sodium Acetate

Sodium acetate is a versatile compound with many uses, most famously in hot packs for providing warmth. In chemical terms, its formula is \(Co_3 - COONa\).
Here’s how it works in hot packs:

• Sodium acetate is dissolved in water at a high concentration.
• Upon activation, it crystallizes and releases heat due to its exothermic reaction.
  Sodium acetate solutions are supercooled - they remain liquid below their freezing point until they are disturbed or 'activated'. At this point, they solidify rapidly, releasing significant heat. This makes sodium acetate ideal for portable heating devices used by skiers and outdoor enthusiasts.