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in terms of energy of reaction, explain how a cold compress used by an athlete works.

A cold instant compress works because it uses an endothermic reaction: the chemicals inside absorb energy (heat) from the surroundings, so the pack (and the athlete’s skin) get colder.

What is inside the cold compress?

Inside a typical disposable cold pack you usually have:

  • One compartment with water.
  • One compartment (or inner pouch) with a solid salt, often ammonium nitrate, urea, or similar.

When the athlete squeezes or strikes the pack, the inner barrier breaks and the solid mixes with the water.

Energy of reaction: step by step

Think of the process in energy terms:

  1. Breaking bonds (requires energy)
    • The ionic crystal of the salt (for example, ammonium nitrate) has ions held together by strong electrostatic forces.
    • To dissolve, those ionic bonds and the attractions in the crystal must be disrupted.
    • This step uses energy; that energy has to come from somewhere, and it comes from the surrounding water and from the athlete’s skin.
  1. Forming new interactions (releases energy)
    • Once the salt dissolves, its ions become surrounded (hydrated) by water molecules.
    • Ion–dipole attractions form between the ions and water, and these new interactions release some energy.
  1. Overall energy balance (endothermic)
    • In a cold pack, the energy needed to break the ionic lattice and separate particles is greater than the energy released when the ions are hydrated.
    • So the net enthalpy change ΔH\Delta HΔH is positive (endothermic): the reaction absorbs heat from the surroundings.
 * As heat flows into the dissolving mixture, the temperature of the pack and the nearby tissue drops, often by several degrees.

An illustrative way to picture it: the dissolving salt is “pulling” thermal energy out of your skin to pay the energetic cost of breaking its crystal structure.

Energy diagram in words

If you drew an energy diagram:

  • Reactants (water + solid salt) would be at a lower energy level.
  • Products (salt solution) would be at a higher energy level.
  • The vertical gap between these levels is ΔH>0\Delta H>0ΔH>0: that extra energy has to be absorbed from the surroundings, which is why you feel cold.

So, in thermochemistry terms:

  • The cold compress relies on an endothermic dissolution reaction with positive enthalpy change.
  • The reaction absorbs heat from the athlete’s body, decreasing the temperature at the injury site and giving the cooling effect.

TL;DR:
A cold compress for athletes works because a salt dissolves in water in an endothermic process; the reaction absorbs heat from the athlete’s skin, lowering the local temperature and making the pack feel cold.

Information gathered from public forums or data available on the internet and portrayed here.