The cold pack: a chilly example of an endothermic reaction

Kelly Resmer
6 November 2018

Above: Image © AndreyPopov,

Has this ever happened to you or someone you know? You’re running in gym class and you sprain your ankle. Your teacher grabs the first aid kit and pulls out an instant cold pack. They press it to your ankle. It feels cold!

An instant cold pack is a plastic package filled with chemicals. When you crunch it it becomes really cold, really fast. What is happening? How can the chemicals mix and make such a chilly reaction so quickly?

The answer can be found in thermodynamics!

Two main types of thermodynamic reactions

Thermodynamics is a branch of science that explores heat and energy. In thermodynamics, chemical reactions can be classified as endothermic or exothermic.

Exothermic reactions are reactions that give off energy in the form of heat. You’ve probably seen many examples of them. For example, a fire is one type of exothermic reaction.  A fire gives off heat when you burn a fuel source - like wood - in the presence of oxygen.

Endothermic reactions are the opposite of exothermic reactions. They take in heat energy from their surroundings. This means that the surroundings of endothermic reactions are colder at the end of the reaction.

Getting back to that injury...

An instant cold pack is the perfect example of an endothermic reaction. There are many possible ingredients in an instant cold pack, but they often contain ammonium nitrate and water. The ammonium nitrate is stored in a sealed plastic bag that is surrounded by water. When you pop the plastic that contains the ammonium nitrate solid, it comes into contact with water and dissolves. The chemical reaction between ammonium nitrate and water is an endothermic reaction: the reaction takes in heat energy from its surroundings, so the bag feels cold.

But how do you know what type of reaction it is?

There are two ways you can determine if a reaction is exothermic or endothermic:

  1. You can measure the temperature change, or
  2. You can measure the energy used to break and make the reacting chemical bonds.

Measuring the temperature change is the easiest way. Scientists measure the temperature of the reaction before and after the reaction is completed. Since it’s difficult to measure the temperature of the reaction directly, they often measure the temperature of the surroundings instead. If the reaction takes place in a solution, then scientists measure the temperature of the solution before and after the reaction.

If the temperature of the surroundings has increased since before the reaction took place,  that means the reaction gave off energy in the form of heat. This makes it an exothermic reaction. If the temperature decreased after the reaction, then the reaction absorbed energy. This makes it an endothermic reaction.

The other way to determine if the reaction is exothermic or endothermic is to calculate the change in internal energy. This internal energy is called enthalpy of the reaction. The enthalpy change (ΔH) tells us the amount of heat transferred between the system (the reaction) and the surroundings. This value can tell us if the reaction is exothermic or endothermic.

In a chemical reaction, the reacting chemicals need to break their bonds to form new bonds. These newly formed bonds make up new chemicals, the products of the reaction. Making and breaking bonds takes energy.

Scientists have determined how much energy it takes to make or break various bonds. You can calculate enthalpy change by adding up the energies of the different bonds broken and formed in a chemical reaction.

ΔH = energy used in reactant bond breaking + energy released in product bond making

If the enthalpy change (ΔH) of the reaction is negative value then the reaction is exothermic, if it is positive then it is endothermic.

Summing up

Exothermic and endothermic reactions are important for our chemical world. These reactions can help keep us warm by giving off energy (exothermic) or help cool us down by taking in energy (endothermic).

Did you know? Scientists can measure energy in foods by measuring how much heat the food releases when it is burned. They measure this using a tool called a bomb calorimeter.

Learn more!

Science: Energy Changes and Reversible Reactions (2014)

What is Thermodynamics? (2014)
National Aeronautics and Space Administration  (NASA)

Some Like It Hot, Some Like It Cold (2004)
R.G. Silberman, Journal of Chemical Education 81
Link to abstract. Registration or subscription required to view full text.

Exothermic, Endothermic and Chemical Change
American Chemical Society

How Hand Warmers Work (with Thermal Imaging)
Let’s Talk Science

What is an Exothermic Reaction?
Let’s Talk Science

What is an Exothermic Reaction?
Scientific American

Kelly Resmer

Kelly is a chemistry undergraduate laboratory instructor in Halifax.  She loves working with students in the lab, watching chemistry happen! She has a PhD in chemistry and is very interested in studying and learning about bacteria, the good and the bad ones! 

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