The Cold Pack: A Chilly Example of an Endothermic Reaction

Has this ever happened to you? You are running in gym class and you twist your ankle. It hurts and starts to swell up. Your teacher grabs the first aid kit and pulls out an instant cold pack. After one good squeeze, the pack becomes really cold, almost instantly. What is happening? How do the chemicals make the pack cold so quickly? The answer can be found in thermodynamics! This is a branch of science that explores the transfer of energy. In thermodynamics, chemical reactions can be classified as either endothermic or exothermic. 

What are the two main types of thermodynamic reactions?

Exothermic reactions are reactions that release energy in the form of heat. You are probably familiar with many examples of these reactions. For example, burning gasoline in a car’s engine is an exothermic reaction. This particular type of exothermic reaction is known as a combustion reaction. A combustion reaction occurs when a compound, such as the hydrocarbons that make up fuel, react with oxygen to form a new product and produce heat. 

Endothermic reactions are the opposite of exothermic reactions. They absorb heat energy from their surroundings. This means that the surroundings of endothermic reactions are colder as a result of the reaction. Melting ice is an example of this type of reaction.

How do you know what type of thermodynamic reaction is happening?

One way to do this is by looking at the system and surroundings of a reaction. The system is where the reaction takes place, and the surroundings are the area around the system. 

To determine if a reaction is exothermic or endothermic, you could:

1. measure the temperature change of the system or its surroundings, or
2. calculate the energy of the system. 

Of these two methods, measuring the temperature change is easier. To do this, you simply measure the temperature of a reaction before and after it is completed. Since it can sometimes be difficult to measure the temperature within the system of a reaction, scientists often measure the temperature of the surroundings instead.

It is possible to predict whether a reaction will be endothermic or exothermic by doing a little math. For this, it helps to know a bit about chemical reactions and chemical bonds.

There are two sides to any chemical reaction. On one side are the reactants. A reactant is the substance (or substances) that you start with. On the other side are the products. A product is the substance, or substances, that you end up with after the reaction happens. 

In a chemical reaction, the chemical bonds in the reactant molecules are broken. New bonds are formed in the product molecules. An example would be the combustion reaction between methane (CH₄) oxygen (O₂) (the reactants) that produces carbon dioxide (CO₂) and water (H₂0) molecules (the products). Bonds are broken in the methane and oxygen molecules. Bonds are formed in the carbon dioxide and water molecules.

What is important to know is that energy is needed to both make and to break bonds. To determine if a reaction is exothermic or endothermic, you have to compare the amount of energy needed to break the bonds of the reactants to the amount of energy released when new bonds are made. If the amount of energy released when the new bonds are formed in the products is greater, then it is an exothermic reaction. If the amount of energy needed to break the bonds of the reactants is greater, then it is an endothermic reaction. 

One way to show this is using an energy diagram. Energy diagrams show the energy levels of reactants and products in a reaction.

You can see from the diagram above that the energy level of the products of an exothermic reaction is lower than the energy level of the reactants. The difference between the energy levels of reactants and products is called the enthalpy change (ΔH). In an exothermic reaction, the ΔH is NEGATIVE. In an endothermic reaction, the ΔH is POSITIVE.

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.

It is possible to calculate ΔH without even doing an experiment! Scientists have determined experimentally the energies required to make and break specific molecular bonds. These energies are known as average bond energies.

Using the methane combustion example again, the math works like this:

ΔH = [energy used in reactant bond breaking] - [energy released in product bond making]

= [ 4 C-H bonds + 2 O=O bonds] - [4 O-H bonds + 2 C=O bonds]

= [(4 x 99 kcal/mol) + (2 x 119 kcal/mol)] - [(4 x 111 kcal/mol) + (2 x 192 kcal/mol)]

= [396 + 238] - [444 + 384]

= 634 - 828

= - 194 kcal/mol

Since the enthalpy change is negative, we know that the reaction will be exothermic.

How do thermodynamics work in a cold pack?

Now, let’s go back to our instant cold pack. 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 solid ammonium nitrate and water. 

Did you know? 

Ammonium nitrate is a nitrate salt. It is heavily used in agriculture as a fertilizer. It is also used as an explosive in the mining industry.

The ammonium nitrate is stored in a sealed plastic bag that is surrounded by water. When you pop the bag, the ammonium nitrate comes into contact with water and dissolves. 

Dissolving an ionic compound, like table salt or ammonium nitrate, involves energy. Like other types of reactions, heat energy may be given off or taken in when the material dissolves. This energy is called the energy of solution and can be written as ΔH_soln. 

ΔH_soln = ∑ΔH [products] - ∑ΔH [reactants]

Rather than working out the ΔH for the reactants and products using bond energies, scientists often use pre-calculated values on Standard Enthalpy of Formation (ΔH°_f) tables. From such a table we learn that:

Let’s do the math to calculate the energy of solution

ΔH_soln = ∑ΔH [products] - ∑ΔH [reactants]

= [ mol (NH₄⁺_(aq)) + mol (NO₃^-_(aq)) ] - [ mol (NH₄NO_3(s)) ]

= [ (1 mol)(-132.8 kJ/mol) + (1 mol)(-206.6 kJ/mol) ] - [ (1 mol)(-365.1 kJ/mol) ]

= - 339.4 + 365.1

= 25.7 kJ

Remember at the beginning we said that if ΔH is NEGATIVE the reaction is exothermic and that if ΔH is POSITIVE the reaction is endothermic? Well, that also applies to energy of solution problems. Since we calculated that the ΔH_soln was positive (25.7 kJ), the reaction must be endothermic. We know this to be true because the cold pack made its surroundings very cold!

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).