When a reaction draws in energy to help it happen, this is endothermic. A vital part of understanding how chemistry works, endothermic reactions play an important role across industrial processes, natural reactions, and even in everyday life.
Learning how you can identify an endothermic reaction is a key part of educating yourself on the world of chemistry, but it can be difficult to make sense of the moving parts.
That’s why we’re here to walk you through what defines an endothermic reaction, examine real-life examples, and uncover the many ways these reactions are applied in everyday life and industry.
In this post:
Key Takeaways
Endothermic reactions absorb energy, including heat, from their surroundings to drive chemical changes.
These reactions are characterised by a positive enthalpy change (ΔH > 0) and often result in a cooling effect in the surrounding environment.
Common examples include the melting of ice, evaporation, and sublimation, all of which involve breaking molecular bonds using heat.
Endothermic reactions play a role in industrial processes such as the production of lime, hydrogen, and ammonium nitrate, as well as in refrigeration and cooling systems.
The difference between endothermic and exothermic reactions is that endothermic reactions absorb energy, leaving the surrounding area colder, while exothermic reactions release it, giving off heat.
How Do Endothermic Reactions Work?
Endothermic reactions are chemical processes that require an external input of energy to happen. To do this, these reactions draw in heat from their surroundings, in a process that has a positive enthalpy change – usually written as ΔH>0.
This absorption is necessary to overcome the activation energy barrier and enable the breaking of chemical bonds within the reactants.
A defining feature of endothermic reactions is their positive enthalpy change (ΔH > 0), which signifies that the energy required for the reaction exceeds the energy released.
For example, when you touch an object undergoing an endothermic reaction, such as a chemical cold pack, it feels cold to the touch because the reaction is drawing heat away from your skin and the surrounding environment. This cooling effect is a direct result of the energy transfer involved.
Definition and Basic Principles
An endothermic reaction is defined by its ability to absorb energy to drive chemical changes. This process begins when the energy needed to break the reactants’ bonds is greater than the energy released as they make new bonds.
The difference in these energy levels is supplied by heat from the environment, making heat absorption an essential feature of the reaction.
The basic principles governing these reactions revolve around thermodynamics and energy conservation. The energy absorbed is not lost but transformed, as it facilitates changes in the molecular structure of the reactants, enabling new chemical compounds to form.
This process is governed by the laws of enthalpy and the overall energy balance in the reaction system.
Common Examples of Endothermic Reactions
Theory is fantastic, but grasping complex concepts is easier when you can relate it to real-life examples. Thankfully, there are some very simple ways we see endothermic reactions happen, with many of them being something seen every day.
Melting of ice to form water
When ice meets heat, it melts to become water. This heat is needed to break the bonds holding the H2O together as a solid into a liquid, which in turn cools down the surroundings.
When the freezing point is reached, the molecules in this solid form begin to break down, becoming more loosely spaced out, turning to water.
Evaporation of liquid water to vapour
Evaporation is often associated with heat thanks to boiling water turning into vapour, but despite this it is actually an endothermic process. Water molecules require extra heat to continue to space out from each other, similar to how they turn from solid to liquid.
With enough heat taken from the surroundings, these water molecules increase their kinetic energy enough to become vapour, rising into the air.
Sublimation
Sublimation is when a substance changes straight from a solid to a gas without ever being in a liquid state. While this may seem an uncommon trait, there are many household and everyday items that use it to work. Examples include:
- Bathroom fresheners – The scents from these are often carried in alcohols such as ethanol, which sublimate into the air, carrying the fragrance.
- Mothballs – Made using naphthalene, or C10H8, this chemical sublimates into the air to act as a deterrent to moths and other insects.
- Dry ice – A solid form of carbon dioxide, or CO2, used for quick freezing.
What Are the Applications of Endothermic Reactions?
Now we understand the processes behind endothermic reactions, as well as some day-to-day examples, we can look into the high-end applications of this process. From high end industrial manufacturing to being a critical component in refrigeration, here’s some ways this process is used.
Industrial and Chemical Manufacturing
One example in this space is the processing of limestone into lime. Limestone, otherwise known as calcium carbonate (CaCO3) is burnt off in an industrial-sized kiln at about 900°C, producing both carbon dioxide and calcium oxide (CaO).
This leftover material, known also as quicklime, has several uses across agriculture and manufacturing, including:
- Manufacturing: Used in producing iron, steel, paper, pulp, porcelain, and glass
- Water treatment: Softens water and treats water and flue gases
- Agriculture: Used in liming to neutralise acidic soils and add nutrients
- Construction: A key ingredient in mortars and cement
- Dental materials: Used in dental bases, restorations, and prostheses
Another comes from the production of hydrogen (H), often made by the steam-methane reforming method. As part of the process, heat is applied to methane mixed with steam under very high pressure, with a catalyst to produce three new component parts: hydrogen, carbon monoxide, and carbon dioxide. This reaction can be written as CH + H2O (+ heat) → CO + 3H2.
Last but not least of these is ammonium nitrate (NH4NO3) where endothermic reactions between ammonia solutions, heated into a gas, and nitric acid, which is concentrated, leads to the formation of this chemical. It is another chemical that finds mass use in the agricultural industry, making it fairly lucrative to produce.
Refrigerants
From the fridges we have in our homes, to large-scale industrial refrigeration, air conditioning units and more, endothermic reactions are a cornerstone of cooling and freezing systems around the world.
When these reactions draw in heat, they leave the spaces around them cooler. With a large enough reaction, this can keep spaces cooler consistently, helping with preservation of food, heat-sensitive elements, and more.
Refrigeration works cyclically, with liquids turning to gas and back again, starting and ending with a compressor. The refrigerant, usually a fluid, gets compressed in this unit into a hot gas before being sent to a condenser, which turns it into liquid.
After that, it is sent to an expansion valve, which helps reduce the pressure and expel the heat. At this point the refrigerant is very cold from the process, helping to maintain steady low or sub-freezing temperatures inside of the required area.
This process repeats itself, with heat being expelled outside of the unit, and cooling held within.
There are a number of different groups of refrigerants recognised in industrial settings, including:
- CFCs (Chlorofluorocarbons): Made of chlorine, fluorine, and carbon, and phased out for damaging the ozone layer
- HCFCs (Hydrochlorofluorocarbons): Created with hydrogen, chlorine, fluorine, and carbon. Less harmful than CFCs, but also being phased out
- HFCs (Hydrofluorocarbons): Contain hydrogen, fluorine, and carbon, it is ozone-safe but contributes to global warming.
- HFOs (Hydrofluoroolefins): Formed of hydrogen, fluorine, and carbon with an ‘olefin‘ bond. Ozone-safe, and has a lower contribution to global warming
There are also natural refrigerants, including ammonia (NH3), carbon dioxide, propane (C₃H₈), and isobutane (C4H10), and while these are more eco-friendly they carry other risks such as high flammability.
What’s the Difference between Endothermic and Exothermic Reactions?
If endothermic reactions are defined by feeling cold and needing energy to happen, exothermic reactions are the opposite: they release heat, often making their surroundings warmer. This key distinction comes from the direction that energy flows during the process.
A good example of an exothermic reaction is epoxy resin. Once it is mixed with a hardener, it generates a large amount of heat that helps strengthen the material and give it strength once set.
Both reactions are complementary when it comes to energy dynamics, with endothermic reactions having a positive enthalpy change (ΔH), and exothermic reactions having a negative enthalpy change. This shows that the reaction releases more energy than it absorbs.
Conclusion
Endothermic reactions are vital to both the natural world and human innovation. Whether it’s the melting of ice, the operation of a refrigerator, or large-scale industrial manufacturing, endothermic reactions demonstrate the importance of energy absorption in creating and sustaining change.
This in turn helps us understand more about the world around us, from the science behind temperature changes as a whole to the intricate process of chemical change at a molecular level. Opening the door on endothermic reactions helps us better comprehend the other elements that surround us.
