What Is An Energy Absorbing Reaction?

Definition of Energy Absorbing Reactions

Energy absorbing reactions are chemical reactions that require energy input to proceed. These are also known as endothermic reactions. The energy is absorbed from the surroundings in the form of heat, which causes the temperature of the surroundings to decrease. The energy input is used to break bonds in the reactant molecules.

In an energy absorbing or endothermic reaction, the amount of energy required to break bonds is greater than the amount of energy released when new bonds are formed in the products. This results in a net absorption of energy from the surroundings. The opposite of an endothermic reaction is an exothermic reaction, which releases energy.

Some key features of endothermic reactions:

  • Require an input of energy, usually in the form of heat
  • Cause a drop in temperature of the surroundings as heat is absorbed
  • Energy is used to break bonds in reactants
  • Energy absorbed is greater than energy released when new bonds form
  • ∆H (enthalpy change) is positive

In summary, energy absorbing or endothermic reactions use energy from the surroundings to fuel the reaction, resulting in a net absorption of energy. This energy is utilized to break bonds in the reactant molecules.

Types of Energy Absorbing Reactions

There are several major types of chemical reactions that absorb energy:

Endothermic reactions – These reactions require an input of energy, usually in the form of heat, to drive the reaction forward. Examples include the thermal decomposition of calcium carbonate and the reaction between barium hydroxide and ammonium chloride. The absorption of energy causes the reactants to break apart and form new products.

Polymerization reactions – Many polymerization reactions that link monomers together into polymers are endothermic and require energy input. Examples include styrene polymerizing to polystyrene and ethylene polymerizing to polyethylene.

Hydrogenation reactions – In hydrogenation, hydrogen gas is added to unsaturated hydrocarbons, often with the use of a catalyst. This saturates carbon-carbon double or triple bonds and is endothermic. Hydrogenation is used to transform vegetable oils into saturated fats.

Hydration reactions – When an anhydrous compound reacts with water, it often absorbs heat. Hydration of anhydrous copper sulfate and calcium oxide (quicklime) are two examples of endothermic hydration reactions.

Neutralization reactions – When an acid and a base react and form water and a salt, this exothermic reaction releases heat. However, if dilute acid and base solutions are used, the ionization of water molecules to create hydronium and hydroxide ions requires energy input, making it slightly endothermic.

Everyday Examples

Energy absorbing reactions are commonplace in our everyday lives, though we may not always think of them in those terms. Here are some examples of energy absorbing reactions people regularly encounter:

Photosynthesis – Plants absorb light energy from the sun and use it to convert carbon dioxide and water into glucose and oxygen. This process stores energy in the glucose molecules for plants to use.

Cooking – Applying heat to food is an energy absorbing process, as the food uses that heat energy to undergo physical and chemical changes like melting, caramelizing, coagulating, and browning.

Hand Warmers – Those portable hand warmers contain ingredients like iron powder, water, salt, activated charcoal, and vermiculite. When exposed to air, these undergo an exothermic oxidation reaction that releases energy in the form of heat that you can hold in your hands.

So in summary, energy absorbing reactions are happening all around us – in plants during photosynthesis, when we cook food, and even in the handy little hand warmers we use on cold days.


Energy absorbing reactions have many important applications in various areas. Some of the key uses are:

Safety Gear – Energy absorbers are commonly used in helmets, pads, and other protective gear. For example, football helmets contain pads that can absorb and dissipate impact energy to reduce the forces on a player’s head during collisions.

Flame Retardants – Some flame retardant materials work by absorbing heat energy through endothermic reactions, preventing or delaying the spread of fire. These chemicals are applied to furniture, electronics, building materials and more.

Spacecraft – Spacecraft shields and surfaces are designed with energy absorbing materials to dissipate the extreme heat generated from atmospheric entry or rocket exhaust. This protects sensitive components and payloads.

Automotive – Cars incorporate energy absorbing front and rear crumple zones that dissipate kinetic energy during collisions to protect occupants. Energy absorbers are also used in bumpers and interior padding.

Infrastructure – Shock absorbers and other dampening systems that utilize energy absorbing reactions are important for structures like buildings, bridges, and dams. They allow the structure to withstand seismic activity, strong winds, or impacts.

The ability to absorb and dissipate energy through chemical reactions makes these incredibly useful in a wide range of applications where protection from impacts, heat, and kinetic energy is critical.

How Energy Absorption Works

Energy absorbing reactions function through thermodynamic processes that absorb heat or other forms of energy from the surrounding environment. This occurs because the reactions are endothermic, meaning they require an input of energy to proceed.

On a molecular level, endothermic reactions involve the breaking of bonds between atoms in the reactant molecules. This bond breaking step requires energy input. The energy is provided by absorption of heat from the surroundings, resulting in a decrease in thermal energy and temperature.

Some examples of endothermic bond breaking include the dissociation of gases like nitrogen or hydrogen, the vaporization of liquids, and the decomposition of salts like ammonium nitrate. In each case, energy is consumed to break intermolecular attractions and provide the activation energy to enable the reaction.

The absorbed energy causes the systems to transition to higher energy and more disordered states. This aligns with the second law of thermodynamics, which states that the entropy or disorder of an isolated system will increase over time. Endothermic reactions increase entropy through the energy they absorb from the surroundings.

The capacity for energy absorption is an intrinsic property of the chemical system determined by thermodynamic parameters like enthalpy and free energy changes. By selecting reactants with favorable thermodynamic profiles, chemical engineers can design specialized energy absorbing materials and reactions.

Energy Absorption vs Energy Release

Energy absorbing reactions, also known as endothermic reactions, are processes that absorb heat energy from the surroundings. This contrasts with exothermic reactions, which release energy into the environment in the form of heat. While exothermic reactions are more common in everyday life, endothermic reactions play important roles as well.

A classic example of an endothermic process is photosynthesis. Plants use the energy from sunlight to convert carbon dioxide and water into glucose and oxygen. This process stores energy in the glucose molecules for later use. Other endothermic reactions include thermal decompositions, where heat is absorbed to break chemical bonds, and dissolving salts in water.

In contrast, exothermic reactions like combustion, rust formation, and the reaction between acids and bases release energy. These reactions give off heat and light to the surroundings. Exothermic reactions power many familiar processes – fire, digestion, and the burning of fuels all rely on the release of stored chemical energy. While endothermic reactions absorb energy for later use, exothermic reactions tap into stored energy right away to do work or give off heat. Both play critical, complementary roles in physical and biological systems.

Kinetics and Equilibrium

The rate and extent of energy absorption in chemical reactions depends on several factors like temperature, concentrations of reactants, and the presence of catalysts. In general, increasing temperature speeds up the reaction kinetics and pushes the equilibrium towards the products. Higher concentrations or pressures of reactants also favor faster reaction rates and product formation. Catalysts can accelerate reaction rates by lowering the activation energy barrier without being consumed. However, catalysts do not affect the position of equilibrium.

The kinetics follow the Arrhenius equation, where the rate constant increases exponentially with temperature. The thermodynamic equilibrium is described by the reaction quotient, Q, which depends on the reactant and product concentrations. At equilibrium, Q = Keq, the equilibrium constant. Le Chatelier’s principle states that increasing the temperature or reactant concentrations will shift the equilibrium to counteract the change. This allows more products to form and absorb energy until a new equilibrium state is established at the higher temperature or reactant levels.

The kinetics determine how quickly equilibrium is reached, while the thermodynamics determine the maximum extent of the reaction and energy absorption. By optimizing these factors, the rate and capacity of energy absorbing reactions can be maximized for practical applications and devices.

Energy Storage

Some reagents have the ability to absorb and store energy, rather than immediately releasing it. These energy storing reagents can have a high energy, unstable intermediate form. The absorbed energy gets locked into straining chemical bonds or altering molecular geometries.

For example, photochromic compounds can absorb light energy and temporarily change color. The energy gets stored as a structural change in the molecule. When the energy is released, the molecules revert back to their original state. The energy absorbing and releasing process can repeat over and over.

Rechargeable batteries also rely on energy storage and release. Electrochemical reactions during charging force electrons into high energy positions. The battery stores this potential energy until it is tapped and released to power devices.

Energy storage allows a delay between when energy is absorbed and when it is used. By slowly releasing the energy over time, these reagents create a more controlled, sustainable output.

Latest Research

Researchers are actively exploring new ways to design and optimize energy absorbing chemical reactions. Some key areas of focus include:

Metal-organic frameworks (MOFs) – These porous structures can absorb large amounts of energy through structural changes and guest molecule adsorption. Strategies are being developed to fine-tune MOFs to maximize energy absorption capacity.

Molecular switch reactions – Reactions that can reversibly make and break bonds upon stimulation can effectively absorb and store energy. Work is being done to identify molecular switch mechanisms responsive to heat, light, electricity etc.

Reversible covalent bond forming reactions – Covalent bonds store large amounts of energy. Chemists are engineering reversible covalent reactions that can absorb energy through bond breaking and re-formation.

Ionic liquids – The tunable properties of ionic liquids make them promising as energy absorbing media. Approaches using ionic liquids include absorbing heat of reaction energy and phase change energy.

Kinetic stabilization – Chemical species can be kinetically trapped in high energy states. Research on slowing down energy releasing steps aims to improve energy absorption capacities.

Supramolecular polymers – These polymers assembled through non-covalent interactions can be designed to absorb large amounts of mechanical energy through reversible bond breaking.

In summary, scientists are taking inspiration from diverse fields like materials science, nanotechnology, and biology to come up with innovative ways to maximize energy absorption during chemical reactions.


This article has provided an overview of energy absorbing reactions. We began by defining what an energy absorbing reaction is – a chemical reaction that requires energy input to proceed and stores some of that energy within the bonds of the products.

Some key types of energy absorbing reactions were discussed, including endothermic chemical reactions, adsorption, absorption, and photosynthesis. Examples of energy absorbing reactions we encounter every day were provided, like photosynthesis in plants and the evaporation of sweat from our skin.

We explored some applications of energy absorbing reactions, like their use in thermal energy storage and heat pumps. The mechanics of how energy absorption works on a molecular level were explained, contrasting energy absorbing reactions with energy releasing reactions.

The kinetics and equilibrium principles that govern these reactions were reviewed. We also discussed how energy can be stored through chemical bonds, stressed materials, electric fields and more. Finally, current areas of research related to energy absorption such as developing new phase change materials were highlighted.

In summary, this article has covered the fundamentals of energy absorbing reactions across definition, types, examples, applications, mechanisms, principles, energy storage and latest research.

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