What Are Some Examples Of Chemical Reactions That Require Energy?

A chemical reaction is a process where one set of chemical substances (reactants) are transformed into a different set of substances (products). Chemical reactions involve the breaking and formation of chemical bonds, which requires an input or release of energy. The energy change in a chemical reaction is known as the enthalpy change (ΔH).

There are two main types of chemical reactions based on their energy requirements:

Exothermic reactions – These reactions release energy, resulting in products that have less energy than the reactants. As bonds are formed in the products, energy is released, generally in the form of heat. Exothermic reactions have a negative enthalpy change (ΔH is negative).

Endothermic reactions – These reactions require an input of energy to proceed, resulting in products that have more energy than the reactants. As bonds are broken in the reactants, energy must be absorbed, usually in the form of heat. Endothermic reactions have a positive enthalpy change (ΔH is positive).

In this article, we will look at some common examples of exothermic and endothermic chemical reactions.

Combustion Reactions

Combustion reactions are a type of exothermic chemical reaction where a fuel is oxidized and energy is released in the form of heat and light. The most common example of a combustion reaction is fire. Forest fires, burning wood in a fireplace, and burning gasoline in a car engine are all examples of combustion reactions.

In a combustion reaction, the fuel and an oxidant (usually oxygen gas) react to generate significant amounts of energy. The fuels can be wood, coal, gasoline, methane, or any other hydrocarbon. Here is the general combustion reaction:

Fuel + Oxygen → Carbon Dioxide + Water + Heat

For example, when wood burns in a fireplace, the hydrocarbon fuel that makes up wood reacts with oxygen from the air to produce carbon dioxide, water vapor, and release a large amount of heat energy.

CH4 + 2 O2 → CO2 + 2 H2O

The combustion of gasoline in car engines follows a similar process, with gasoline as the fuel reacting with oxygen to drive the motion of the vehicles.

Decomposition Reactions

Decomposition reactions are chemical reactions in which a single compound breaks down into two or more simpler substances. These reactions require an input of energy to break bonds and split the compound apart. Some examples of decomposition reactions include:

  • Thermal decomposition – Heat is used to break down compounds. For example, calcium carbonate (CaCO3) breaks down into calcium oxide (CaO) and carbon dioxide (CO2) when heated.
  • Electrolysis – Electricity is used to decompose compounds. A common example is the electrolysis of water (H2O) into hydrogen (H2) and oxygen (O2) gases.
  • Photolysis – Light energy breaks bonds in a chemical compound. An example is the photolysis of silver chloride (AgCl) into silver (Ag) and chlorine (Cl2).

In electrolysis of water specifically, an electric current is passed through water, which decomposes the water molecules into oxygen gas forming at the anode and hydrogen gas forming at the cathode. This decomposition reaction requires an input of energy from the current to break the bonds between the hydrogen and oxygen atoms.

a diagram showing the process of electrolysis of water, with electricity splitting water molecules into hydrogen and oxygen gas.

Single Displacement Reactions

Single displacement reactions, also called replacement reactions, occur when an active metal displaces a less active metal from a compound. The more active metal replaces the less active metal in the compound, taking its place. For example:

Zn + CuSO4 → ZnSO4 + Cu

In this reaction, solid zinc metal displaces copper ions (Cu2+) in copper sulfate solution. The zinc metal is more reactive than copper, so it can replace copper in the compound, forming zinc sulfate. The copper that was displaced appears as solid copper metal. Other examples of active metals that can displace less active metals include iron displacing copper, and aluminum displacing zinc.

Single displacement reactions are common examples of redox reactions, where reduction (electron gain) and oxidation (electron loss) are occurring simultaneously. The active metal is oxidized when it loses electrons, while the less active metal that is displaced gains those electrons and is reduced.

Double Displacement Reactions

Double displacement reactions, also known as double replacement reactions, involve the exchange of cations between two ionic compounds. In these reactions, the cations and anions of two ionic compounds “switch partners”, forming two entirely new compounds.

Here are some examples of double displacement reactions:

  • When a solution of sodium chloride is mixed with a solution of silver nitrate, the sodium (Na+) and nitrate (NO3-) ions exchange partners, precipitating solid silver chloride and leaving aqueous sodium nitrate.
  • Calcium chloride mixed with sodium carbonate forms solid calcium carbonate and aqueous sodium chloride through an exchange of calcium (Ca2+) and carbonate (CO32-) ions.
  • Solutions of barium chloride and potassium sulfate react through a double displacement, producing an insoluble precipitate of barium sulfate and a solution of potassium chloride.

In each of these examples, mixing two ionic compounds results in the two cations swapping anions and forming new ionic compounds – one of which is often insoluble and precipitates out as a solid.

Endothermic Reactions

An endothermic reaction is a type of chemical reaction that requires an input of energy to proceed. Endothermic means “heat absorbing”, so during an endothermic reaction, heat energy is absorbed from the surroundings which causes the temperature of the surroundings to decrease. This contrasts with exothermic reactions, which release energy in the form of heat to the surroundings causing the temperature to rise.

Some examples of endothermic reactions include:

  • Dissolving ammonium nitrate in water – this process requires heat energy input to break apart the ammonium nitrate crystals into ions which can dissolve in water. The absorption of heat makes the solution cooler.
  • Thermite reaction between aluminum and iron(III) oxide to produce aluminum oxide and molten iron. This requires a very high temperature to initiate the reaction.
  • Decomposition of calcium carbonate to form calcium oxide and carbon dioxide. Limestone is heated to drive off carbon dioxide gas and convert the limestone to lime, which requires heat input.
  • The reaction between barium hydroxide and ammonium thiocyanate to precipitate barium thiocyanate. This reaction absorbs heat as it proceeds.

Endothermic reactions require an energy input because energy is required to break the bonds in the reactant molecules. This absorbed energy provides the activation energy needed for the reaction to occur. The absorption of heat energy also reflects that the products of the reaction have a higher enthalpy than the reactants.


Photosynthesis is the process plants use to convert light energy from the sun into chemical energy in the form of glucose. This process requires light energy to drive a series of chemical reactions. During the light-dependent reactions of photosynthesis, light energy is absorbed by chlorophyll and converted into stored chemical energy in the form of ATP and NADPH. The light energy excites electrons in chlorophyll and leads to the splitting of water molecules, releasing oxygen as a byproduct. The high-energy electrons then move through an electron transport chain, and their energy is used to power ATP synthase and generate ATP. Meanwhile, NADP+ is reduced to NADPH by absorbing energized electrons. The ATP and NADPH generated in the light reactions then provide the energy and electrons needed to fix carbon dioxide and produce glucose in the light-independent reactions, also known as the Calvin cycle. Overall, photosynthesis requires light energy from the sun to generate the ATP and NADPH needed to build carbohydrates from carbon dioxide and water.

Cellular Respiration

Cellular respiration is the process that cells use to break down glucose and produce energy in the form of ATP. This is an endothermic reaction that requires energy input. The overall chemical reaction for cellular respiration is:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP)

There are three main stages in cellular respiration:

Glycolysis: This initial step splits the 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing a net gain of 2 ATP molecules. This stage does not require oxygen.

Krebs Cycle: The pyruvate enters the mitochondria where it is further broken down into CO2, producing NADH and FADH2 molecules that carry energy. This stage requires oxygen.

Electron Transport Chain: The NADH and FADH2 molecules donate their electrons to generate a proton gradient across the mitochondrial membrane. This gradient powers ATP synthase to produce the majority of ATP. Oxygen is the final electron acceptor at the end of the chain.

In summary, cellular respiration uses oxygen to break down glucose and harvest electrons, generating ATP. This endothermic reaction requires an input of energy to break apart the glucose molecule and produce biological energy that powers cells.


Polymerization is a chemical reaction that joins together many small molecules known as monomers into a large chainlike molecule known as a polymer. During this reaction, the monomers must absorb energy in order to break their double bonds and link together to form the polymer. For example, polyethylene, one of the most common plastics, is produced by the polymerization of the monomer ethylene, which is an alkene with the formula C2H4. The double bonds in ethylene molecules break, allowing the molecules to link together into long polyethylene chains. This polymerization reaction requires a heat source to provide the necessary energy for the bonds to break and reform. As the polyethylene chains grow longer, more energy is required to keep the reaction going, so polymerization is an endothermic process that absorbs heat. Many other important plastics, fibers, and rubbers are also formed through endothermic polymerization reactions that require an input of energy.


In summary, there are a number of chemical reactions that require an input of energy, known as endothermic reactions. We discussed several examples such as:

  • Combustion reactions like burning fossil fuels
  • Decomposition reactions like the electrolysis of water
  • Single displacement reactions like the reaction between copper and silver nitrate
  • Double displacement reactions like the reaction between sodium carbonate and hydrochloric acid

Other important endothermic reactions we covered include photosynthesis in plants, cellular respiration for energy production, and polymerization to form large molecules. All of these reactions and more require an input of energy to proceed, often in the form of heat, light, or electricity. The energy is used to break bonds and drive the reactions forward. Understanding endothermic chemical processes helps illuminate where energy flows in important systems like living organisms and industrial processes.

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