Exothermic and Endothermic Reactions with Examples

Introduction:  

A chemical change is always accompanied by chemical reactions. In a chemical reaction, one or more substances combine to form a new substance entirely different in properties from the original substance. In every chemical reaction, a change in energy is involved, i.e., the energy difference between the chemical energies of reactants and products. This energy can be in the form of heat, light, and electricity. 

Every substance has a fixed amount of energy, which is stored in the form of potential energy and is known as its chemical energy. 

A chemical reaction requires the breaking of chemical bonds between atoms resulting in absorption of energy in the form of heat and the formation of new bonds with the release of energy. These two types of energy are different from each other. 

Therefore, in a chemical reaction, either energy is released, or energy is absorbed. 

Chemical reactions are of two types based on the energy released or absorbed. 

1. Exothermic Reactions 

2. Endothermic Reactions 

Let’s learn more about these reactions in this session. 

Exothermic Reactions 

Exothermic reactions are chemical reactions that release energy in the form of heat or light. 

1. As heat is given out in the surroundings in this reaction, it causes a rise in temperature. 

2. In the exothermic reaction, energy is released because the total energy of the product is less than the total energy of the reactants. 

3. That means it takes less energy to break bonds in the reactants than is released when new bonds form in the product 

Examples of Exothermic Reactions: 

Example1. Combustion 

Combustion is a process in which a substance reacts rapidly with oxygen to release heat and energy. 

All combustion reactions are exothermic reactions.  

Look at the big bonfire in the Figure below. The combustion of wood is an exothermic reaction that releases a lot of energy as heat and light. You can see the light energy the fire is giving off. If you were standing near the fire, you would also feel its heat. 

These reactions release fair amounts of energy in the form of heat but also form some by-products such as smoke 

An example of combustion is the combustion of methane which can be represented as follows:

Another example of a combustion reaction is the combustion of glucose, given by the following reaction:   

Example 2: Neutralization Reactions 

These are those reactions where acids and bases react to give salt and water. 

Example:  When sodium hydroxide(NaOH), a base reacts with hydrochloric acid (HCl), it forms salt, i.e., common salt and water. 

This reaction produces 57.2 kJ of energy for each molecule of salt produced in the reaction. Therefore neutralization reactions are considered exothermic reactions. 

Example 3: Sodium Metal in Water 

The reaction of sodium metal with water is highly exothermic because enough heat is given  

off during the exothermic reaction to ignite the produced H₂​(g).  

The reaction is especially vigorous when warm water is used.  

Sodium reacts strongly with water. The reaction can be written as follows: 

The above change is an exothermic reaction in which the sodium metal is heated, and it burns with a typical orange flame. 

Example 3:  Making Hot Ice  

Hot ice is obtained when we solidify sodium acetate from a supercooled solution.  

The resultant crystals seem like water ice, except they are hot instead of cold. This reaction is used for making chemical hand warmers. 

 Example 4: Detonation of Nitro-glycerine 

Nitro-glycerine is one of the hottest detonating high explosives. 

Nitro-glycerine has strong explosive powers, and it is used via its detonation at ordinary temperature and pressure and would occupy over 1,200 times the original volume. 

It detonates at temperatures up to 5000^oC. Therefore, it is an exothermic reaction. 

Example 5: Nuclear Fission of Uranium-235 

Nuclear fission: It is the process in which a large nucleus splits into two smaller nuclei with the release of energy.  

Nuclear Fission of the atom of uranium-235 releases more than 2.5 million times the energy of burning coal. If one fission reaction produces two neutrons, these two neutrons 

 can cause two additional fissions.  
If those two fissions release four neutrons, those four neutrons 

split other nuclei and could produce four more fissions, and 

 so on, resulting in a nuclear chain reaction, as shown in this figure. 

This uranium is used in nuclear power plants where the fission reaction is controlled with the help of control rods. 

Some Other Examples of Exothermic Reactions 

1. Formation of CO₂: When carbon burns in oxygen to form carbon dioxide, much heat is produced. 

2. Formation of water: When hydrogen is burnt in the oxygen, water is formed, and heat is released. 

3. Formation of Ammonia: Nitrogen reacts with hydrogen in the presence of a catalyst (fine powder of iron) at around 500℃ and above 200 atm of pressure to form ammonia. 

4. When water is added to the quick lime solution, a lot of heat is produced during the reaction with the formation of calcium hydroxide. (slaked lime) 

Examples of Exothermic Reaction in Everyday Life 

1. Respiration 

Respiration is a process where humans take the oxygen and release the carbon dioxide. The chemical equation of this process can be written as: 

Respiration is an important exothermic reaction that occurs in every cell and gives energy to cells to maintain all the important functions of our body.  

2. Burning of Candle 

The burning of the candle is also an exothermic reaction. 

The candle is a paraffin wax that is made from hydrocarbons.  

When the candle burns, it reacts with atmospheric oxygen and breaks hydrocarbon bonds. These changes release a good amount of energy, and therefore, it feels warm around the burning candle. 

3. Lighting of Matchbox: 

Matchstick is made of phosphorous, sulphur, and other substances. When the matchstick is burned, a chemical reaction between the phosphorous, sulphur, and atmospheric air causes fire and releases heat. 

Other Examples are the formation of snow in clouds, Burning of fuels, thermite reactions,  freezing, condensation, and deposition. 

Endothermic Reactions 

A chemical reaction in which the heat is absorbed is called an endothermic reaction. 

1. This reaction causes a fall in temperature. 

2. Exothermic reactions cannot occur spontaneously.  

3. For this reaction to occur, external energy must be supplied. 

Examples of Endothermic Reactions 

Example 1: Photosynthesis  

Photosynthesis is an endothermic chemical reaction. 

1. It is a necessary process for the plants to prepare their food by themselves. 

In this process, plants use the energy from the sun in the presence of carbon dioxide and water to produce glucose and oxygen. 
The reaction can be summarized as: 

sunlight + 6CO₂(g) + H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g) 

Example 2: Cooking an Egg 

During cooking the egg, the energy(heat) is absorbed from the pan to cook the egg. 

Example 3: Melting Ice Cubes 

For making a cool drink, generally, ice is added. The ice takes out the heat from the water to melt, making the drink nice and cold. 

Example 4: Evaporation of Water 

Evaporation is a type of process in which a substance directly changes from its liquid state to its gaseous state. 

An example of Evaporation is the drying of wet clothes.  

In this case, the water droplets inside the clothes (system) pull the heat from the surroundings. As a result, the system’s energy (wet clothes) increases. 

Example 5: Electrolysis process 

Electrolysis is a process where the decomposition of a molecule into its constituent ions takes place by supplying electricity. 

The energy required for it in this reaction is supplied externally; therefore, it is an endothermic Reaction. 

Example: Electrolytic decomposition of sodium chloride into sodium hydroxide and hydrogen chloride. 

The overall reaction is the breakdown of sodium chloride into its elements: 

2NaCl→2Na(s)+Cl2(g) 

Other Examples of Endothermic Reactions 

1. Formation of Carbon disulphide: 

Carbon is heated with sulphur at high temperatures; the product obtained in this reaction is liquid carbon disulphide. 

2. Formation of Nitric oxide:  

When nitrogen and oxygen are heated together at a temperature of around  3000℃, Nitric oxide gas is formed.  

3. Decomposition of Calcium Carbonate: 

When Calcium carbonate is heated, it decomposes into CaO and carbon dioxide when it is heated at 1000℃. 

Some other examples are splitting a gas molecule, Baking bread, Sublimation of dry ice, and melting solid salts. Cracking of alkanes, melting ice, Evaporation of water 

Difference Between Exothermic and Endothermic Reactions 

Summary:

• In every chemical change/ reaction, a change in energy is involved.
• Chemical reactions are divided into two types depending on the energy released or absorbed.
  • Exothermic Reaction
  • Endothermic Reaction
• Exothermic Reaction: A reaction where the energy is liberated in the form of heat, or light, is called an exothermic reaction.
• It causes an increase in the temperature.
• This reaction occurs spontaneously.
• Endothermic Reaction: A reaction where the heat is absorbed s called an endothermic reaction. It causes a fall in temperature.
• This reaction is non-spontaneous since the energy must be supplied from outside to proceed with this reaction.