Exothermic reactions: Definition, Examples and Everyday Applications

Chemical reactions are not just about forming new substances; they are also about energy changes. One such important type is the exothermic reaction, where energy is released into the surroundings. These reactions are a part of our everyday life, from burning fuel to breathing and producing energy inside our bodies. This guide is about an exothermic reaction, how energy is released, why the enthalpy change is negative and where such reactions occur in real life. 

Table of Content

• What is an Exothermic Reaction
• Measurement of Heat in Exothermic Reactions
• Examples of Exothermic Reactions

What is an Exothermic Reaction

An exothermic reaction is a type of chemical reaction in which energy is released into the surroundings. This energy usually comes out in the form of heat, light, or sometimes sound. These reactions are very common and can be seen easily in our daily lives, such as burning fuel, lighting a candle, or even breathing.

The general form of an exothermic reaction is:

Reactants → Products + Energy

An exothermic reaction is a chemical reaction in which more energy is released than consumed. During the reaction, energy stored in chemical bonds is released when new bonds are formed. As a result, heat or light is given out to the surroundings.

Importantly! Exothermic reactions are the opposite of endothermic reactions, where energy is absorbed from the surroundings. In an exothermic reaction, the surroundings become warmer because energy moves out of the system.

Following are the characteristics which make easy in identifying the exothermic reactions: 

• Energy is released into the surroundings
• Surroundings become warmer
• ΔH value is negative
• Products are more stable than reactants
• Very common in nature and industries

In thermochemistry, the energy change of a reaction is measured using the enthalpy change (ΔH).
For an exothermic reaction:

• The value of ΔH is negative (ΔH < 0)
• A negative ΔH clearly shows that energy is released

Exothermic reactions are important because they:

• Provide energy for living organisms
• Help in electricity generation
• Are used in heating and cooking
• Play a key role in industries and power plants

But How Does Energy Change in an Exothermic Reaction ? 

Every chemical reaction involves breaking old bonds and forming new ones.

• Breaking bonds requires energy
• Forming bonds releases energy

In an exothermic reaction, the energy released during bond formation is greater than the energy required to break the bonds of the reactants. The extra energy is released as heat or light.

The enthalpy change can be explained using the formula:

ΔH = Energy required to break bonds - Energy released during bond formation

Since more energy is released, the overall ΔH becomes negative.

Measurement of Heat in Exothermic Reactions

The heat released during an exothermic reaction is measured using an instrument called a calorimeter. A calorimeter helps determine how much heat is transferred from the reaction to the surroundings by recording the temperature change of a known substance, usually water.

Importantly, in exothermic reactions, the heat released by the reaction is equal to the negative of the energy change of the system, meaning the enthalpy change (ΔH) has a negative value.

However, It is very difficult to measure the total energy of a system directly. Therefore, scientists measure enthalpy change (ΔH) instead, which gives accurate information about the energy released.

Examples of Exothermic Reactions

Exothermic reactions occur everywhere from simple household activities to powerful industrial and nuclear processes.

• Combustion

Combustion reactions is the burning of a substance in the presence of oxygen, producing heat and light.

Example:

  CH4+2O2→CO2+2H2O+Heat

Another important combustion reaction occurs in living organisms:

  C6H12O6+6O2→6CO2+6H2O+Energy

These combustion fuel reactions release large amounts of energy and are widely used for cooking, heating, and power generation.

• Detonation of Nitroglycerin

Nitroglycerin is a highly explosive substance. When it detonates, it releases a huge amount of heat and gases in a very short time.

4C3H5N3O9→12CO2+10H2O+6N2+O2+Energy

Gases expand rapidly and temperature can reach nearly 5000°C with Very high negative ΔH value.This makes it a powerful exothermic reaction.

• Nuclear Fission of Uranium-235

In nuclear power plants, uranium-235 undergoes nuclear fission.

U235+n→Ba141+Kr92+3n+Energy

 A single atom releases enormous energy and Energy released is millions of times more than that of coal combustion which are controlled using control rods to absorb neutrons,This reaction is used to generate electricity.

• Neutralisation Reactions

A neutralisation reaction occurs when an acid reacts with a base to form a salt and water.

Example:
  HCl+NaOH→NaCl+H2O+Heat

The standard enthalpy change of neutralisation is about -57.3 kJ/mol, which proves that neutralisation reactions are exothermic in nature.

Do you know Why is respiration considered to be an exothermic reaction?

Respiration is the process by which living organisms release energy from food.

Chemical equation:
  HCl+NaOH→NaCl+H2O+Heat

In this process, glucose reacts with oxygen to produce carbon dioxide, water, and energy. This released energy is used for growth, movement, and maintaining body temperature.

Since energy is released, respiration is an exothermic reaction.

Read More: Heat of Combustion

Exothermic reaction is a chemical reaction that releases energy in the form of heat or light. These reactions in chemistry have a negative enthalpy change and occur when bond formation releases more energy than bond breaking requires.