Why do reactions release or absorb energy?

Short Answer

Reactions release or absorb energy because chemical bonds must break and new bonds must form during a reaction. Breaking bonds requires energy, while forming new bonds releases energy. If more energy is released in forming products than is used to break reactant bonds, the reaction releases energy (exothermic). If more energy is absorbed in breaking bonds than is released by making new ones, the reaction absorbs energy (endothermic).

This energy change helps explain why some reactions feel hot and others feel cold. It also determines reaction behaviour in nature, industry, and daily life.

Detailed Explanation :

Energy changes in reactions

Chemical reactions always involve changes in energy. Every substance is made of atoms held together by chemical bonds. During a reaction, these bonds break and new bonds form to create new substances. These processes require or release energy, which is why reactions either absorb or release heat.

To break a chemical bond, energy must be supplied. This energy is usually absorbed from the surroundings. On the other hand, when new bonds form, energy is released because the atoms move to a more stable, lower-energy arrangement. The balance between these two processes determines whether the overall reaction releases or absorbs energy.

If the energy released during bond formation is greater than the energy absorbed in bond breaking, the reaction becomes exothermic and releases energy. If the energy absorbed in breaking bonds is greater than the energy released, the reaction becomes endothermic and absorbs energy.

This process of energy exchange helps maintain natural processes such as respiration, photosynthesis, plant growth, cooking, and temperature regulation.

Why reactions release energy

Reactions release energy when the products formed are more stable than the reactants. Stable molecules have strong bonds that require less internal energy. When these strong bonds form, extra energy is released into the surroundings.

Examples:

  • Combustion: Burning fuels like wood, LPG, or petrol releases large amounts of heat and light because strong new bonds are formed.
  • Neutralization: When acids and bases react, heat is released as stable salt and water molecules form.
  • Respiration: The breakdown of glucose in living organisms releases energy needed for life processes.

In all these cases, bond formation releases more energy than what was required to break the original bonds.

Why reactions absorb energy

Some reactions absorb energy because they require more energy to break bonds than the energy released when new bonds form. This happens when the products are less stable or have higher energy than the reactants.

Examples:

  • Photosynthesis: Plants absorb sunlight to turn carbon dioxide and water into glucose and oxygen.
  • Melting of ice: Energy from the surroundings is absorbed to break hydrogen bonds between water molecules.
  • Evaporation: Water absorbs heat to change from liquid to gas.

These reactions cool the surroundings because they take in energy from the environment.

Role of bond energy in reactions

Bond energy is the amount of energy needed to break one mole of a bond. Different bonds have different strengths. Breaking a strong bond needs more energy, while forming a strong bond releases more energy.

Chemical reactions depend on comparing:

  1. Energy absorbed (bond breaking)
  2. Energy released (bond formation)
  • If bond formation energy > bond breaking energy → energy released
  • If bond breaking energy > bond formation energy → energy absorbed

This comparison explains the energy patterns in all reactions.

Activation energy and reaction start

Before any reaction begins, reactant molecules must overcome a minimum energy barrier known as activation energy. This energy allows the bonds in reactants to start breaking.

  • If activation energy is low, reactions start easily.
  • If activation energy is high, reactions need heat, light, or a spark to begin.

This explains why:

  • A matchstick needs friction to ignite.
  • Some food items spoil faster in warmth.
  • Catalysts help speed reactions by lowering activation energy.

Although activation energy influences how fast a reaction starts, the energy released or absorbed depends on bond energies of reactants and products.

Importance of energy changes in reactions

Energy changes during chemical reactions are important because they:

  • Help maintain life processes (respiration, photosynthesis).
  • Allow industries to design efficient processes (combustion, manufacturing).
  • Explain natural temperature regulation (evaporation, condensation).
  • Help predict reaction behaviour and stability.
  • Form the basis of thermodynamics, energy flow, and chemical kinetics.

Without energy changes, no reaction could proceed, and life would not exist.

Conclusion

Reactions release or absorb energy because breaking bonds needs energy while forming new bonds releases energy. If the energy released during bond formation is greater, the reaction becomes exothermic and gives off heat. If more energy is needed to break bonds, the reaction becomes endothermic and absorbs heat. These energy changes control reaction behaviour, influence natural processes, and play an essential role in both living systems and industrial applications.