Discover “Energy Release in Bond Formation” and how combustion releases energy through bond energy changes, producing heat and light.

Basics of Combustion

Combustion occurs when a chemical reaction breaks existing bonds in reactants and forms new bonds in products. This process involves changes in bond energy, playing a key role in whether the reaction releases or absorbs energy. Typically, combustion is an exothermic reaction, meaning energy is released during the process.

What Is Bond Energy?

Bond energy refers to the energy required to either break or form a chemical bond. During a reaction:

1. Energy is absorbed to break the bonds in the reactants.
2. Energy is released when new bonds are formed in the products.
   The difference between these energies determines whether the reaction is exothermic (releases energy) or endothermic (absorbs energy).

Example: Reaction of Carbon and Oxygen

A classic example of combustion is the reaction between carbon (C) and oxygen (O₂):

C + O₂ → CO₂ + Energy

This reaction involves the following energy changes:

1. Breaking bonds (energy absorbed):
   The oxygen molecule (O₂) must break its bond, which requires energy.
   O₂ → 2O (ΔE > 0)
2. Forming bonds (energy released):
   Carbon forms strong double bonds with oxygen in carbon dioxide (CO₂), releasing a significant amount of energy.
   C + O + O → CO₂ (ΔE < 0)

Energy Balance in Combustion

The net energy released during combustion is determined by comparing:

1. Energy absorbed to break bonds in reactants.
2. Energy released when forming bonds in products.

For example, in the reaction of carbon and oxygen:

• Energy required to break the O₂ bond: ~498 kJ/mol.
• Energy released when forming CO₂ bonds: ~1598 kJ/mol (799 kJ/mol per bond × 2 bonds).

Since the energy released is far greater than the energy absorbed, the reaction is exothermic, resulting in the release of heat and light.

Key Numbers (Approximate):

• Oxygen molecule (O₂) bond energy: ~498 kJ/mol.
• Carbon-oxygen bond energy in CO₂: ~799 kJ/mol (per bond), ~1598 kJ/mol (for both bonds).

Thus, the formation of CO₂ releases much more energy than is consumed to break the O₂ bond, making combustion a strong energy-releasing reaction.

Summary

The release of energy in combustion is governed by the balance between:

• Energy absorbed to break reactant bonds.
• Energy released when product bonds form.

This energy difference explains why combustion reactions release significant heat and light, enabling continuous burning.

Reference page: “Key fire characteristics: combustion elements, smoke risks, toxic gases, and safe firefighting techniques.”

References　National Fire Protection Association (NFPA)

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