6.1 Energy in Chemical Reactions

This topic explains how energy is involved in chemical reactions. It describes how energy is either evolved or absorbed during reactions due to bond breaking and bond formation.

Definition

Energy in Chemical Reactions: The energy in the form of heat that is either evolved or absorbed as a result of a chemical reaction.

Main Idea

In chemical reactions, old bonds are broken and new bonds are formed. Bond breaking always consumes energy, and bond making always releases energy. When the energy released by bond formation is greater than the energy consumed by bond breaking, there is a net transfer of chemical energy from the system to the surroundings. When the energy consumed by bond breaking is greater than the energy released by bond formation, energy is transferred from the surroundings to the system.

Core Concept

• Bond breaking: Requires energy (endothermic).
• Bond formation: Releases energy (exothermic).
• Net energy change: Determines whether the reaction is exothermic or endothermic.
• Exothermic: Energy released (system → surroundings).
• Endothermic: Energy absorbed (surroundings → system).

Detailed Explanation

Energy Changes in Chemical Reactions

• The energy in the form of heat is either evolved or absorbed as a result of chemical reaction.
• This is because, in chemical reactions, old bonds are broken and new bonds are formed.
• Bond breaking always consumes energy, and bond making always releases energy.
• When the energy released by bond formation is greater than the energy consumed by bond breaking, there is a net transfer of chemical energy from system to the surroundings.
• Whereas energy is transferred from surroundings to the system, when the energy consumed by bond breaking is greater than the energy released by bond formation.
• Thus, in chemical reactions, energy transfer occurs between a system and the surroundings.