Enthalpy Change & Activation Energy | Chemistry for Grade 11 (IGCSE) PDF Download

Enthalpy Change & Activation Energy

• Chemical reactions require atoms or particles to collide for them to occur within a system.
• Several factors influence collisions, including energy, orientation, and collision frequency.
• Each collision must possess a minimum energy threshold for successful reaction, termed activation energy (Ea).
• Activation energy varies among reactions, depending on the specific chemical species involved.
• Reactions with higher activation energies necessitate more initial energy input compared to those with lower activation energies.
• The exchange of thermal energy during a reaction is known as the enthalpy change (ΔH), which can be either positive (endothermic) or negative (exothermic) based on the reaction type.

Reaction Pathway Diagrams

Exothermic Reactions

• Exothermic reactions release more energy than they absorb.
• Energy released during the formation of new bonds exceeds the energy needed to break bonds in the reactants.
• The energy change is negative as the products possess less energy than the reactants.
• Consequently, exothermic reactions are characterized by a negative ΔH value.
• The reaction pathway diagram illustrates the process of an exothermic reaction.
• The diagram represents the pathway for exothermic reactions:
  

Endothermic Reactions

• Endothermic reactions absorb more energy to break bonds than they release to form new bonds.
• The energy change is positive because the products possess more energy than the reactants.
• Consequently, an endothermic reaction is characterized by a positive ΔH value, evident in energy level diagrams and calculations.

Exothermic Reactions:

• Release more energy than they absorb.
• Energy is released during bond formation compared to bond breaking in the reactants.
• The energy change is negative because products have lower energy levels than reactants.
• This results in a negative ΔH value for exothermic reactions.

The Reaction Pathway for Endothermic Reactions

• If more energy is required to break bonds than is released during bond formation, the reaction is termed endothermic overall.
• Endothermic reactions exhibit a positive energy change since products possess higher energy levels than reactants.
• A positive ΔH value characterizes endothermic reactions, evident in energy level diagrams and calculations.
• The reaction pathway diagram above illustrates endothermic reactions.