Energy and Change in Chemical Reactions

Introduction

• Discussion on important definitions and concepts related to energy and change in chemical reactions.
• Key terms include heat of reaction, exothermic, endothermic, activation energy, activated complex, and catalyst.

Key Definitions and Concepts

Enthalpy Change (Heat of Reaction)

• Symbol: ΔH
• Represents the total internal energy change in a reaction.
• Difficult to measure directly, easier to measure change.
• Enthalpy change occurs when energy is transferred into or out of a system.
• Accompanied by heat transfer, hence the term "heat of reaction."
• Unit: kJ/mol
• Formula: Enthalpy of products - Enthalpy of reactants
• Example: ΔH = 100 (products) - 400 (reactants) = -300 kJ/mol

Activation Energy

• Energy required to start a chemical reaction.
• Minimum energy needed for a reaction to occur.
• Determined from a potential energy diagram (reactants to activated complex).
• Example: From 400 to 900 kJ/mol = 500 kJ/mol for forward reaction.
• Activation energy differs for forward and reverse reactions.
• Reverse reaction example: From 100 to 900 kJ/mol = 800 kJ/mol.

Activated Complex

• Unstable transition state between reactants and products.
• Located at the peak of the potential energy curve.
• Energy absorbed (activation energy) breaks reactant bonds, leading to this state.

Exothermic vs. Endothermic Reactions

• Exothermic:
  • Heat exits the system.
  • Net release of energy, products have lower energy than reactants.
  • Example: Energy of products < Energy of reactants.
• Endothermic:
  • Heat enters the system.
  • Net intake of energy, products have higher energy than reactants.
  • Example: Energy of products > Energy of reactants.

Catalyst

• Substance that increases the rate of a chemical reaction without changing itself.
• Speeds up the reaction.
• Lowers the activation energy, providing an alternative pathway for the reaction.
• Remains unchanged after the reaction.
• Comparison on energy diagrams:
  • Without catalyst: High activation energy.
  • With catalyst: Lower activation energy.

Conclusion

• These definitions are crucial for understanding energy changes in chemical reactions.
• Further detail on topics like exothermic vs. endothermic reactions is available in additional videos.
• Encouragement to practice with past paper questions.
• Reminder to subscribe and engage with the content.