Lecture Notes: Enthalpy and State Functions

Introduction

• Topic: Enthalpy
• Pre-requisite: Understanding of State Functions

State Functions

• Definition: Describes the current state of a system, independent of the path taken
• Analogy: Displacement vs. Distance
  • Displacement: Direct path from point A to point B (State Function)
  • Distance: Actual path taken, may be indirect (Path Function)

Energy Diagrams in Chemistry

• Axes:
  • Y-axis: Energy
  • X-axis: Reaction Coordinate (can also be time)
• Reactants to Products: Potential Energy Changes
  • High potential energy reactants transition to low potential energy products
  • State Function: Change in energy (ΔE) between reactants and products

Enthalpy (H)

• Definition: Transfer of heat to/from a system at constant pressure
• Symbol: H
• Common Condition: Constant pressure, usually atmospheric in open systems
• Chemists' Interest: Flow of heat at constant pressure

Relating Enthalpy to Internal Energy (ΔE)

• Equation: H = E + PV
• Working Equation: ΔH = ΔE + PΔV
• Relevant Equations:
  • ΔE = q + w
  • w = -PΔV
• Derivation: ΔH
  • Substitute ΔE: ΔH = q + w + PΔV
  • Adjust w: ΔH = q - w + w ➡ ΔH = q (at constant pressure)
• Key Concept: ΔH is the heat energy at constant pressure

Potential Energy Diagrams

Exothermic Processes

• ΔH < 0 (Negative ΔH) : Energy released
• Equation Example: 2H₂ + O₂ → 2H₂O + 510 kJ
• Energy Diagrams:
  • Reactants start at high potential energy, products at low potential energy
  • Negative ΔH ➡ Initial energy (reactants) higher than final energy (products)

Endothermic Processes

• ΔH > 0 (Positive ΔH): Energy absorbed
• Equation Example: 510 kJ + 2H₂O → 2H₂ + O₂
• Energy Diagrams:
  • Reactants start at low potential energy, products at high potential energy
  • Positive ΔH ➡ Initial energy (reactants) lower than final energy (products)

Key Takeaways

• Exothermic:
  • Negative ΔH
  • Energy released as a product
  • Graph shows decrease in potential energy
• Endothermic:
  • Positive ΔH
  • Energy absorbed as a reactant
  • Graph shows increase in potential energy
• Enthalpy Focus: Mostly on heat transfer at constant pressure

Next Steps

• Future videos will cover calculations involving enthalpy.