Transferring Energy: Thermochemistry

Jun 13, 2024

Transferring Energy: Thermochemistry Lecture Notes

Introduction

  • Thermochemistry: Study of energy transfer during physical and chemical processes.

Ways of Transferring Energy

  1. Work
    • Definition: Energy transferred when a force moves an object a certain distance.
    • Equations:
      • Work (W) = Force (F) × Distance (d)
      • In physics, units: Joules (J), Newton (N), and meter (m).
    • Unit Analysis:
      • Joule = Newton × meter
      • Newton = kg·m/s²
      • Joule also equals kg·m²/s² (same as potential and kinetic energy units).
    • Chemistry Equation: Work = -Pressure (P) × Change in Volume (ΔV)
      • Unit Conversion: 1 liter·atmosphere = 101.325 Joules
  2. Heat
    • Defined as energy transferred from a hotter object to a colder one.
    • Always flows from high temperature to low temperature.
    • Molecular Explanation: Faster-moving (higher temperature) molecules collide with slower-moving (lower temperature) molecules, transferring energy until thermal equilibrium is reached.

Internal Energy

  • Represented by E or U (interchangeable).
  • Definition: Sum of all kinetic and potential energies in a system.
  • Often discussed in terms of change in energy,
    • ΔE (Energy Final - Energy Initial) = Energy of Products - Energy of Reactants.
    • First Law of Thermodynamics: ΔE_system = -ΔE_surroundings (Energy cannot be created or destroyed).

Energy Transfer Mechanisms

  • Energy can be transferred through work (W) and heat (Q).
    • ΔE = Q + W
      • Internal energy change (ΔE) relies on heat and work done on the system.
    • Positive Q: Heat flows into the system (endothermic).
    • Negative Q: Heat flows out of the system (exothermic).
    • Work Dynamics:
      • Positive W: Work done on the system (compression).
      • Negative W: Work done by the system (expansion).

Important Concepts

  • Thermodynamic quantities have two parts: magnitude and sign.
  • Always use proper units (Joules).
  • Signs must be from the system's point of view:
    • Energy into system: Positive sign.
    • Energy out of system: Negative sign.
  • System vs. surroundings dynamics agree with the first law of thermodynamics:
    • Q_system = -Q_surroundings
    • W_system = -W_surroundings

Worked Example

  • Problem: A reaction releases 890 Joules of heat to surroundings, and the gas produced does 450 Joules of work on surroundings.
  1. Closed System: Energy but not matter can flow between system and surroundings.
  2. Signs:
    • Q = -890 J (energy released)
    • W = -450 J (work done by system)
  3. Change in Energy:
    • ΔE = Q + W = -890 J + (-450 J) = -1340 J or -1.34 kJ

Summary

  • Energy can be transferred through work and heat.
  • Equations and units must be consistent with thermodynamic principles.
  • Next lecture: Focus solely on heat as a form of energy transfer.