Lecture Notes: First Law of Thermodynamics

Key Concepts

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transferred from one place to another.
• Internal Energy: Denoted by U, it is the energy within a system.
• Energy Transfer: Occurs through heat and work.

Energy Flow

• Energy can flow into or out of a system:
  • Heat (Q): Energy flow into the system increases internal energy.
  • Work (W): Surroundings performing work on a system increases internal energy.

Illustrative Example

• Like money, energy must come from somewhere:
  • If a system gains 300J, surroundings lose 300J.
  • Energy transfer is a zero-sum game: gain for one is a loss for another.

Types of Systems

1. Open System
   • Matter and energy can be transferred in/out.
2. Closed System
   • Only energy (not matter) can be transferred in/out.
3. Isolated System
   • Neither matter nor energy can be transferred in/out.

Equation for Internal Energy Change

• Chemistry: ( \Delta U = Q + W )
• Physics: ( \Delta U = Q - W )

Point of View Differences

• Chemistry: System's perspective
  • ( W ) is negative when work is done by the system (system loses energy).
  • ( W ) is positive when work is done on the system (system gains energy).
• Physics: Surroundings' perspective
  • ( W ) is positive when work is done by the system (surroundings gain energy).
  • ( W ) is negative when work is done on the system (surroundings lose energy).

Heat Transfer

• Endothermic Process: ( Q ) is positive, system absorbs heat.
• Exothermic Process: ( Q ) is negative, system releases heat.

Sign Conventions in Chemistry

• W:
  • Positive when work is done on the system.
  • Negative when work is done by the system.
• Q:
  • Positive for heat absorbed by the system.
  • Negative for heat released by the system.

Practice Problems

• Future exercises will involve calculating changes in internal energy using the concepts of heat and work.