First Law of Thermodynamics

Basic Principle

• Energy cannot be created or destroyed.
• Energy is transferred from one place to another, not lost.

Energy Transfer

• System vs. Surroundings: Energy can flow into or out of a system through:
  • Heat (Q): When heat flows into a system, it increases the system's internal energy (U).
  • Work (W): When work is done on a system by the surroundings, it also increases the system's internal energy.

Illustration with Money

• Analogous to a financial transaction:
  • If you sell a laptop for $500, your account increases by $500, while the buyer's decreases by the same amount.
  • This illustrates energy transfer: when a system gains energy, the surroundings lose an equivalent amount.

Types of Systems

1. Open System:
   • Matter and energy can transfer in and out.
2. Closed System:
   • Only energy can transfer in and out; matter cannot.
3. Isolated System:
   • Neither matter nor energy can transfer in or out.
   • Mass and total energy remain constant.

Equation for Internal Energy

• Chemistry: ∆U = Q + W
• Physics: ∆U = Q - W
• Differences arise due to different viewpoints:
  • Chemistry focuses on the system's viewpoint.
  • Physics considers the surroundings' viewpoint.

Work and Energy in Chemistry vs. Physics

• Chemistry:
  • W is negative when work is done by the system (energy is expended).
  • W is positive when work is done on the system (energy is gained).
• Physics:
  • W is positive when work is done by the system (surroundings gain energy).
  • W is negative when work is done on the system (system loses energy).

Sign Conventions

• Q (Heat):
  • Positive when absorbed by the system (endothermic).
  • Negative when released by the system (exothermic).
• W (Work in Chemistry):
  • Positive when work is done on the system.
  • Negative when work is done by the system.

Applications

• In future videos: practice problems on calculating changes in internal energy using Q and W.