Energy of a Chemical System Notes

Internal energy, the first law, and state functions

• The energy of a chemical system is governed by the first law: energy cannot be created or destroyed.
• The internal energy of a system, denoted $u$, is a state function that, for idealized conditions, depends on temperature. In general, it is the energy stored in bonds, vibrations, and composition of the system.
• Change in internal energy is given by the first law: $\Delta U = q + w,$ where:
  • $q$ is heat transferred into the system (positive when heat enters the system),
  • $w$ is work done on the system (positive when work is done on the system).
• Sign convention used in this lecture:
  • If the surroundings do work on the system (compression), then w > 0.
  • If the system does work on the surroundings (expansion), then w < 0.
• For isolated systems, total energy is constant; energy can flow between system and surroundings, but the total remains unchanged.
• In measuring chemical thermodynamics, we track changes that transfer energy between system and surroundings (heat and work).