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Chemical Thermodynamics and Energetic - First Law of Thermodynamics

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Estimated time: 7 minutes
Maharashtra State Board: Class 11

Definition: Thermodynamic State Variables

The specific values of macroscopic variables that completely describe every equilibrium state of a thermodynamic system are called thermodynamic state variables.

Maharashtra State Board: Class 11

Definition: Extensive Variables

The thermodynamic state variables that depend on the size of the system (e.g., internal energy, volume) are called extensive variables.

Maharashtra State Board: Class 11

Definition: Intensive Variables

The thermodynamic state variables that do not depend on the size of the system (e.g., pressure, temperature) are called intensive variables.

Maharashtra State Board: Class 11

Law: First Law of Thermodynamics

Statement:
The net heat energy supplied to a system is equal to the sum of the change in internal energy of the system and the work done by the system. It is based on the law of conservation of energy.

Formula:

Q = ΔU + W

where Q = heat added, ΔU = change in internal energy, W = work done by the system.

Maharashtra State Board: Class 12

Key Points: First Law of Thermodynamics

First Law: Energy of system + surroundings remains constant → ΔU = q + W

ΔU: change in internal energy, q: heat, W: work done on system

Sign convention:

  • Work by system (−)
  • on system (+)
  • Heat absorbed (+)
  • released (−)

ΔU > 0: energy enters system; ΔU < 0: energy leaves system

  • Isothermal: ΔU = 0 → q = −W
  • Adiabatic: q = 0 → ΔU = W
  • Isochoric: W = 0 → ΔU = q
  • Isobaric: ΔU = q + W

Video Tutorials

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Shaalaa.com | First Law of Thermodynamics: Internal Energy, Heat, and Work

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First Law of Thermodynamics: Internal Energy, Heat, and Work [00:13:16]
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