Q.Which of the following is not a thermodynamic function?

Entropy, internal energy, and enthalpy are all state (thermodynamic) functions; frictional energy depends on the path/process and hence is not a thermodynamic function.

A thermodynamic function (state function) is a property whose value depends only on the current state of the system (defined by variables like pressure, volume, temperature, composition) -- not on the path taken to reach that state. Internal energy (U), enthalpy (H), and entropy (S) are all classic examples: their change between two states is fixed, regardless of how the system got there.

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Among G, U, H and q, only heat (q) is a path function — the other three are state functions.

A state function is a thermodynamic quantity whose value depends only on the current state of the system (defined by variables like temperature, pressure, volume, composition), not on how that state was reached. Internal energy (U), enthalpy (H) and Gibbs free energy (G) are all state functions: their change between two states (delta-U, delta-H, delta-G) is fixed regardless of the path taken.

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