To determine which of the following is not a thermodynamic function, we need to identify the characteristics of thermodynamic functions. Thermodynamic functions are state functions, which means they are path-independent and depend only on the initial and final states of a system, not on the path taken to get from one state to another.
### Step-by-Step Solution:
1. **Understanding Thermodynamic Functions**:
- Thermodynamic functions are properties that depend only on the state of the system. Examples include internal energy (U), enthalpy (H), and Gibbs free energy (G). These functions are path-independent.
2. **Identifying the Given Functions**:
- The question provides several options, including internal energy, enthalpy, Gibbs free energy, and work done.
3. **Analyzing Internal Energy (U)**:
- Internal energy is a state function. It depends only on the state of the system (temperature, pressure, volume) and is path-independent.
4. **Analyzing Enthalpy (H)**:
- Enthalpy is also a state function. It is defined as H = U + PV, where P is pressure and V is volume. Like internal energy, it is path-independent.
5. **Analyzing Gibbs Free Energy (G)**:
- Gibbs free energy is another state function, defined as G = H - TS (where T is temperature and S is entropy). It is also path-independent.
6. **Analyzing Work Done (W)**:
- Work done is not a state function. It depends on the path taken to go from one state to another. For example, the work done during expansion or compression can vary based on the specific process followed (isothermal, adiabatic, etc.).
7. **Conclusion**:
- Since work done is path-dependent, it is not a thermodynamic function. Therefore, the answer to the question is that work done is not a thermodynamic function.
### Final Answer:
The option that is not a thermodynamic function is **work done**.
