List the characteristics of Gibbs free energy.

#### Solution

**Characteristics of Gibbs free energy:**

**1. **Gibbs free energy is defined as the part of the total energy of a system that can be converted (or) available for conversion into work.

G = H – TS ………..(1)

Where H = enthalpy, T = temperature and S = entropy

**2. **G is a state function and is a single value function.

**3. **G is an extensive property, whereas ∆G becomes intensive property for a closed system. Both G and ∆G values correspond to the system only.

**4.** ∆G gives criteria for spontaneity at constant pressure and temperature.

- If ∆G is negative (∆G < O), the process is spontaneous.
- If ∆G is positive (∆G > O), the process is non-spontaneous.
- If ∆G is zero (AG = O), the process is equilibrium.

**5. **For any system at constant pressure and temperature,

∆G = ∆H – T∆S ……….. (2)

We know AH = ∆U + P∆V

∆G = ∆U + P∆V – T∆S ………(3)

**6. **For the first law of thermodynamics, ∆U = q + w

∆G = q + w + P∆V – T∆S …………(4)

For the second law of thermodynamics, ∆S = `"q"/"T"`

∆G = q + w + P∆V – T`"q"/"T"`

∆G = w + P∆V …………(5)

∆G = – w – P∆V ……….(6)

**7.** – P∆V represent the work done due to expansion against constant external pressure. Therefore, it is clear that the decrease in free energy (– ∆G) accompanying a process taking place at constant temperature and pressure is equal to the maximum work obtainable from the system other than the work of expansion.

**8. **Unit of Gibb’s free energy is J mol^{–1}