Recall the criterion for spontaneity using Gibbs free energy

At constant temperature and pressure:

• If $\Delta G < 0$, the process is spontaneous.
• If $\Delta G > 0$, the process is non-spontaneous.
• If $\Delta G = 0$, the system is at equilibrium (reversible condition).

Check each option

Option A: $\Delta G$ is positive for a spontaneous reaction

• This is incorrect because spontaneous reactions have: $\Delta G < 0$

Option B: $\Delta G$ is positive for a non-spontaneous reaction

• This is correct because non-spontaneous reactions have: $\Delta G > 0$

Option C: $\Delta G$ is zero for a reversible reaction

• This is correct in the sense that for a reversible process at equilibrium: $\Delta G = 0$

Option D: $\Delta G$ is negative for a spontaneous reaction

• This is correct because: $\Delta G < 0$

Identify the statement which is not correct

The only incorrect statement is: $\boxed{\text{A}}$