Question

Which of the following is not correct?

• A. ΔG is negative for a spontaneous reaction
• B. ΔG is zero for a reversible reaction
• C. ΔG is positive for a spontaneous reaction
• D. ΔG is positive for a non-spontaneous reaction

Answer 1

The Correct Option is C

To determine which of the given options is incorrect, we need to understand the concept of Gibbs free energy change (\(\Delta G\)) and its relation to reaction spontaneity in thermodynamics. 

The Gibbs free energy change (\(\Delta G\)) is a thermodynamic quantity that helps predict the spontaneity of a chemical reaction at constant temperature and pressure. Here's how \(\Delta G\) is interpreted:

• \(\Delta G < 0\): The reaction is spontaneous.
• \(\Delta G = 0\): The reaction is at equilibrium, and it is reversible.
• \(\Delta G > 0\): The reaction is non-spontaneous.

Now, let's evaluate the given options:

1. Option 1: \(\Delta G\) is negative for a spontaneous reaction — This is correct because, for a reaction to occur spontaneously, it must have a negative Gibbs free energy change.
2. Option 2: \(\Delta G\) is zero for a reversible reaction — This is correct as a reaction at equilibrium or reversible state has a Gibbs free energy change of zero.
3. Option 3: \(\Delta G\) is positive for a spontaneous reaction — This is incorrect. A positive \(\Delta G\) indicates that the reaction is non-spontaneous.
4. Option 4: \(\Delta G\) is positive for a non-spontaneous reaction — This is correct. A positive Gibbs free energy change indicates that the reaction will not occur spontaneously under the given conditions.

Thus, the correct answer is: \(\Delta G\) is positive for a spontaneous reaction, as it contradicts the fundamental principle of thermodynamics regarding reaction spontaneity. A positive \(\Delta G\) signifies a non-spontaneous process.

Answer 2

For a process to be spontaneous, the Gibbs free energy change (ΔG) must be negative at constant temperature and pressure, indicating that the reaction proceeds forward without external energy input. Conversely, for a non-spontaneous process, ΔG is positive, meaning the reaction does not proceed unless energy is supplied. For a reaction in equilibrium (reversible), ΔG = 0.
Thus, statement (3) is incorrect as it suggests ΔG is positive for a spontaneous reaction, which contradicts the condition for spontaneity.

(ΔG)_P,T = (+) for non-spontaneous process
So, the correct answer is : Option 3.