Question

If $\Delta G$ is positive, the reaction is:

• A. Spontaneous
• B. Non-spontaneous
• C. At equilibrium
• D. Reversible

Hint:

Remember the thermodynamic spontaneity criteria at constant $T$ and $P$:
$\Delta G < 0 \rightarrow$ Spontaneous
$\Delta G > 0 \rightarrow$ Non-spontaneous
$\Delta G = 0 \rightarrow$ Equilibrium
This is one of the most frequently asked basic questions in thermodynamic exams.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks about the spontaneity of a chemical reaction when the Gibbs free energy change ($\Delta G$) is positive.
Gibbs free energy is a thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at constant temperature and pressure.

Step 2: Key Formula or Approach:
The change in Gibbs free energy is given by the relation:
\[ \Delta G = \Delta H - T\Delta S \]
where:
$\Delta H$ is the change in enthalpy.
$T$ is the absolute temperature in Kelvin.
$\Delta S$ is the change in entropy.
The sign of $\Delta G$ determines the thermodynamic feasibility of a process at constant temperature and pressure:
1. If $\Delta G < 0$, the process is spontaneous (exergonic).
2. If $\Delta G > 0$, the process is non-spontaneous (endergonic).
3. If $\Delta G = 0$, the system is in dynamic equilibrium.

Step 3: Detailed Explanation:

• A positive value of $\Delta G$ indicates that the free energy of the products is greater than the free energy of the reactants.

• For such a reaction to occur, an external input of energy is required to drive the process forward.

• Consequently, the forward reaction cannot proceed on its own under the specified conditions, making it non-spontaneous.

• Conversely, the reverse reaction under these conditions would have a negative $\Delta G$ and would be spontaneous.

Step 4: Final Answer:
Since $\Delta G$ is positive, the reaction is non-spontaneous.