For chemical reaction to be spontaneous, ∆G must be

1. Analyze the question

The question asks for the condition of the Gibbs Free Energy change ($\Delta G$) for a chemical reaction to occur spontaneously. In thermodynamics, spontaneity refers to whether a reaction will proceed on its own without continuous external input of energy.

2. Explain the concept

Gibbs Free Energy ($\Delta G$) is the energy associated with a chemical reaction that can be used to do work. It is defined by the equation:

Where:

• $\Delta H$ is the change in enthalpy (heat content).
• $T$ is the absolute temperature in Kelvin.
• $\Delta S$ is the change in entropy (disorder).

The criteria for spontaneity are:

• If $\Delta G < 0$ (negative): The reaction is spontaneous (it happens naturally).
• If $\Delta G > 0$ (positive): The reaction is non-spontaneous (it requires energy input to occur).
• If $\Delta G = 0$: The reaction is at equilibrium.

3. Show the solution

Since the question specifically asks for the condition under which a reaction is spontaneous, we look for the value that indicates the system is moving toward a lower energy state, which is the hallmark of a natural, spontaneous process.

According to the Second Law of Thermodynamics, for any spontaneous process, the total entropy of the universe must increase, which mathematically translates to $\Delta G$ being less than zero. Therefore, $\Delta G$ must be negative.

4. Identify the answer

Comparing our explanation to the given options: A. positive (Non-spontaneous) B. negative (Spontaneous) C. zero (Equilibrium) D. equal to the enthalpy change (Only true if $T\Delta S = 0$)

The correct option is B.

5. Tip

Think of $\Delta G$ like a ball rolling down a hill. A spontaneous reaction is like the ball rolling down naturally—it releases energy, so the change is negative. A non-spontaneous reaction is like trying to push the ball back up the hill—you have to put energy in, so the change is positive.