The free energy change for a reversible reaction at equilibrium is:
zero
small positive
small negative
large positive.

Options

A. zero
B. small positive
C. small negative
D. large positive.

Explanation

To determine the free energy change for a reversible reaction at equilibrium, we can follow these steps:

### Step 1: Understand the Concept of Gibbs Free Energy
Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work that can be performed by a thermodynamic system at constant temperature and pressure.

**Hint:** Recall that Gibbs free energy is a measure of the energy available to do work in a system.

### Step 2: Identify the Equation for Gibbs Free Energy Change
The change in Gibbs free energy (ΔG) for a process is given by the equation:
\[ \Delta G = \Delta H - T \Delta S \]
where:
- ΔH = change in enthalpy
- T = absolute temperature
- ΔS = change in entropy

**Hint:** Remember that ΔG depends on both enthalpy and entropy changes.

### Step 3: Analyze the Conditions at Equilibrium
At equilibrium, the system is in a state where the forward and reverse reactions occur at the same rate. This means that there is no net change in the concentrations of reactants and products.

**Hint:** Consider what happens to the system when it reaches a state of balance.

### Step 4: Determine the Value of ΔG at Equilibrium
At equilibrium, the change in Gibbs free energy (ΔG) is zero:
\[ \Delta G = 0 \]
This indicates that the system is at a state of maximum stability, and no net work can be done.

**Hint:** Think about what "equilibrium" means in terms of energy changes.

### Step 5: Conclude the Answer
Based on the analysis, the free energy change for a reversible reaction at equilibrium is zero.

**Final Answer:** The correct option is **zero**.