A chemical system is at equilibrium when

Analyze the question: We need the thermodynamic condition that tells us a system is at equilibrium.

Concept: The Gibbs free energy change (ΔG) tells us the direction of a process at constant temperature and pressure:

• ΔG < 0: the reaction is spontaneous in the forward direction.
• ΔG > 0: the forward reaction is non-spontaneous (the reverse is favoured).
• ΔG = 0: no net change occurs, so the system is at equilibrium.

The relationship is ΔG = ΔH − TΔS. At equilibrium, the forward and backward reactions occur at the same rate, so there is no net driving force. This means ΔG = 0, which gives ΔH = TΔS.

Checking the options:

• A. ΔG positive: the reaction is non-spontaneous, so this is not equilibrium.
• B. ΔG zero: this is the equilibrium condition.
• C. ΔS negative: entropy change alone does not tell us about equilibrium, because ΔH also matters.
• D. ΔS positive: same reason as C.
• E. ΔS zero: ΔS of the system need not be zero at equilibrium. For example, at the melting point of ice, ΔS is not zero, but ΔG is zero.

Tip: Remember "G for Go": if ΔG is negative, the reaction goes. If ΔG is zero, it has nowhere to go because it is at equilibrium. For NECO and JAMB, any question on equilibrium and free energy will almost always point to ΔG = 0 (and ΔG° = −RT ln K is related to this).