Explain the change of energy with a graph for the internal rotation of ethane about the $C-C$ bond after a $120^{\circ}$ rotation.

(N/A) The internal rotation of ethane about the $C-C$ bond leads to different conformational isomers.
$1$. The staggered conformation is the most stable form with minimum energy due to minimum torsional strain.
$2$. The eclipsed conformation is the least stable form with maximum energy due to maximum torsional strain.
$3$. $A$ rotation of $60^{\circ}$ from staggered to eclipsed increases the energy by $12.5 \ kJ \ mol^{-1}$.
$4$. $A$ further rotation of $60^{\circ}$ (total $120^{\circ}$) brings the molecule back to a staggered conformation,which is equivalent to the initial state in terms of energy.
Thus,after a $120^{\circ}$ rotation,the molecule returns to a staggered conformation,which is a minimum energy state.