119.

Out of staggered and eclipsed conformations of ethane, which is more stable and why ?

Staggered conformation of ethane is more stable than the eclipsed conformation.
Reason. When the staggered conformation is rotated through an angle of 60°, it changes to eclipsed conformation and the eclipsed conformation when further rotated through 60° gives back the staggered conformation.

These two conformations differ in their energy contents and also in their relative stabilities. The staggered conformation with minimum repulsions between C-H bonding electrons and hydrogens of one methyl group and those of other has lesser energy than the eclipsed conformation where the force of repulsion between C-H bonding electrons and hydrogen of one methyl group and those of other is maximum.

The energy difference between staggered and eclipsed conformation of ethane is 12-55 kJ/mole. Hence staggered conformation is more stable than the eclipsed conformation.