See, in ethane, the hydrogen atoms of one methyl group try to stay away from the other. Ka seng! So the staggered form, where they are farthest, has less steric hindrance and also less torsional strain. That’s why it’s more stable, ha, by that 12.5 kJ per mole.
Honestly, I think of it like my bicycle tyre and the wet uphill path to Mawphlang. When the tyre is old and bulging in the wrong place—like the eclipsed conformation—it rubs hard against the mudguard, wasting my energy. But when the tube sits nice and staggered inside the rim, the ride is smooth, and I save my strength. That’s the stability, you see?
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