Answer:
5J/mol
Explanation:
H-H eclipsed = Torsional strain = 4.0 kJ/mol
H-CH3 eclipsed = Mostly torsional strain = 6.0 kJ/mol
CH3-CH3 eclipsed =Torsional and steric strain = 11.0 kJ/mol
CH3-CH3 gauche = Steric strain = 3.8 kJ/mol
You know each H-H eclipsed is 4 kJ/mol (8 kJ/mol total) and the reported barrier is 13 kJ/mol. Therefore the H-I eclipsed interaction must be 5 kJ/mol, (13-8 = 5).
