The barrier to C-C bond rotation in iodoethane is 13 kJ/mol. What energy can you assign to an H-I eclipsing interaction?

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).