Energy of eclipsed and staggered conformations of ethane and propane Which points on the given energy diagram represents eclipsed confirmation of ethane and staggered confirmation of propane respectively?


Attempt: Eclipsed configuration is unstable because of repulsions between filled $\sigma$ or bonding orbital and the anti or staggered configuration is more stable due to interactions between filled $\sigma$ orbital and empty $\sigma^*$ orbital.
From the given diagram, either R or S could represent eclipsed confirmation of ethane and P or Q could represent staggered confirmation of propane.
Since they are different compounds, they should lie on different energy diagrams. So the answer could be (R,Q) or (S,P). Which one is correct?
That diagram is horribly drawn.  The way it is drawn, the 13.6 kJ/mol number is meaningless - it measures from the maximum of one curve to the minimum of the other curve.  

I think that you are supposed to assume that for one of the molecules, the one for the lower curve, the torsional strain is 12.6 kJ/mol and for the other molecule the torsional strain is 13.6 kJ/mol.  

Do you have some reason to think that the barrier to free rotation for propane would be higher or lower than for ethane?  Perhaps your book gives the value for the torsional strain of ethane/propane? (through a quick google search I was able to find the torsional strain for both of these)