Part 8: THERMODYNAMICS

This should have been a reasonably straight forward calculation, but care needs to be taken to get the right structures and the right molecular formula, then do the math correctly.

a. C₆H₁₂ + 9 O₂ --> 6 CO₂ + 6 H₂O

b. The calculation is shown on the figure below.

c.

d. See diagram above.

e. The difference between the 3 isomers is in the degree of branching. Hex-1-ene i is the least branched and the least stable (DH_f = -10.0 kcal/mol) in contrast to isomer iii 3,3-dimethylbut-1-ene (DH_f = -14.3 kcal/mol) which is more branched. A more branched isomer has more primary C-H bonds which are stronger than secondary C-H which in turn are stronger than tertiary C-H.

f.

Common errors:

In part a:

• did not show the right products from the combustion reaction (i.e. CO₂ and H₂O)
• did not balance the coefficients
• wrong molecular formula for hex-1-ene

In part d :

• Incorrect math, e.g. using 18 H₂ instead of 9.
• getting the signs wrong