Heat of hydrogenation from heat of combustion and heat of formation?

[Andrew Polyak]

The heats of combustion (to CO₂ and H₂O) of some C₅ compounds are given below.

Z-2-pentene –3369 kJ/mol

E-2-pentene –3365 kJ/mol

1-pentene –3376 kJ/mol

pentane–3536 kJ/mol
The heat of formation of CO₂ is –393 kJ/mol and that of H₂O is –285 kJ/mol.

What is the heat of hydrogenation of 1-pentene (in kJ/mol)?

 

There are three questions on my OCHEM practice exam like this, and I can't understand any of them. I understand everything else, but I want the extra 50 points. I know how to calculate delta h using BDE, but that's where my knowledge stops.

[Enthalpy]

1) What do you obtain by hydrogenating 1-pentene?

2) Can you find a series of transformations, each having a known heat, which is equivalent to this hydrogenation: same beginning, same end? Can you then combine the individual heats? Don't forget the additional H₂O.

 

Keep in mind: there is a gross and a net heat of combustion (translation to other languages may not be direct). One makes liquid water at rooms temperature, the other gaseous water at room temperature. The difference is big and is a very frequent source of mistakes. Always check the consistency of the heats of combustion. -285 kJ/mol is liquid water.

 

Added warning: heats of formation are often measured by burning the compounds. It's universal and apparatus exists for that purpose BUT the combustion heat of hydrocarbons exceeds vastly their heat of formation, so that a slightly imprecise heat of combustion reults in a botched heat of formation or hydrogenation.

[John Cuthber]

You also need to be aware of this

http://en.wikipedia.org/wiki/Hess's_law