Organic hydrogenation reactions, in which H2 and an \"unsaturated\" organic comp

Question

Organic hydrogenation reactions, in which H2 and an "unsaturated" organic compound combine, are used in the food, fuel, and polymer industries. In the simplest case, ethene (C2H4) and H2 form ethane (C2H6). If 159 kJ is given off per mole of C2H4 reacting, how much heat is released when 14.9 kg of C2H6 forms?

Explanation / Answer

Organic hydrogenation reactions, in which H2 and an "unsaturated" organic compound combine, are used in the food, fuel, and polymer industries. In the simplest case, ethene (C2H4) and H2 form ethane (C2H6). If 115 kJ is given off per mole of C2H4 reacting, how much heat is released when 15.0 kg of C2H6 forms? ( in kJ) It's a little hard to tell from the wording, but I think the final sentence is referring to the compounds forming from scratch (aka., heats of formation). This is certainly a standard organic question! In this case you need to look at all the reactions involved and look for common points between them; you will line up and cancel the reactions exactly as you did in intro chem. (If you don't know what I'm talking about look up Hess' Law). I *think* the part about C2H4 "reacting" is referring to the hydrogenation, though the question should make this more clear. So here are your basic reactions: C2H4 + H2 -> C2H6 delta H: 115 kJ/mol C2H4 -> 2C's and 4 H's delta H: look it up in your bond dissociation table or heats of formation table 2 C's and 6 H's -> C2H6 delta H: unknown (you are solving for this) So rearrange these three reactions so that you can get them to cancel, remembering to change the sign of delta H when you reverse an equation. Then add up the delta H's and this will give you your heat of formation of C2H6. 2C's and 4 H's -> C2H4 C2H4 + H2 -> C2H6 ___________________ 2C's and 4 H's +H2 -> C2H6 But you'll have to do it yourself to account for the stoichiometry- I ignored the given mass.

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