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Benzene hydrogenation enthalpy

Hi,
I'm struggling to understand why benzene is proven to be more stable when it has a lower enthalpy of hydrogenation (208 kJ mol^-1) value than the theoretical compound cyclohexa-1,3,5-triene (360kJ mol^1).

Surely if the bonds are stronger more energy would be required to break them, so more energy would be released? If that were the case then wouldn't cyclohexa-1,3,5-triene be more stable? We know it isn't but I can't understand why.

I know why benzene is more stable as a structure, but I don't understand how the hydrogenation values support this. Help? :/
Original post by brrrigid
Hi,
I'm struggling to understand why benzene is proven to be more stable when it has a lower enthalpy of hydrogenation (208 kJ mol^-1) value than the theoretical compound cyclohexa-1,3,5-triene (360kJ mol^1).

Surely if the bonds are stronger more energy would be required to break them, so more energy would be released? If that were the case then wouldn't cyclohexa-1,3,5-triene be more stable? We know it isn't but I can't understand why.

I know why benzene is more stable as a structure, but I don't understand how the hydrogenation values support this. Help? :/


Your problem lies with a fundamental misunderstanding of stability.

When bonds are broken energy is NEEDED and USED not released.

Think of a cup. Without applying energy to the cup it is not possible to break it. Use energy and you can smash it into a thousand pieces.

Making bonds releases energy
Breaking bonds requires energy

Say it out loud and learn it.

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All substances have chemical POTENTIAL energy. This is due to their potential to change when interacting with other substances or changing state.

A stable substance has very low potential energy.

It is not possible to absolutely measure chemical potential energy, but it is possible to measure a change in chemical potential energy because of the law of conservation of energy.

If heat energy is released then potential energy must go down (and vice versa).

If benzene releases less energy on hydrogenation then it must start from a lower potential energy position. i.e. it is more stable to begin with.

Check out this interactive on benzene that I prepared. (hover your mouse over the data on page 2)
(edited 6 years ago)
Reply 2
Original post by charco
Your problem lies with a fundamental misunderstanding of stability.

When bonds are broken energy is NEEDED and USED not released.

Think of a cup. Without applying energy to the cup it is not possible to break it. Use energy and you can smash it into a thousand pieces.

Making bonds releases energy
Breaking bonds requires energy

Say it out loud and learn it.

------------------------------------------------------------------

All substances have chemical POTENTIAL energy. This is due to their potential to change when interacting with other substances or changing state.

A stable substance has very low potential energy.

It is not possible to absolutely measure chemical potential energy, but it is possible to measure a change in chemical potential energy because of the law of conservation of energy.

If heat energy is released then potential energy must go down (and vice versa).

If benzene releases less energy on hydrogenation then it must start from a lower potential energy position. i.e. it is more stable to begin with.

Check out this interactive on benzene that I prepared. (hover your mouse over the data on page 2)


The way you explained it made it so much easier than what I've been looking at! Thank-you so much, I feel like I finally understand :smile:
Reply 3
Original post by brrrigid
it has a lower enthalpy of hydrogenation (208 kJ mol^-1) value than the theoretical compound cyclohexa-1,3,5-triene (360kJ mol^1).


Better to say it has a less exothermic value (-208) than theoretical (-360).

The theoretical is therefore the lower value.
Reply 4
Original post by Pigster
Better to say it has a less exothermic value (-208) than theoretical (-360).

The theoretical is therefore the lower value.


Got it, thankyou :smile:

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