Reaction enthalpy involving H-H

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Andrew Nguyen 2I
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Joined: Fri Sep 29, 2017 7:07 am

Reaction enthalpy involving H-H

Postby Andrew Nguyen 2I » Thu Jan 25, 2018 7:30 pm

Why are the bond enthalpies for H-H about 300-400 kJ/mol, but the enthalpy of formation for H-H zero kj/mol?

Tiffany Dao 1A
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am

Re: Reaction enthalpy involving H-H

Postby Tiffany Dao 1A » Thu Jan 25, 2018 7:33 pm

Bond enthalpy and enthalpy of formation are two different things. So the bond enthalpy is the energy for the bond between the two hydrogen atoms. However, the enthalpy of formation is for the standard state. In lecture, the example states that H2 -> H2 requires no energy because it's already in its simplest state.

AtreyiMitra2L
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Re: Reaction enthalpy involving H-H

Postby AtreyiMitra2L » Thu Jan 25, 2018 8:04 pm

Bond enthalpy is the amount of energy to break one mole of the substance. On the other hand, enthalpy of formation is the amount of energy required to make the substance by substances in their standard states (the state at which they will be found at 1 atm). H2 at one atm is in gas. Therefore, because it is already in the standard state, no extra energy is required.


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