enthalpy of formation vs enthalpy of reaction

[thebillsfan]

What is the enthalpy of formation, ∆Hf, of hydrogen
iodide at 298 K? (Note: The bond energies of H2(g), I2(g), and HI(g) at 298 K are 436 kJ/mol, 151 kJ/mol, and 298 kJ/mol, respectively.)

Okay so the relevant equation is H2+I2-->2HI. Now I got this wrong because I calculated the total energy released, which is something like -9 kJ. When they say enthalpy of formation, are they just talking about ONE MOLE of product formed? So I shouldve divided my answer in two?

So do we need to make a distinction between the total heat released/heat of reaction AND heat of formation? One is extrinsic and the latter is intrinsic (without regard to number of moles)?

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[RogueUnicorn]

standard enthalpies are per mol values.

 

[RogueUnicorn]

What is the enthalpy of formation, ∆Hf, of hydrogen
iodide at 298 K? (Note: The bond energies of H2(g), I2(g), and HI(g) at 298 K are 436 kJ/mol, 151 kJ/mol, and 298 kJ/mol, respectively.)

Okay so the relevant equation is H2+I2-->2HI. Now I got this wrong because I calculated the total energy released, which is something like -9 kJ. When they say enthalpy of formation, are they just talking about ONE MOLE of product formed? So I shouldve divided my answer in two?

So do we need to make a distinction between the total heat released/heat of reaction AND heat of formation? One is extrinsic and the latter is intrinsic (without regard to number of moles)?

heat of a rxn is often reported as a per mol value as well

 

[wanderer]

Okay so the relevant equation is H2+I2-->2HI. Now I got this wrong because I calculated the total energy released, which is something like -9 kJ. When they say enthalpy of formation, are they just talking about ONE MOLE of product formed? So I shouldve divided my answer in two?

Yes. 1/2 H2 + 1/2 I2 --> HI. Hf is the enthalpy of this reaction.

 

[thebillsfan]

heat of a rxn is often reported as a per mol value as well

so how will I know when it is and is not supposed to be given as a per mol value? if they don't specify, what are you to assume?

 

[RogueUnicorn]

so how will I know when it is and is not supposed to be given as a per mol value? if they don't specify, what are you to assume?

units are always specified

 

[ratatat]

Yeah I'm pretty sure they will always specify, ie. say something like 'molar heat of enthalpy'.

 

[pnoybballin]

So the total energy released is -9 kJ but because they are asking for the enthalpy of formation, ∆Hf, the answer is -9/2 kJ = -4.5 kJ?

 

[thebillsfan]

correct. i swear they didnt say molar. they just said enthalpy of formation. i think if it's something "of formation" we can assume its molar. if it says total heat released or "heat released" i'm not going to do it per mole.

 

[RogueUnicorn]

correct. i swear they didnt say molar. they just said enthalpy of formation. i think if it's something "of formation" we can assume its molar. if it says total heat released or "heat released" i'm not going to do it per mole.

yes