enthalpy of formation vs enthalpy of reaction

thebillsfan

Unseasoned Veteran
10+ Year Member
5+ Year Member
What is the enthalpy of formation, &#8710;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)?

RogueUnicorn

rawr.
7+ Year Member
standard enthalpies are per mol values.

RogueUnicorn

rawr.
7+ Year Member
What is the enthalpy of formation, &#8710;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

10+ Year Member
5+ Year Member
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.

OP
T

thebillsfan

Unseasoned Veteran
10+ Year Member
5+ Year Member
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

rawr.
7+ Year Member
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

Junior Member
10+ Year Member
5+ Year Member
So the total energy released is -9 kJ but because they are asking for the enthalpy of formation, &#8710;Hf, the answer is -9/2 kJ = -4.5 kJ?

OP
T

thebillsfan

Unseasoned Veteran
10+ Year Member
5+ Year Member
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

rawr.
7+ Year Member
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