# 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