[joey11223]

Hey guys i have two quick questions concerning module 2, revising it now for January re-take...

1) Just so i know, when it asks questions like define enthalpy of combustion or formation, can i say " when blah blah.. under standard conditions" or do i actually have to put "under 298k and 100KPa?"

2) Random question i can't figure out..

the Hc for diamond= -397Kj mol-1 and the Hc for graphite = -394KJ mol-1

how much energy would be required to change 10g of graphite into diamond? The answer is +2.5KJ but i don't understand how that is found exactly...

any help would be great

[shengoc]

Originally Posted by joey11223

Hey guys i have two quick questions concerning module 2, revising it now for January re-take...

1) Just so i know, when it asks questions like define enthalpy of combustion or formation, can i say " when blah blah.. under standard conditions" or do i actually have to put "under 298k and 100KPa?"

2) Random question i can't figure out..

the Hc for diamond= -397Kj mol-1 and the Hc for graphite = -394KJ mol-1

how much energy would be required to change 10g of graphite into diamond? The answer is +2.5KJ but i don't understand how that is found exactly...

any help would be great

For question one, standard conditions does imply 298 K and stable states, but the temperature of 298 K is not implied, but i guess for a level, you should probably include it anyway.

For question 2, the enthalpy is in kJ per mole, so simply convert the 10g of graphite/diamond(both are carbon of Mr roughly 12 g/mol), so you get mole of C = 10/12 = 5/6 mol

so energy difference between them = 5/6 mol x (-394 -[-397]) kJ/mol = 5/6 x 3 kJ = +5/2 kJ = +2.5 kJ(this is for the change graphite to diamond, therefore the enthalpy is that of diamond minus that of graphite.

Hope that helps.

[joey11223]

thanks a lot, gah its like i knew i would need to use the 12 from Carbon and 10/12 and get the differnece between em -394--397 so 10/12x 3kj but for some weird reason them both being made of carbon put me off doing that. lolz

So you think putting under standard conditions isn't enough and i should at least put standard pressure and 298k?

[shengoc]

Originally Posted by joey11223

thanks a lot, gah its like i knew i would need to use the 12 from Carbon and 10/12 and get the differnece between em -394--397 so 10/12x 3kj but for some weird reason them both being made of carbon put me off doing that. lolz

So you think putting under standard conditions isn't enough and i should at least put standard pressure and 298k?

Alright, last year when i was in a level, the mark scheme insisted on the 298 K, but only now in uni, i am told(and read through in text as well), standard state simply implies pressure of 1 bar(=10^5 Pa) and also standard(normal) states. You should realise that enthalpies/entropy/free energy values might change with temperature.

[shengoc]

Originally Posted by joey11223

thanks a lot, gah its like i knew i would need to use the 12 from Carbon and 10/12 and get the differnece between em -394--397 so 10/12x 3kj but for some weird reason them both being made of carbon put me off doing that. lolz

So you think putting under standard conditions isn't enough and i should at least put standard pressure and 298k?

Also, carbon and graphite are only written as C(s), they differ mainly in density, but in 1 mol, their masses would be the same(or very similar)