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Enthalpy of formation

This is something I really should be able to do by now, but I've never fully got to grips with it. What do I do? I know it should be roughly +460kjmol^-1image.jpg
Original post by The Wavefunction
This is something I really should be able to do by now, but I've never fully got to grips with it. What do I do? I know it should be roughly +460kjmol^-1image.jpg


Is that the answer which was given or did you calculate the enthalpy? if you calculated, could I have a look on your calculation, please?
Original post by Kallisto
Is that the answer which was given or did you calculate the enthalpy? if you calculated, could I have a look on your calculation, please?


Do you know the definition of enthalpy of formation?
Original post by charco
Do you know the definition of enthalpy of formation?


Do you mean the kind of enthalpy where the conditions are standard, so 25 °C (298,15 K) and 1013 kPa? is that right?
Original post by Kallisto
Is that the answer which was given or did you calculate the enthalpy? if you calculated, could I have a look on your calculation, please?


Wikipedia states that the value is +460kjmol^-1.
Original post by The Wavefunction
Wikipedia states that the value is +460kjmol^-1.


I have got +466 kjmol^-1:

E = E(S-N) + E(S=N) - E(S4N4) = 226 kjmol^-1 + 328 kjmol^-1 - 88 kjmol^-1 = 466 kjmol^-1

This result is very close to Wikipedia's given value. Seems to be plausible, as S4N4 consists of S-N and S=N bonds.
Original post by Kallisto
I have got +466 kjmol^-1:

E = E(S-N) + E(S=N) - E(S4N4) = 226 kjmol^-1 + 328 kjmol^-1 - 88 kjmol^-1 = 466 kjmol^-1

This result is very close to Wikipedia's given value. Seems to be plausible, as S4N4 consists of S-N and S=N bonds.

Wouldn't you have to take into account the number of S=N bonds and S-N bonds?
Original post by Kallisto
Do you mean the kind of enthalpy where the conditions are standard, so 25 °C (298,15 K) and 1013 kPa? is that right?


No.

I mean that the enthalpy of formation is the energy change when 1 mol of a substance is formed from its constituent elements in the standard states.

While bond enthalpy terms are always from molecules in the gaseous state.

Hence you must take into account the enthalpy of vapourisation of sulphur etc.
Original post by charco
No.

I mean that the enthalpy of formation is the energy change when 1 mol of a substance is formed from its constituent elements in the standard states.

While bond enthalpy terms are always from molecules in the gaseous state.

Hence you must take into account the enthalpy of vapourisation of sulphur etc.


You seem to be pretty clued up on this, any idea what I do?
Original post by The Wavefunction
You seem to be pretty clued up on this, any idea what I do?


You write an equation for the formation of your product. From the elements (A) in their standard states to the product (B).

You can then use the enthalpy of vapourisation to get everything into gas that needs to be.

Then you construct an energy cycle to get from A to B in your equation.

The sum of the steps from A to B is always the same no matter what route you take (Hess).

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