Calculate standard enthalpy of formation from thermodynamic data?

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020200
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Hi,

I am a bit confused as to how to do this question:

Use data from the Tables 1 and 2 to calculate the standard enthalpy of formation for Calcium fluoride.

How can you calculate the standard enthalpy of formation if calcium is not in its standard state? How would you do this question anyway? Can someone please explain the steps?

Thanks!
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HiggsBoson
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(Original post by 020200)
Hi,

I am a bit confused as to how to do this question:

Use data from the Tables 1 and 2 to calculate the standard enthalpy of formation for Calcium fluoride.

How can you calculate the standard enthalpy of formation if calcium is not in its standard state? How would you do this question anyway? Can someone please explain the steps?

Thanks!
I think you will need to draw a Born-Haber cycle.
Once you have that drawn, the formation enthalpy should be equal to all the other values going clockwise.
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username3718068
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(Original post by 020200)
Hi,

I am a bit confused as to how to do this question:

Use data from the Tables 1 and 2 to calculate the standard enthalpy of formation for Calcium fluoride.

How can you calculate the standard enthalpy of formation if calcium is not in its standard state? How would you do this question anyway? Can someone please explain the steps?

Thanks!
You would have to construct a Born - Haber cycle as HiggsBoson has said.
btw calcium is in its standard state, and the data for that is included in the table.

as to how to solve the question, you start with the equation:
Ca(s) + F2(g) => CaF2(s)

you also know that Ca has lost 2 electrons and each fluorine atom has gained one.
So you need to think of a way from the given data to convert Ca(s) to Ca2+(s)
(hint: you first convert the calcium into gaseous calcium first then do enthalpy of ionisation of Ca, then Ca+). Don't use ionisation enthalpy of Ca and multiply by 2 as that will give you the wrong answer.

As for flourine, you need to use bond dissociation enthalpy of 1 mole of diatomic flourine, then use enthalpy of electron affinity (multiplied by 2 as there are 2 moles of flourine now).

Then do lattice formation enthalpy. (note: this is the negative of lattice dissociation in the given data).

Once you have constructed the born Haber cycle with these numbers, you can then easily calculate enthalpy of formation.
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020200
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(Original post by dip0)
You would have to construct a Born - Haber cycle as HiggsBoson has said.
btw calcium is in its standard state, and the data for that is included in the table.

as to how to solve the question, you start with the equation:
Ca(s) + F2(g) => CaF2(s)

you also know that Ca has lost 2 electrons and each fluorine atom has gained one.
So you need to think of a way from the given data to convert Ca(s) to Ca2+(s)
(hint: you first convert the calcium into gaseous calcium first then do enthalpy of ionisation of Ca, then Ca+). Don't use ionisation enthalpy of Ca and multiply by 2 as that will give you the wrong answer.

As for flourine, you need to use bond dissociation enthalpy of 1 mole of diatomic flourine, then use enthalpy of electron affinity (multiplied by 2 as there are 2 moles of flourine now).

Then do lattice formation enthalpy. (note: this is the negative of lattice formation in the given data).

Once you have constructed the born Haber cycle with these numbers, you can then easily calculate enthalpy of formation.
Thanks! It had just been a while since I had done thermodynamics so forgot that you could just work out enthalpy of formation from using the cycle.
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