Chemistry A-Level Born Haber Cycles

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N70
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My understanding of enthalpy change of atomisation is that it is the enthalpy change when one mole of gaseous atoms are formed from an element (e.g 1/2 Cl2 (g) = Cl (g) )
So when you create a born haber cycle for MgCl2, you double the enthalpy change of atomisation (as you are forming 2 moles of Cl)

But why is it that when you create a born haber cycle for MgO, you halve the enthalpy change of atomisation?
I have this step as 1/2O2 (g) = O(g) which I thought was the enthalpy change of atomisation of oxygen? Is this a mistake in the mark scheme or have I made a mistake?
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gingur
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To my best understanding, you are correct. The Chemguide revision sheet for thermodynamics has 1.2O2 (g) -> O (g) as the example for enthalpy of atomisation (https://chemrevise.files.wordpress.c...namics-aqa.pdf)

Excuse my formatting lmao
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charco
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(Original post by N70)
My understanding of enthalpy change of atomisation is that it is the enthalpy change when one mole of gaseous atoms are formed from an element (e.g 1/2 Cl2 (g) = Cl (g) )
So when you create a born haber cycle for MgCl2, you double the enthalpy change of atomisation (as you are forming 2 moles of Cl)

But why is it that when you create a born haber cycle for MgO, you halve the enthalpy change of atomisation?
I have this step as 1/2O2 (g) = O(g) which I thought was the enthalpy change of atomisation of oxygen? Is this a mistake in the mark scheme or have I made a mistake?
You have to be careful about which energy changes you are given, as the definitions are all-important.

As you say, the enthalpy of atomisation refers to the formation of 1 mol of gaseous atoms from an element in its standard state.

However, the bond enthalpy term refers to the energy needed to break 1 mol of covalent bonds in a gaseous molecule.

So:
The bond dissociation enthalpy
Cl2(g) ==> 2Cl(g) - the energy change = 242 kJ mol-1

Whereas the atomisation enthalpy
1/2Cl2(g) ==> Cl(g) - the energy change = 121 kJ mol-1

I expect that you have been given the bond enthalpy of oxygen, not the enthalpy of atomisation.

Here's a run through of the MgO Born-Haber cycle:




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Last edited by charco; 4 months ago
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N70
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(Original post by gingur)
To my best understanding, you are correct. The Chemguide revision sheet for thermodynamics has 1.2O2 (g) -> O (g) as the example for enthalpy of atomisation (https://chemrevise.files.wordpress.c...namics-aqa.pdf)

Excuse my formatting lmao
(Original post by charco)
You have to be careful about which energy changes you are given, as the definitions are all-important.

As you say, the enthalpy of atomisation refers to the formation of 1 mol of gaseous atoms from an element in its standard state.

However, the bond enthalpy term refers to the energy needed to break 1 mol of covalent bonds in a gaseous molecule.

So:
The bond dissociation enthalpy
Cl2(g) ==> 2Cl(g) - the energy change = 242 kJ mol-1

Whereas the atomisation enthalpy
1/2Cl2(g) ==> Cl(g) - the energy change = 121 kJ mol-1

I expect that you have been given the bond enthalpy of oxygen, not the enthalpy of atomisation.

Here's a run through of the MgO Born-Haber cycle:




---------------------------------------------------------------
Signature
Colourful Solutions Chemistry Videos on YouTube
Thank you!! This makes a lot more sense!
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