The Student Room Group

Help lattice Enthalpy question

Hey, I don’t understand why the Ca(g) goes back to being a solid Ca(s). Because the first Ca(s) to Ca(g) is atomisation of Calcium which I understand and the 0.5O2 (g) to O(g) is atomisation of O.
Plz help me
C808CC6A-14B9-4AA1-AC89-613EF84BBC49.jpeg247D5683-5B91-49E6-9143-0EBF2F09473D.jpeg
And I don’t get the markscheme too. Can someone send worked solutions to this question. It’s not against guidelines since I told you what I know and that’s my working out part (I just write that on my paper). People learn in different ways I learn by observing/ by seeing the working out my brain can work through it and understand at a steady pace.
(edited 1 year ago)
Original post by Alevelhelp.1
Hey, I don’t understand why the Ca(g) goes back to being a solid Ca(s). Because the first Ca(s) to Ca(g) is atomisation of Calcium which I understand and the 0.5O2 (g) to O(g) is atomisation of O.
Plz help me
C808CC6A-14B9-4AA1-AC89-613EF84BBC49.jpeg247D5683-5B91-49E6-9143-0EBF2F09473D.jpeg
And I don’t get the markscheme too. Can someone send worked solutions to this question. It’s not against guidelines since I told you what I know and that’s my working out part (I just write that on my paper). People learn in different ways I learn by observing/ by seeing the working out my brain can work through it and understand at a steady pace.


I think that’s a typo and the top line should read “Ca (g) + O (g)”. Even so, it wouldn’t affect your answer in any way, as to adjust the calculation to account for the apparent typo, you’d subtract and then add the atomisation enthalpy of Ca, which effectively cancels out the effect.

The mark scheme simply requires you to complete the Born-Haber cycle with the following lines (given in the same order as the MS, as this order makes the most sense):

“Ca^+ (g) + O (g) + e^-“ (The arrow to platform with these formulae is pointing up)

”Ca^2+ (g) + O (g) + 2e^-“ (The arrow to platform with these formulae is pointing up)

”Ca^2+ (g) + O^- (g) + e^-“ (The arrow to the platform with these formulae is pointing down, because this process is exothermic)

”Ca^2+ (g) + O^2- (g)“ (The arrow to the platform with these formulae is pointing up)

You should then draw an arrow down labelled “lattice enthalpy (of formation)” between the platform with “Ca^2+ (g) + O^2- (g)“ and the line at the bottom.

With Born-Haber cycles, the enthalpy change of formation is the sum of all the other enthalpy changes, as per Hess’ law. You can therefore set up an equation and rearrange it:

Using the fact that the enthalpy change of formation is the sum of everything you else: (ΔHf CaO) = (atomisation of Ca) + (atomisation of O) + (1st Ei of Ca) + (2nd Ei of Ca) + (1st Ea of O) + (2nd Ea of O) + (Lattice enthalpy of formation of CaO)

==> (Lattice enthalpy of formation of CaO) = (atomisation of Ca) + (atomisation of O) + (1st Ei of Ca) + (2nd Ei of Ca) + (1st Ea of O) + (2nd Ea of O) - (ΔHf CaO)
(edited 1 year ago)

Quick Reply

Latest