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OCR AS level chemistry A Jan 2012 F322 question 4.

I am doing the January 2012 paper before examination on Tuesday. I can't work out why my answer to this question is incorrect. Chains, Energy and Resources.

Question 4,Hydrogen and chlorine are reacted together to form hydrogen chloride.H2(g) + Cl2(g) -> 2HCl(g) delta H = -184 kJ per mol.

(a) Calculate the bond enthalpy for the H-Cl bond using the information in the table below.

Bond bond enthalpy/kJ per mol
H-H +436
Cl-Cl +243

I get the answer of 'bond enthalpy = 247.5 kJ per mol', but the mark scheme says 431.5.Any help greatly appreciated.
(H-H) + (Cl-Cl) = 2(Cl-H) + Delta H
The delta H is the energy released when forming these new bond Cl-H
Reply 2
Avatar for Duckdrake
Duckdrake
OP
Original post by jonnypdot
(H-H) + (Cl-Cl) = 2(Cl-H) + Delta H
The delta H is the energy released when forming these new bond Cl-H


Yes, I don't need the delta H. I need the bond enthalpy, which I have worked out using that formula that you have said, and I have got the answer as 247.5 kJ per mol.

I have looked at the mark scheme and I have put the calculation into my calculator exactly the way they have said it, and I get the answer 247.5 kJ per mol; while they get 431.5 kJ per mol. I have no idea how to get this answer. Is the mark scheme wrong?
Okay took longer than I'd like to admit:

Enthalpy on product side is 679
679-2x=-184
Therefore 2x = 863
x = 431.5kJ per mole.
(edited 8 years ago)
Original post by Duckdrake
Yes, I don't need the delta H. I need the bond enthalpy, which I have worked out using that formula that you have said, and I have got the answer as 247.5 kJ per mol.

I have looked at the mark scheme and I have put the calculation into my calculator exactly the way they have said it, and I get the answer 247.5 kJ per mol; while they get 431.5 kJ per mol. I have no idea how to get this answer. Is the mark scheme wrong?


you have to use the delta h to work out the bond enthalpy
Reply 5
Avatar for Duckdrake
Duckdrake
OP
Original post by bluespiked
Okay took longer than I'd like to admit:

Enthalpy on product side is 679
679-2x=-184
Therefore 2x = 863
x = 431.5kJ per mole.


Thank you very much, I can see now, I missed a minus.

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