Born haber cycle help

How would I calculate the Cl-Cl bond enthalpy from a MgCl2 born haber cycle with all the other values given to us in the question?

Reply 1

apply_medicine

Original post

by Erd_g

How would I calculate the Cl-Cl bond enthalpy from a MgCl2 born haber cycle with all the other values given to us in the question?

depends what other information you are given, (enthalpy of lattice formation/dissociation). just remember that you have to times the electon affinity of chlorine by 2. replace where chlorine is getting “atomised” as the bond enthalpy of Cl-Cl is the same as 2x the atomisation of chlorine.

replace the born haber arrows with the values given, and where chlorine is getting atomised, replace with X and solve to find X

sorry if that made no sense

Reply 2

TypicalNerd

Original post

by Erd_g

How would I calculate the Cl-Cl bond enthalpy from a MgCl2 born haber cycle with all the other values given to us in the question?

If I am correct in assuming this is a typical question where you have the standard formation enthalpy of MgCl₂(s), atomisation enthalpy of Mg, the first and second ionisation energies of Mg etc etc, then you should note that the standard formation enthalpy is the sum of all the other processes occurring as per the Born-Haber cycle.

In this particular case, you have

Δ_fH^o [MgCl₂(s)] = Δ_atmH [Mg] + IE1(Mg) + IE2(Mg) + Δ_bH [Cl-Cl] + 2 x Ea [Cl] + U [MgCl₂]

Where Δ_fH^o denotes a standard formation enthalpy, Δ_atmH denotes an atomisation enthalpy, IE denotes an ionisation energy, Δ_bH denotes a bond enthalpy, Ea denotes an electron affinity and U denotes a lattice formation enthalpy/energy.

You would simply define the bond enthalpy as some unknown (e.g. “x”, like in GCSE maths), plug in the numbers you know and calculate the value of x from there.

If you are with AQA or Edexcel, you do have to be careful with the lattice enthalpy/energy though, as it can be defined either as the formation enthalpy (exothermic) or the dissociation enthalpy (endothermic), though it is generally more common at A level for the first definition to be used. OCR only uses lattice formation enthalpy in their questions, which is the correct definition to work with when carrying out the above calculations.

(edited 1 month ago)

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