Born haber cycles I just can't do them

Seaders

I am ok up till the the delta H3 part however I dont understand MG2+ + 2E- +2CL Part.

Reply 1

BabyGirl

Ok I think delta H3 is the 2nd Ionisation energy for Mg and then 2 x H4 is the atomisation of chlorine :smile:

Reply 2

Ghetto_Princess12

Delta H3 is second ionisation energy therefore you multiply it by 2, also the electron affirmity (h5) is 2x Cl.

Reply 3

Seaders

Is there a site which gives rules for constructing born haber because I struggle with it.

Reply 4

sez

you should look at getting a copy of the letts chem guide
for a-level its REEEEEEEEEEALLY good

Reply 5

sez

Eg sodium chloride
ok you have 2 route options route 1 or route 2
Route 1

Na(s) + 1/2Cl2(g) ----> Na+Cl-(s) (F- enthalpy change of formation of Nacl)

Route 2

•

-Production of gaseous atoms:

Na(s)---->Na(g) (A- enthalpy change of atomisation of Na)
1/2Cl2(g)---->Cl(g) (B- enthalpy change of atomisation of Cl)

•

-production of gaseous ions

Na(g)---->Na+(g) + e- (C- 1st ionisation energy of Na)
Cl(g) + e- ---->Cl-(g) (D- 1st electron affinity of Cl)

•

production of the solid ionic lattice

Na+(g)+Cl_(g) ----> Na+Cl-(s) (E- lattice enthalpy of NaCl)

Route1:F
Route2:A+B+C+D+E

using Hess's law: A+B+C+D+E=F

Reply 6

sez

basically this applies to any example
let me know if it helps ... it took a while lol

Reply 7

sparklet

(MG2+ + 2E- )+2CL

The bit in brackets represents second ionisation energy equation of magnesiul. The 2CL is written like that as that was the step for enthalpy of atomisation for chlorine, hence it is not written as CL2. The born haber cycles are usually in orderly fashion. One thing you must remember is that, for example CL2. When you atomise it, you will times the value by two (since there are two atoms), when you carry out first electron affinity, that value must be timed by two as well (again two atoms).