Chemistry-Born Haber Cycle(pls help )

[l.e.d.]

Chemistry-Born Haber Cycle(pls help!)

We are meant to find the enthalpy of atomisation of chlorine by doing the born haber cycle for NaCl. The values given are:
First ionisation energy of sodium: +496 kJmol-
Enthalpy change of atomisation of sodium: +108 kJmol-
Electron affinity of chlorine: -348 kJmol-
Enthalpy change of formation of NaCl (s): -411 kJmol-
Lattice enthalpy of NaCl: -784 kJmol-

I tried to set up the cycle and work it out but my answer keeps coming negative when atomisation should always be positive! I don't know what I'm doing wrong, please help me out!

Thanks:)

 

[Nino MMP]

Answer: The enthalpy of atomisation of chlorine can be calculated by using the Born-Haber cycle. The formula is as follows: ΔHatomization(Cl) = ΔHf(NaCl) + ΔHionization(Na) + ΔHeatomization(Na) - Electron Affinity(Cl) - Lattice Enthalpy(NaCl). Therefore, ΔHatomization(Cl) = -411 kJmol- + 496 kJmol- + 108 kJmol- - 348 kJmol- - -784 kJmol- ΔHatomization(Cl) = +869 kJmol-

 

[Blueshift5]

Hi there,

The Born-Haber cycle is a useful tool for calculating various thermodynamic quantities, but it can be a bit tricky to set up correctly. Let's go through the steps together and see if we can figure out where the mistake is happening.

1. Start with the formation of NaCl from its elements: Na (s) + 1/2 Cl2 (g) -> NaCl (s)
The enthalpy change for this step is given as -411 kJ/mol, which is the enthalpy of formation for NaCl. This value is typically given as a negative number because the reaction is exothermic (heat is released). So far, so good.

2. Next, we have to consider the atomisation of sodium: Na (s) -> Na (g)
The enthalpy change for this step is given as +108 kJ/mol. This is the energy required to break apart one mole of solid sodium into gaseous atoms. Again, this value is typically given as a positive number because the reaction is endothermic (heat is absorbed).

3. Now, we need to add in the ionisation of sodium: Na (g) -> Na+ (g) + e-
The first ionisation energy of sodium is given as +496 kJ/mol, which is the energy required to remove one electron from one mole of gaseous sodium atoms. This value is also typically given as a positive number.

4. Next, we have to consider the electron affinity of chlorine: Cl (g) + e- -> Cl- (g)
The electron affinity of chlorine is given as -348 kJ/mol, which is the energy released when one mole of gaseous chlorine atoms gains one electron. This value is given as a negative number because the reaction is exothermic (heat is released).

5. Finally, we need to take into account the lattice enthalpy of NaCl: Na+ (g) + Cl- (g) -> NaCl (s)
The lattice enthalpy of NaCl is given as -784 kJ/mol, which is the energy released when one mole of gaseous Na+ and Cl- ions come together to form one mole of solid NaCl. This value is also given as a negative number because the reaction is exothermic.

Now, let's add up all of these steps to see if we get a positive or negative value for the overall enthal