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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: HSOoi on July 20, 2015, 10:30:46 AM

Title: Stability of FeCl3 vs FeCl2
Post by: HSOoi on July 20, 2015, 10:30:46 AM

Hi there,

From what I have learned:
1. Fe3+ ion have fully Half-filled 4d orbital.
2. Fe2+ ion have one more electron in 4d orbital than Fe3+.
3. Fully filled or Fully Half-Filled orbital configuration is more stable.
4. Therefore, Fe3+ configuration is more stable than Fe2+ configuration.

Question here is:
2FeCl₃ + Fe  :rarrow: 3FeCl₂

How can this be possible? Isn't FeCl₃ more stable because of it's orbital configuration?
Why would it react to form less stable compound?
What am I missing here?
Please advice...

Title: Re: Stability of FeCl3 vs FeCl2
Post by: MrTeo on July 20, 2015, 01:06:42 PM

I'm not sure if the "half-filled" ruled applies that easily also to transition metals, to be honest. Going down the periodic table more effects sum up and the easy rules (nay, approximations) true for the first elements aren't good anymore (even though Fe is from the first transition series so I'd expect a fairly decent beahaviour). What about hydration? I would guess that this aspects plays quite an important role as the hydration shell of Fe³⁺ will be fairly different from Fe²⁺ and generally when we're discussing redox reactions we're talking about water solutions. There's also a good example of this in the first group, when you try to compare the redox potential of Li with the other alkali metals.

Any inorganic chemist who could validate/crush my gibberish?

Title: Re: Stability of FeCl3 vs FeCl2
Post by: Enthalpy on July 21, 2015, 03:41:16 AM

The heat of formation (as solids, not solutions, sorry) of FeCl₃ is -400kJ/mol versus -342kJ/mol for FeCl₂, but the reaction replaces 2 moles by 3 moles.

The reasoning also ignores the fate of chlorine, but it's important besides the iron ions.

Metallic iron too is important. Aggregating it from individual atoms means a huge energy. The heat of formation of the compounds is measured versus their constituent elements in normal state, here solid iron, hence the my comparison at first line holds, but it's conceptually important: comparing only FeCl₃ and FeCl₂ with atomic iron would neglect that Fe is solid.

Title: Re: Stability of FeCl3 vs FeCl2
Post by: Corribus on July 21, 2015, 10:49:22 AM

Breaking up a regular solid like iron is also usually entropically favorable.