[Geddoe]

Atom hybridization changes in many chemical reactions. In each of the following reactions,
determine what change, if any, occurs to the hybridization of the underlined atom.
(a) SO2 + 1/2 O2 -> SO3
(b) PCl3 + Cl- -> PCl4-


For (a) its sp2 hybridized (see http://answers.yahoo.com/question/index?qid=20071026054205AAlS22D, makes sense) for SO2 then in SO3 its sp3 hybridized and for (b) its sp3 as well in PCl3, but I'm unsure of PCl4-....Can't seem to write the proof.
In PCl3 there is a lone pair on P and 3 bonds, so sp3 hybridisation is needed (the 3s2 3p1 3p1 3p1 orbitals produce 4 sp3 hybrid obitals, one of which is a lone pair and the other 3 are half filled and bond to Cl)

In the final structure I/m not sure. I would have though a co-ordinate bond to the extra Cl with the lone pair providing both electrons but that also leaves this Cl with its own 8 electrons and a share of the lone pair. I tried structures with sp3d hybridisation as exists in PCl5 but without success

[orgoclear]

(a) S of SO2 has hybridisation

there are 6 valence electrons out of which 4 are covalently bonded by two sigma and two pi bonds. So, no. of bonded pairs = 2 (no. of sigma bonds)
Now 2 electrons remain

So there is one lone pair. So hybridisation is sp2

SO3 has 3 bond pairs and no lone pairs. so again its hydrization (i.e. of S) is sp2

(b) PCl3

There are 5 valence electrons and 3 bond pairs (all sigma bonds) There is 1 lone pair. So, hybridization is sp3

PCl4-

There are 6 valence electrons (1 extra due to the negative charge)
There are 4 bond pairs and 1 lone pair
So hybridisation is sp3d

[vibhu.kiddy]

please explain me the basic concept of hybridisation.

[AWK]

(b) PCl3 + Cl- -> PCl4-
Are you sure this reaction is possible.
PCl₅ in crystal form has a structure PCl₄⁺PCl₆^-

[tamim83]

(b) PCl3 + Cl- -> PCl4-
Are you sure this reaction is possible.
PCl5 in crystal form has a structure PCl4+PCl6-

I think it may be, it looks like some sort of gas phase reaction.  It looks like a "chemically sound" reaction; the chloride ion would come in opposite the lone pair on phosphorus (back-side attack), and the chlorines already bonded would go from being pyramidal to being planar (can't invert if the lone pair is there).  The result would be a distorted tetrahedral PCl4^- ion.  

please explain me the basic concept of hybridisation

Hybridization is the mixture of orbitals on the same atomic center to produce new orbitals that "point" in the correct angles for bonding.  It was derived by Linus Pauling and was used to explain bond angles for molecules like methane (CH₄) determined spectroscopically.  For methane, the bond angles are all about 109.5 degrees, p orbitals are all perpendicular to one another.  If we mix all of the 2p orbtials with the 2s orbital (takes a lot of math, you have to take linear combinations of wavefunctions) you get 4 sp³ hybrid orbitals that point 109.5 degrees from one another, which matches the experimental data.  Hybridization is part of valence bond theory, which says that orbitals on different atoms must overlap to form a chemical bond.  

Hope this helps

[AWK]

The result would be a distorted tetrahedral PCl4- ion. 

If it can be formed (I doubt) it should be bisphenoidal shape (seesaw)

[tamim83]

The result would be a distorted tetrahedral PCl4- ion. 

If it can be formed (I doubt) it should be bisphenoidal shape (seesaw)

Yes, thats what I meant.  I was taught that the shape was either "distorted tetrahedral" or "see-saw"; I just went with the former.  Like I said, I don't see why this can't form, in the gas phase that is.  I can even think of a molecular beam set up that would possibly give me this (my late Ph. D advisor did things like this and wanted to do more).