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Topic: Three reactions which I can't understand....help please.  (Read 15060 times)

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Offline junbumkim

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Three reactions which I can't understand....help please.
« on: May 27, 2007, 05:02:45 AM »
I am studying for PCAT, and couldn't understand some basic reactions..
I'd appreciate if you one of you experts could give me a detailed explanations on these following reactions..I am studying by myself, so it's hard to look for a person to get help from..

1. An excess amount of cholorine gas is passed over hot iron fillings.
    Cl2 + F --> FeCl3 , in this reaction, why does Fe have +3
   charge in the product? Couldn't it have +2 ?

2. Solid litium hydride is added to water
   LiH + H2O -> Li+ + OH- + H2(g)
   Would I be wrong to say if it just formed LiOH and more water instead of H2
   
gas? How can one know it forms H2?

3. A concentrated solution of ammonia is added to a solution of Zinc Iodide
    NH3 + ZnI2  ---> Zn(OH)2   + NH4+
    Where does OH come from?
 

Offline Yggdrasil

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Re: Three reactions which I can't understand....help please.
« Reply #1 on: May 27, 2007, 12:55:11 PM »
1. An excess amount of cholorine gas is passed over hot iron fillings.
    Cl2 + F --> FeCl3 , in this reaction, why does Fe have +3
   charge in the product? Couldn't it have +2 ?

Yes, the two most common oxidation states of iron are +3 and +2.  I don't know how you would tell which form gets created in your reaction (note, your reaction is not balanced).

Quote
2. Solid litium hydride is added to water
   LiH + H2O -> Li+ + OH- + H2(g)
   Would I be wrong to say if it just formed LiOH and more water instead of H2
   
gas? How can one know it forms H2?

This is an acid/base reaction.  However, it is a strange acid/base reaction because the hydride ion (H-) is acting as your base.  As in a normal acid/base reaction, the base picks up the proton from your acid (H2O), creating H2.  This leaves behind an Li+ ion and a OH- ion.  The reaction you have written will not work (for example, try to balance the reaction that you have written).

Quote
3. A concentrated solution of ammonia is added to a solution of Zinc Iodide
    NH3 + ZnI2  ---> Zn(OH)2   + NH4+
    Where does OH come from?

In the presence of water, ammonia reacts to form NH4OH.

Offline AWK

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Re: Three reactions which I can't understand....help please.
« Reply #2 on: May 28, 2007, 03:37:24 AM »
1. An excess amount of cholorine gas is passed over hot iron fillings.
    Cl2 + F --> FeCl3 , in this reaction, why does Fe have +3
   charge in the product? Couldn't it have +2 ?

Yes, the two most common oxidation states of iron are +3 and +2.  I don't know how you would tell which form gets created in your reaction (note, your reaction is not balanced).
Cl2 is a strong oxidizer and always Fe(III) compound will be obtained in this reaction
But in this reaction
Fe + 2HCl = FeCl2 + H2
in the persence of a strong reductor (H2 in this case ) only Fe(II) compounds can be obtained
]2. Solid litium hydride is added to water
   LiH + H2O -> Li+ + OH- + H2(g)
   Would I be wrong to say if it just formed LiOH and more water instead of H2
   
gas? How can one know it forms H2?

This is an acid/base reaction.  However, it is a strange acid/base reaction because the hydride ion (H-) is acting as your base.  As in a normal acid/base reaction, the base picks up the proton from your acid (H2O), creating H2.  This leaves behind an Li+ ion and a OH- ion.  The reaction you have written will not work (for example, try to balance the reaction that you have written).
This reaction will proceed vigorously
3. A concentrated solution of ammonia is added to a solution of Zinc Iodide
    NH3 + ZnI2  ---> Zn(OH)2   + NH4+
    Where does OH come from?
In the presence of water, ammonia reacts to form NH4OH.

Concentrated water solution of ammonia will form [Zn(NH3)4]2+ cation
Stoichometric amount of diluted ammonia will form:
ZnI2 + 2NH3 +2H2O = Zn(OH)2(s) + 2NH4+ + 2I-
AWK

Offline junbumkim

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Re: Three reactions which I can't understand....help please.
« Reply #3 on: May 28, 2007, 09:24:59 AM »
Thanks a lot for your help.

Offline Borek

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Re: Three reactions which I can't understand....help please.
« Reply #4 on: May 28, 2007, 11:53:24 AM »
Cl2 is a strong oxidizer and always Fe(III) compound will be obtained in this reaction
But in this reaction
Fe + 2HCl = FeCl2 + H2
in the persence of a strong reductor (H2 in this case ) only Fe(II) compounds can be obtained

I would rather say that H+ is too weak oxidizer (E=0) to oxidize Fe2+ further (required potential being 0.77).
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Offline leeeooooo

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Re: Three reactions which I can't understand....help please.
« Reply #5 on: June 02, 2007, 11:41:55 AM »
its because chlorine gas is in excess

Offline sjb

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Re: Three reactions which I can't understand....help please.
« Reply #6 on: June 04, 2007, 04:53:37 AM »
its because chlorine gas is in excess

I wouldn't quite go that far.

Consider for instance the reaction between stilbene (PhCH=CHPh) and Br2. Even if the bromine is in a large excess, under regular conditions there's very little chance you'll get 1,1,2,2-tetrabromo-1,2-diphenylethane (PhCBr2CBr2Ph).

S

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