[antoinetta]

Hi, I'm just wondering if somebody wouldn't mind checking these for me ... I'm supposed to balance these to figure about the number of electrons transferred from the reducing agent to the oxidizing agent for the conventionally balanced equation.  All these reactions occur in basic aqueous solution

Cr(OH)₃ + Br₂ = Br^? + CrO₄^2?
Al + NO₂^? = AlO₂^? + NH₃
Sn(OH)₆^2? + Si = HSnO₂^?  + SiO₃^2?

Be + SO₃^2? = S₂O₃^2? + Be₂O₃^2?

So, I balanced these:

 10 OH^- + 2 Cr(OH)₃ + 3 Br₂ = 6 Br^- + 2 CrO₄^2- + 8 H₂O
   =6 electrons

 OH^- + 2 Al + H₂O + NO₂^- =  2 AlO₂^- + NH₃ 
   =6 electrons

 2 Sn(OH)₆^2- + Si = 2 HSnO₂^- + SiO₃^2- + 5 H₂O
   = 4 electrons

 2 Be + 2 SO₃^2- = S₂O₃^2- + Be₂O₃^2-
   =2 electrons

[DevaDevil]

The best way of doing this is to try and get half reactions before you balance the whole.

10 OH^- + 2 Cr(OH)₃ + 3 Br₂ = 6 Br^- + 2 CrO₄^2- + 8 H₂O
   =6 electrons

half reactions:
2x   5 OH^- + Cr(OH)₃  = CrO₄^2- + 4 H₂O + 3 e^-
3x   Br₂ + 2 e^- = 2 Br^-


and indeed 6 electrons total transfered; 3 per reducing agent (Cr(OH)₃)

OH^- + 2 Al + H₂O + NO₂^- =  2 AlO₂^- + NH₃ 
   =6 electrons

Half reactions are tricky to find here:
2x  Al  =  Al₃⁺ + 3 e^-
1x OH^- + H₂O + NO₂^- + 6 e^- =  4 O^2- + NH₃
and of course
Al³⁺ + 2 O^2- = AlO₂^-

and indeed 6 transfered total, 3 per reducing agent (Al)

2 Sn(OH)₆^2- + Si = 2 HSnO₂^- + SiO₃^2- + 5 H₂O
   = 4 electrons

indeed 4 electrons per Si

2 Be + 2 SO₃^2- = S₂O₃^2- + Be₂O₃^2-
   =2 electrons

2 electrons transfered per Be, 4 total

[antoinetta]

2 Be + 2 SO₃^2- = S₂O₃^2- + Be₂O₃^2-
   =2 electrons

2 electrons transfered per Be, 4 total

Just a quick question, but why do you have to consider the fact that there are 2 Be molecules, for a total of 4 electrons transferred?

[DevaDevil]

well, the half reactions in this case are
(2x)  Be = Be²⁺ + 2 e^-
2 SO₃^2- + 4 e^- = S₂O₃^2- + 3 O^2-

with
2 Be²⁺ + 3 O^2- --> Be₂O₃^2-

the reducing agent in this case is Be, with 2 electrons per beryllium
the oxidising agent is the sulfite, which receives also 2 electrons per sulfate

But the whole reaction as balanced includes 4 electrons.


as i said before, look at the half reactions and you will easily see the amount of electrons in a reaction.
Hope that helps