Dec 14, 2009
Which change requires an oxidizing agent to produce the indicate product?
a) S2O3^2- ---> S4O6^2-
b) ClO- ----> Cl-
c) SO3 ---> SO4^2-
d) Zn -----> ZnSO4

could you be able to state oxidation number for each choices?
Thank you


10+ Year Member
Jun 23, 2007
In a S2O3^2- O is always 2-. So we have 3*2-= -6. The overall charge is -2. So this means S2 must give us +4, or each S is +2.
For the products the same thing. O is -2. This means each S is +2.5. This does not seem right? You can't have a fractional oxidation number. This is because the oxidation number calculated this way is an average. Since there are 4 sulfurs this is an average of them.

If you draw the lewis structure for S4O6^(2-) you get O3-S-S-S-S-O3
In organic chemistry you should have learned how to calculate the oxidation states separately instead of using the average way. The sulfur connected to O3 has an oxidation of +5. To get this look at S in the periodic table. It has 6 valance electrons normally. It is bonded to 3 oxygen's, the oxygens are more electronegative and pull the electrons away from S, so we pretend as though it lost the electrons. The s is also bond to another s. Since they are the same element they share equally and each s gets 1 electron from the bond. So after doing all of this you will see that that the S attached to the oxygens only has 1 electron from the s-s bond, but it should have a valence of 6. Its oxidation number is +5. The S on the other side is also +5. The S-S in the middle have an oxidation state of 0. They have 2 lone pairs on each s and form 2 bonds to other s elements. So 2 lone pairs + 2 equal bonds = 6 electrons. Oxidation state is 0. So now we have 5+5+0+0=10 and 10/4 = 2.5 (the average oxidation state).

I know that was kind of long and maybe confusing. Try and google another source if that was bad. Here is one maybe Also know that there are some cases where oxygen is not -2. Like in peroxides I think.

For B. The reactants ClO-. O is 2-. So Cl has to be +1.

For C. In the reactants O is 2-. S is +6. In the products O is 2- and S is +6.

For D. Zn is 0 in the reactants. Elements are 0. In the products you should have memorized the polyatomic ions and know that SO4 is 2- overall. This gives us s of +6 and o 2-. If SO4^(2-) then Zn has to be 2+.

Let me know if I got anything wrong.
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10+ Year Member
Sep 4, 2006
Dental Student
kpark, your post always does this, but what is the actual answer?

when something requires oxidating agent, then the reaction must be reduced, right? when reaction is reduced, it loses electron, so whatever must become more positive, right?

I would think D is the answer, but I am horrible at chemistry, so please enlighten me.


10+ Year Member
5+ Year Member
Jun 14, 2008
I think the answer is A Cl is going from a + charge to a - charge so it will gain electrons (reduction is gain). I agree about needing to post answers though too.