[BaO]

i need your opinions , here is the question
A reducing agent will cause which of the following changes?
 
A  ClO3^- --> ClO2
B  HS2O4---> H2SO3
C  H3PO3 --> H3PO4
D  NO3^- --> N2O2
 
i picked C , but the answer was A, what do you think ?

thanks in advance
 

[Yggdrasil]

In a redox reaction, a reducing agent becomes oxidized and the other compound in the rection becomes reduced.  Therefore, look for the reactant whose oxidation number is higher than that of the reactant.

If you look at all of the oxidation number of the reactions, you see that A is the only one in which the oxidation number of the products (the ON of the Cl is 4) is lower than the oxidation state of the reactants (the ON of the Cl is 5).  In C, the ON of P in H3PO3 is 3, whereas the ON of P in H3PO4 is 5, so this is an oxidation, not a reduction.

[BaO]

but i thought reducing agent means oxidation. i dont get it.

[wereworm73]

To look at it another way, a reducing agent gives up electrons while the oxidizing agent takes them.

When ClO₃^- was converted to ClO₂, it had to take an electron.  A reducing agent could give this electron.  So, this answer was the correct one.

However, when H₃PO₃ was converted to H₃PO₄, it had to give up a pair of electrons to an oxidizing agent. 

[BaO]

or maybe i dont understand the question because i think the question asks us to find which one undergoes oxidation . is it?


thank you

[wereworm73]

No, the question is asking which one is the reduction reaction.  A reducing agent is going to reduce another substance, not oxidize it.

[BaO]

it's so weird!, because my book says :" the oxidizing angent is reduced during the reaction . the reducing agent is oxidized during the reaction."

 i'm not gonna say anyone wrong , may be i didnt really understand the lesson?

[wereworm73]

The book is correct, but I can see how that might be confusing.

So, I'll try to explain this a little differently...

When something is "reduced", it gained electrons.
And when something is "oxidized", it lost electrons.

Now when a reducing agent gives away its electrons to another substance, it's the same thing as saying that the reducing agent lost electrons, which is the same thing as saying the reducing agent was "oxidized".   Meanwhile, that other substance in the reaction was "reduced" because it gained electrons (from the reducing agent).

Likewise, when a oxidizing agent takes electrons from another substance, it's the same thing as saying that the oxidizing agent is gaining electrons, which is the same thing as saying the oxidizing agent was "reduced".  Meanwhile, the other substance in the reaction was "oxidized" because it lost electrons (to the oxidizing agent).

[BaO]

so the question means for "other substance" right ? thank you very much wereworm73, i should've read the question more carefully

[skyglow1]

A reducing agent reduces something else. Think about things like cleaning agent, it cleans something else. When the reducing agent reduces something else, it gets oxidized itself. In this question, they ask you which of the changes could be that "something else" that is being reduced by a reducing agent.

Looking at the oxidiation numbers, if one of the elements in the compounds has decreased its oxidation number, say from +2 to +1, then it has been reduced and that would be the answer.

[wave]

how about other two answers?  I think they are also right.Is that right?

[wereworm73]

Good eye, wave!

Yeah, a reducing agent would give the nitrate ion an electron to become N₂O₄.   And if "HS₂O₄" was really supposed to be H₂SO₄, then that too would be a reduction reaction, since the sulfur gained 2 electrons from a reducing agent.

[wave]

thank you for your approval,wereworm73.
I am very glad to discuss together with you.So it seems the question itself is something wrong.

[BaO]

i did check the question from the book where i got it.
youre right, it really needs to be edited again!