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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: kenhung123 on May 26, 2010, 06:34:26 pm

Title: Why not a redox?
Post by: kenhung123 on May 26, 2010, 06:34:26 pm
ZnSO3(s) + 2H+(aq) => Zn2+(aq) + SO2(g) + H2O(l)

Zn oxidation number left=+8 on the right +2
S oxidation number left=-2 on the right +4

Title: Re: Why not a redox?
Post by: Martoman on May 26, 2010, 06:46:09 pm
I actually want to know how you got 8 on the left as I struggle with this. I know you have oxygen as -2 so its -6 overall but then what? think of the ions?
Title: Re: Why not a redox?
Post by: kenhung123 on May 26, 2010, 06:54:46 pm
I assigned S as -2, is that a wrong assumption?
Title: Re: Why not a redox?
Post by: Martoman on May 26, 2010, 07:09:20 pm
I don't know enough about oxidation numbers to authrotatively say, but isn't O more electronegative and thus it gets the -ve oxidation number? like in oxygen is +2 coz fluorine is more electronegative. This line of reasoning suggests that S cannot be negative.
Title: Re: Why not a redox?
Post by: Potter on May 26, 2010, 07:16:43 pm
Quote from: kenhung123 on May 26, 2010, 06:34:26 pm
ZnSO3(s) + 2H+(aq) => Zn2+(aq) + SO2(g) + H2O(l)

Zn oxidation number left=+8 on the right +2
S oxidation number left=-2 on the right +4



SO3 has a 2- charge. Since ZnSO3 is an ionic substance the charges have to balance. Such as NaCl or MgCl2. So, the charge on the Zn is 2+ and if you look at the list of rules for oxidation the charge of an ion = it's oxidation number.

Of course there are exceptions like peroxides and superoxides etc..

SO3 has a 2- charge, therefore the oxidation numbers = the charge.

3 x -2 = -6, -6 + 4 = -2

so S has a +4 Oxidation number

And like what you worked out.. Zn has a 2+ charge on the other side and S has a 4+ therefore, it's not an oxidation reaction.

Title: Re: Why not a redox?
Post by: vexx on May 26, 2010, 07:16:56 pm
Quote from: kenhung123 on May 26, 2010, 06:34:26 pm
ZnSO3(s) + 2H+(aq) => Zn2+(aq) + SO2(g) + H2O(l)

Zn oxidation number left=+8 on the right +2
S oxidation number left=-2 on the right +4



hmm i would have thought about it like this..
The ion bonds to Zn, which indicates the Zn is a 2+ ion, and as seen on the right of this equation, the Zn is still 2+ therefore hasn't changed.
And the S remains 4+ throughout, same with the H+.
:. no redox reaction
Title: Re: Why not a redox?
Post by: vexx on May 26, 2010, 07:17:32 pm
Quote from: Potter on May 26, 2010, 07:16:43 pm
Quote from: kenhung123 on May 26, 2010, 06:34:26 pm
ZnSO3(s) + 2H+(aq) => Zn2+(aq) + SO2(g) + H2O(l)

Zn oxidation number left=+8 on the right +2
S oxidation number left=-2 on the right +4



SO3 has a -2 charge. Since ZnSO3 is an ionic substance the charges have to balance. Such as NaCl or MgCl2. So, the charge on the Zn is 2+ and if you look at the list of rules for oxidation the charge of an ion = it's oxidation number.

Of course there are exceptions like peroxides and superoxides etc..

SO3 has a 2- charge, therefore the oxidation numbers = the charge.

3 x -2 = -6, -6 + 4 = -2

so S has a +4 Oxidation number

And like what you worked out.. Zn has a 2+ charge on the other side and S has a 4+ therefore, it's not an oxidation reaction.



beaten;P
Title: Re: Why not a redox?
Post by: kenhung123 on May 26, 2010, 07:21:01 pm
Thanks alot
Title: Re: Why not a redox?
Post by: Martoman on May 26, 2010, 09:08:41 pm
mmmmmmm i was kinda right with the sulfur and ions then. Thats good.
Title: Re: Why not a redox?
Post by: m@tty on May 26, 2010, 11:51:16 pm
Like with NH4NO3 you can't just say:

"H=+1 , O=-2 therefore N=+1"...

Each nitrogen is in a different oxidation state...

Oxidation numbers are more accurately derived from the actual structure of the molecule. For every bond assign -1 to the more electronegative partner and +1 for the other less electronegative member. A faster way of doing this is by the ions as said above, but still there is room for error there...

So for the above you split into ions:
 NH4+ => x+4=1 => x=-3

 NO3- => x-6=-1 =>x=5

...
Title: Re: Why not a redox?
Post by: Mao on May 27, 2010, 07:21:35 pm
When you do oxidation numbers, it helps to bring electron configuration into the picture. If Zn was +8, its valence shell configuration would be crazy. On the other hand, it would make a lot more sense if it was +2 (you often see Zn2+).